mod.rs

// ignore-tidy-filelength

//! String manipulation.
//!
//! For more details, see the [`std::str`] module.
//!
//! [`std::str`]: self

#![stable(feature = "rust1", since = "1.0.0")]

use self::pattern::Pattern;
use self::pattern::{DoubleEndedSearcher, ReverseSearcher, Searcher};

use crate::char;
use crate::fmt::{self, Write};
use crate::iter::{Chain, FlatMap, Flatten};
use crate::iter::{Copied, Filter, FusedIterator, Map, TrustedLen, TrustedRandomAccess};
use crate::mem;
use crate::ops::Try;
use crate::option;
use crate::slice::{self, SliceIndex, Split as SliceSplit};

pub mod pattern;

#[unstable(feature = "str_internals", issue = "none")]
#[allow(missing_docs)]
pub mod lossy;

/// Parse a value from a string
///
/// `FromStr`'s [`from_str`] method is often used implicitly, through
/// [`str`]'s [`parse`] method. See [`parse`]'s documentation for examples.
///
/// [`from_str`]: FromStr::from_str
/// [`parse`]: str::parse
///
/// `FromStr` does not have a lifetime parameter, and so you can only parse types
/// that do not contain a lifetime parameter themselves. In other words, you can
/// parse an `i32` with `FromStr`, but not a `&i32`. You can parse a struct that
/// contains an `i32`, but not one that contains an `&i32`.
///
/// # Examples
///
/// Basic implementation of `FromStr` on an example `Point` type:
///
/// ```
/// use std::str::FromStr;
/// use std::num::ParseIntError;
///
/// #[derive(Debug, PartialEq)]
/// struct Point {
///     x: i32,
///     y: i32
/// }
///
/// impl FromStr for Point {
///     type Err = ParseIntError;
///
///     fn from_str(s: &str) -> Result<Self, Self::Err> {
///         let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
///                                  .split(',')
///                                  .collect();
///
///         let x_fromstr = coords[0].parse::<i32>()?;
///         let y_fromstr = coords[1].parse::<i32>()?;
///
///         Ok(Point { x: x_fromstr, y: y_fromstr })
///     }
/// }
///
/// let p = Point::from_str("(1,2)");
/// assert_eq!(p.unwrap(), Point{ x: 1, y: 2} )
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub trait FromStr: Sized {
    /// The associated error which can be returned from parsing.
    #[stable(feature = "rust1", since = "1.0.0")]
    type Err;

    /// Parses a string `s` to return a value of this type.
    ///
    /// If parsing succeeds, return the value inside [`Ok`], otherwise
    /// when the string is ill-formatted return an error specific to the
    /// inside [`Err`]. The error type is specific to implementation of the trait.
    ///
    /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    ///
    /// # Examples
    ///
    /// Basic usage with [`i32`][ithirtytwo], a type that implements `FromStr`:
    ///
    /// [ithirtytwo]: ../../std/primitive.i32.html
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// let s = "5";
    /// let x = i32::from_str(s).unwrap();
    ///
    /// assert_eq!(5, x);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl FromStr for bool {
    type Err = ParseBoolError;

    /// Parse a `bool` from a string.
    ///
    /// Yields a `Result<bool, ParseBoolError>`, because `s` may or may not
    /// actually be parseable.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// assert_eq!(FromStr::from_str("true"), Ok(true));
    /// assert_eq!(FromStr::from_str("false"), Ok(false));
    /// assert!(<bool as FromStr>::from_str("not even a boolean").is_err());
    /// ```
    ///
    /// Note, in many cases, the `.parse()` method on `str` is more proper.
    ///
    /// ```
    /// assert_eq!("true".parse(), Ok(true));
    /// assert_eq!("false".parse(), Ok(false));
    /// assert!("not even a boolean".parse::<bool>().is_err());
    /// ```
    #[inline]
    fn from_str(s: &str) -> Result<bool, ParseBoolError> {
        match s {
            "true" => Ok(true),
            "false" => Ok(false),
            _ => Err(ParseBoolError { _priv: () }),
        }
    }
}

/// An error returned when parsing a `bool` using [`from_str`] fails
///
/// [`from_str`]: FromStr::from_str
#[derive(Debug, Clone, PartialEq, Eq)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct ParseBoolError {
    _priv: (),
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for ParseBoolError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        "provided string was not `true` or `false`".fmt(f)
    }
}

/*
Section: Creating a string
*/

/// Errors which can occur when attempting to interpret a sequence of [`u8`]
/// as a string.
///
/// As such, the `from_utf8` family of functions and methods for both [`String`]s
/// and [`&str`]s make use of this error, for example.
///
/// [`String`]: ../../std/string/struct.String.html#method.from_utf8
/// [`&str`]: from_utf8
///
/// # Examples
///
/// This error type’s methods can be used to create functionality
/// similar to `String::from_utf8_lossy` without allocating heap memory:
///
/// ```
/// fn from_utf8_lossy<F>(mut input: &[u8], mut push: F) where F: FnMut(&str) {
///     loop {
///         match std::str::from_utf8(input) {
///             Ok(valid) => {
///                 push(valid);
///                 break
///             }
///             Err(error) => {
///                 let (valid, after_valid) = input.split_at(error.valid_up_to());
///                 unsafe {
///                     push(std::str::from_utf8_unchecked(valid))
///                 }
///                 push("\u{FFFD}");
///
///                 if let Some(invalid_sequence_length) = error.error_len() {
///                     input = &after_valid[invalid_sequence_length..]
///                 } else {
///                     break
///                 }
///             }
///         }
///     }
/// }
/// ```
#[derive(Copy, Eq, PartialEq, Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Utf8Error {
    valid_up_to: usize,
    error_len: Option<u8>,
}

impl Utf8Error {
    /// Returns the index in the given string up to which valid UTF-8 was
    /// verified.
    ///
    /// It is the maximum index such that `from_utf8(&input[..index])`
    /// would return `Ok(_)`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::str;
    ///
    /// // some invalid bytes, in a vector
    /// let sparkle_heart = vec![0, 159, 146, 150];
    ///
    /// // std::str::from_utf8 returns a Utf8Error
    /// let error = str::from_utf8(&sparkle_heart).unwrap_err();
    ///
    /// // the second byte is invalid here
    /// assert_eq!(1, error.valid_up_to());
    /// ```
    #[stable(feature = "utf8_error", since = "1.5.0")]
    pub fn valid_up_to(&self) -> usize {
        self.valid_up_to
    }

    /// Provides more information about the failure:
    ///
    /// * `None`: the end of the input was reached unexpectedly.
    ///   `self.valid_up_to()` is 1 to 3 bytes from the end of the input.
    ///   If a byte stream (such as a file or a network socket) is being decoded incrementally,
    ///   this could be a valid `char` whose UTF-8 byte sequence is spanning multiple chunks.
    ///
    /// * `Some(len)`: an unexpected byte was encountered.
    ///   The length provided is that of the invalid byte sequence
    ///   that starts at the index given by `valid_up_to()`.
    ///   Decoding should resume after that sequence
    ///   (after inserting a [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD]) in case of
    ///   lossy decoding.
    ///
    /// [U+FFFD]: ../../std/char/constant.REPLACEMENT_CHARACTER.html
    #[stable(feature = "utf8_error_error_len", since = "1.20.0")]
    pub fn error_len(&self) -> Option<usize> {
        self.error_len.map(|len| len as usize)
    }
}

/// Converts a slice of bytes to a string slice.
///
/// A string slice ([`&str`]) is made of bytes ([`u8`]), and a byte slice
/// ([`&[u8]`][byteslice]) is made of bytes, so this function converts between
/// the two. Not all byte slices are valid string slices, however: [`&str`] requires
/// that it is valid UTF-8. `from_utf8()` checks to ensure that the bytes are valid
/// UTF-8, and then does the conversion.
///
/// [`&str`]: str
/// [byteslice]: ../../std/primitive.slice.html
///
/// If you are sure that the byte slice is valid UTF-8, and you don't want to
/// incur the overhead of the validity check, there is an unsafe version of
/// this function, [`from_utf8_unchecked`][fromutf8u], which has the same
/// behavior but skips the check.
///
/// [fromutf8u]: fn.from_utf8_unchecked.html
///
/// If you need a `String` instead of a `&str`, consider
/// [`String::from_utf8`][string].
///
/// [string]: ../../std/string/struct.String.html#method.from_utf8
///
/// Because you can stack-allocate a `[u8; N]`, and you can take a
/// [`&[u8]`][byteslice] of it, this function is one way to have a
/// stack-allocated string. There is an example of this in the
/// examples section below.
///
/// [byteslice]: ../../std/primitive.slice.html
///
/// # Errors
///
/// Returns `Err` if the slice is not UTF-8 with a description as to why the
/// provided slice is not UTF-8.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // some bytes, in a vector
/// let sparkle_heart = vec![240, 159, 146, 150];
///
/// // We know these bytes are valid, so just use `unwrap()`.
/// let sparkle_heart = str::from_utf8(&sparkle_heart).unwrap();
///
/// assert_eq!("💖", sparkle_heart);
/// ```
///
/// Incorrect bytes:
///
/// ```
/// use std::str;
///
/// // some invalid bytes, in a vector
/// let sparkle_heart = vec![0, 159, 146, 150];
///
/// assert!(str::from_utf8(&sparkle_heart).is_err());
/// ```
///
/// See the docs for [`Utf8Error`][error] for more details on the kinds of
/// errors that can be returned.
///
/// [error]: struct.Utf8Error.html
///
/// A "stack allocated string":
///
/// ```
/// use std::str;
///
/// // some bytes, in a stack-allocated array
/// let sparkle_heart = [240, 159, 146, 150];
///
/// // We know these bytes are valid, so just use `unwrap()`.
/// let sparkle_heart = str::from_utf8(&sparkle_heart).unwrap();
///
/// assert_eq!("💖", sparkle_heart);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
    run_utf8_validation(v)?;
    // SAFETY: Just ran validation.
    Ok(unsafe { from_utf8_unchecked(v) })
}

/// Converts a mutable slice of bytes to a mutable string slice.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // "Hello, Rust!" as a mutable vector
/// let mut hellorust = vec![72, 101, 108, 108, 111, 44, 32, 82, 117, 115, 116, 33];
///
/// // As we know these bytes are valid, we can use `unwrap()`
/// let outstr = str::from_utf8_mut(&mut hellorust).unwrap();
///
/// assert_eq!("Hello, Rust!", outstr);
/// ```
///
/// Incorrect bytes:
///
/// ```
/// use std::str;
///
/// // Some invalid bytes in a mutable vector
/// let mut invalid = vec![128, 223];
///
/// assert!(str::from_utf8_mut(&mut invalid).is_err());
/// ```
/// See the docs for [`Utf8Error`][error] for more details on the kinds of
/// errors that can be returned.
///
/// [error]: struct.Utf8Error.html
#[stable(feature = "str_mut_extras", since = "1.20.0")]
pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
    run_utf8_validation(v)?;
    // SAFETY: Just ran validation.
    Ok(unsafe { from_utf8_unchecked_mut(v) })
}

/// Converts a slice of bytes to a string slice without checking
/// that the string contains valid UTF-8.
///
/// See the safe version, [`from_utf8`][fromutf8], for more information.
///
/// [fromutf8]: fn.from_utf8.html
///
/// # Safety
///
/// This function is unsafe because it does not check that the bytes passed to
/// it are valid UTF-8. If this constraint is violated, undefined behavior
/// results, as the rest of Rust assumes that [`&str`]s are valid UTF-8.
///
/// [`&str`]: str
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // some bytes, in a vector
/// let sparkle_heart = vec![240, 159, 146, 150];
///
/// let sparkle_heart = unsafe {
///     str::from_utf8_unchecked(&sparkle_heart)
/// };
///
/// assert_eq!("💖", sparkle_heart);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
    // SAFETY: the caller must guarantee that the bytes `v`
    // are valid UTF-8, thus the cast to `*const str` is safe.
    // Also, the pointer dereference is safe because that pointer
    // comes from a reference which is guaranteed to be valid for reads.
    unsafe { &*(v as *const [u8] as *const str) }
}

/// Converts a slice of bytes to a string slice without checking
/// that the string contains valid UTF-8; mutable version.
///
/// See the immutable version, [`from_utf8_unchecked()`] for more information.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// let mut heart = vec![240, 159, 146, 150];
/// let heart = unsafe { str::from_utf8_unchecked_mut(&mut heart) };
///
/// assert_eq!("💖", heart);
/// ```
#[inline]
#[stable(feature = "str_mut_extras", since = "1.20.0")]
pub unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
    // SAFETY: the caller must guarantee that the bytes `v`
    // are valid UTF-8, thus the cast to `*mut str` is safe.
    // Also, the pointer dereference is safe because that pointer
    // comes from a reference which is guaranteed to be valid for writes.
    unsafe { &mut *(v as *mut [u8] as *mut str) }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for Utf8Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(error_len) = self.error_len {
            write!(
                f,
                "invalid utf-8 sequence of {} bytes from index {}",
                error_len, self.valid_up_to
            )
        } else {
            write!(f, "incomplete utf-8 byte sequence from index {}", self.valid_up_to)
        }
    }
}

/*
Section: Iterators
*/

/// An iterator over the [`char`]s of a string slice.
///
///
/// This struct is created by the [`chars`] method on [`str`].
/// See its documentation for more.
///
/// [`chars`]: str::chars
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Chars<'a> {
    iter: slice::Iter<'a, u8>,
}

/// Returns the initial codepoint accumulator for the first byte.
/// The first byte is special, only want bottom 5 bits for width 2, 4 bits
/// for width 3, and 3 bits for width 4.
#[inline]
fn utf8_first_byte(byte: u8, width: u32) -> u32 {
    (byte & (0x7F >> width)) as u32
}

/// Returns the value of `ch` updated with continuation byte `byte`.
#[inline]
fn utf8_acc_cont_byte(ch: u32, byte: u8) -> u32 {
    (ch << 6) | (byte & CONT_MASK) as u32
}

/// Checks whether the byte is a UTF-8 continuation byte (i.e., starts with the
/// bits `10`).
#[inline]
fn utf8_is_cont_byte(byte: u8) -> bool {
    (byte & !CONT_MASK) == TAG_CONT_U8
}

#[inline]
fn unwrap_or_0(opt: Option<&u8>) -> u8 {
    match opt {
        Some(&byte) => byte,
        None => 0,
    }
}

/// Reads the next code point out of a byte iterator (assuming a
/// UTF-8-like encoding).
#[unstable(feature = "str_internals", issue = "none")]
#[inline]
pub fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) -> Option<u32> {
    // Decode UTF-8
    let x = *bytes.next()?;
    if x < 128 {
        return Some(x as u32);
    }

    // Multibyte case follows
    // Decode from a byte combination out of: [[[x y] z] w]
    // NOTE: Performance is sensitive to the exact formulation here
    let init = utf8_first_byte(x, 2);
    let y = unwrap_or_0(bytes.next());
    let mut ch = utf8_acc_cont_byte(init, y);
    if x >= 0xE0 {
        // [[x y z] w] case
        // 5th bit in 0xE0 .. 0xEF is always clear, so `init` is still valid
        let z = unwrap_or_0(bytes.next());
        let y_z = utf8_acc_cont_byte((y & CONT_MASK) as u32, z);
        ch = init << 12 | y_z;
        if x >= 0xF0 {
            // [x y z w] case
            // use only the lower 3 bits of `init`
            let w = unwrap_or_0(bytes.next());
            ch = (init & 7) << 18 | utf8_acc_cont_byte(y_z, w);
        }
    }

    Some(ch)
}

/// Reads the last code point out of a byte iterator (assuming a
/// UTF-8-like encoding).
#[inline]
fn next_code_point_reverse<'a, I>(bytes: &mut I) -> Option<u32>
where
    I: DoubleEndedIterator<Item = &'a u8>,
{
    // Decode UTF-8
    let w = match *bytes.next_back()? {
        next_byte if next_byte < 128 => return Some(next_byte as u32),
        back_byte => back_byte,
    };

    // Multibyte case follows
    // Decode from a byte combination out of: [x [y [z w]]]
    let mut ch;
    let z = unwrap_or_0(bytes.next_back());
    ch = utf8_first_byte(z, 2);
    if utf8_is_cont_byte(z) {
        let y = unwrap_or_0(bytes.next_back());
        ch = utf8_first_byte(y, 3);
        if utf8_is_cont_byte(y) {
            let x = unwrap_or_0(bytes.next_back());
            ch = utf8_first_byte(x, 4);
            ch = utf8_acc_cont_byte(ch, y);
        }
        ch = utf8_acc_cont_byte(ch, z);
    }
    ch = utf8_acc_cont_byte(ch, w);

    Some(ch)
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for Chars<'a> {
    type Item = char;

    #[inline]
    fn next(&mut self) -> Option<char> {
        next_code_point(&mut self.iter).map(|ch| {
            // SAFETY: `str` invariant says `ch` is a valid Unicode Scalar Value.
            unsafe { char::from_u32_unchecked(ch) }
        })
    }

    #[inline]
    fn count(self) -> usize {
        // length in `char` is equal to the number of non-continuation bytes
        let bytes_len = self.iter.len();
        let mut cont_bytes = 0;
        for &byte in self.iter {
            cont_bytes += utf8_is_cont_byte(byte) as usize;
        }
        bytes_len - cont_bytes
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.iter.len();
        // `(len + 3)` can't overflow, because we know that the `slice::Iter`
        // belongs to a slice in memory which has a maximum length of
        // `isize::MAX` (that's well below `usize::MAX`).
        ((len + 3) / 4, Some(len))
    }

    #[inline]
    fn last(mut self) -> Option<char> {
        // No need to go through the entire string.
        self.next_back()
    }
}

#[stable(feature = "chars_debug_impl", since = "1.38.0")]
impl fmt::Debug for Chars<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Chars(")?;
        f.debug_list().entries(self.clone()).finish()?;
        write!(f, ")")?;
        Ok(())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for Chars<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<char> {
        next_code_point_reverse(&mut self.iter).map(|ch| {
            // SAFETY: `str` invariant says `ch` is a valid Unicode Scalar Value.
            unsafe { char::from_u32_unchecked(ch) }
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Chars<'_> {}

impl<'a> Chars<'a> {
    /// Views the underlying data as a subslice of the original data.
    ///
    /// This has the same lifetime as the original slice, and so the
    /// iterator can continue to be used while this exists.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut chars = "abc".chars();
    ///
    /// assert_eq!(chars.as_str(), "abc");
    /// chars.next();
    /// assert_eq!(chars.as_str(), "bc");
    /// chars.next();
    /// chars.next();
    /// assert_eq!(chars.as_str(), "");
    /// ```
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    #[inline]
    pub fn as_str(&self) -> &'a str {
        // SAFETY: `Chars` is only made from a str, which guarantees the iter is valid UTF-8.
        unsafe { from_utf8_unchecked(self.iter.as_slice()) }
    }
}

/// An iterator over the [`char`]s of a string slice, and their positions.
///
/// This struct is created by the [`char_indices`] method on [`str`].
/// See its documentation for more.
///
/// [`char_indices`]: str::char_indices
#[derive(Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct CharIndices<'a> {
    front_offset: usize,
    iter: Chars<'a>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for CharIndices<'a> {
    type Item = (usize, char);

    #[inline]
    fn next(&mut self) -> Option<(usize, char)> {
        let pre_len = self.iter.iter.len();
        match self.iter.next() {
            None => None,
            Some(ch) => {
                let index = self.front_offset;
                let len = self.iter.iter.len();
                self.front_offset += pre_len - len;
                Some((index, ch))
            }
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<(usize, char)> {
        // No need to go through the entire string.
        self.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for CharIndices<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<(usize, char)> {
        self.iter.next_back().map(|ch| {
            let index = self.front_offset + self.iter.iter.len();
            (index, ch)
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for CharIndices<'_> {}

impl<'a> CharIndices<'a> {
    /// Views the underlying data as a subslice of the original data.
    ///
    /// This has the same lifetime as the original slice, and so the
    /// iterator can continue to be used while this exists.
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    #[inline]
    pub fn as_str(&self) -> &'a str {
        self.iter.as_str()
    }
}

/// An iterator over the bytes of a string slice.
///
/// This struct is created by the [`bytes`] method on [`str`].
/// See its documentation for more.
///
/// [`bytes`]: str::bytes
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone, Debug)]
pub struct Bytes<'a>(Copied<slice::Iter<'a, u8>>);

#[stable(feature = "rust1", since = "1.0.0")]
impl Iterator for Bytes<'_> {
    type Item = u8;

    #[inline]
    fn next(&mut self) -> Option<u8> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }

    #[inline]
    fn count(self) -> usize {
        self.0.count()
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        self.0.last()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        self.0.nth(n)
    }

    #[inline]
    fn all<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    {
        self.0.all(f)
    }

    #[inline]
    fn any<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    {
        self.0.any(f)
    }

    #[inline]
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.0.find(predicate)
    }

    #[inline]
    fn position<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
    {
        self.0.position(predicate)
    }

    #[inline]
    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
    {
        self.0.rposition(predicate)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl DoubleEndedIterator for Bytes<'_> {
    #[inline]
    fn next_back(&mut self) -> Option<u8> {
        self.0.next_back()
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        self.0.nth_back(n)
    }

    #[inline]
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.0.rfind(predicate)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl ExactSizeIterator for Bytes<'_> {
    #[inline]
    fn len(&self) -> usize {
        self.0.len()
    }

    #[inline]
    fn is_empty(&self) -> bool {
        self.0.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Bytes<'_> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl TrustedLen for Bytes<'_> {}

#[doc(hidden)]
unsafe impl TrustedRandomAccess for Bytes<'_> {
    unsafe fn get_unchecked(&mut self, i: usize) -> u8 {
        // SAFETY: the caller must uphold the safety contract
        // for `TrustedRandomAccess::get_unchecked`.
        unsafe { self.0.get_unchecked(i) }
    }
    fn may_have_side_effect() -> bool {
        false
    }
}

/// This macro generates a Clone impl for string pattern API
/// wrapper types of the form X<'a, P>
macro_rules! derive_pattern_clone {
    (clone $t:ident with |$s:ident| $e:expr) => {
        impl<'a, P> Clone for $t<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                let $s = self;
                $e
            }
        }
    };
}

/// This macro generates two public iterator structs
/// wrapping a private internal one that makes use of the `Pattern` API.
///
/// For all patterns `P: Pattern<'a>` the following items will be
/// generated (generics omitted):
///
/// struct $forward_iterator($internal_iterator);
/// struct $reverse_iterator($internal_iterator);
///
/// impl Iterator for $forward_iterator
/// { /* internal ends up calling Searcher::next_match() */ }
///
/// impl DoubleEndedIterator for $forward_iterator
///       where P::Searcher: DoubleEndedSearcher
/// { /* internal ends up calling Searcher::next_match_back() */ }
///
/// impl Iterator for $reverse_iterator
///       where P::Searcher: ReverseSearcher
/// { /* internal ends up calling Searcher::next_match_back() */ }
///
/// impl DoubleEndedIterator for $reverse_iterator
///       where P::Searcher: DoubleEndedSearcher
/// { /* internal ends up calling Searcher::next_match() */ }
///
/// The internal one is defined outside the macro, and has almost the same
/// semantic as a DoubleEndedIterator by delegating to `pattern::Searcher` and
/// `pattern::ReverseSearcher` for both forward and reverse iteration.
///
/// "Almost", because a `Searcher` and a `ReverseSearcher` for a given
/// `Pattern` might not return the same elements, so actually implementing
/// `DoubleEndedIterator` for it would be incorrect.
/// (See the docs in `str::pattern` for more details)
///
/// However, the internal struct still represents a single ended iterator from
/// either end, and depending on pattern is also a valid double ended iterator,
/// so the two wrapper structs implement `Iterator`
/// and `DoubleEndedIterator` depending on the concrete pattern type, leading
/// to the complex impls seen above.
macro_rules! generate_pattern_iterators {
    {
        // Forward iterator
        forward:
            $(#[$forward_iterator_attribute:meta])*
            struct $forward_iterator:ident;

        // Reverse iterator
        reverse:
            $(#[$reverse_iterator_attribute:meta])*
            struct $reverse_iterator:ident;

        // Stability of all generated items
        stability:
            $(#[$common_stability_attribute:meta])*

        // Internal almost-iterator that is being delegated to
        internal:
            $internal_iterator:ident yielding ($iterty:ty);

        // Kind of delegation - either single ended or double ended
        delegate $($t:tt)*
    } => {
        $(#[$forward_iterator_attribute])*
        $(#[$common_stability_attribute])*
        pub struct $forward_iterator<'a, P: Pattern<'a>>($internal_iterator<'a, P>);

        $(#[$common_stability_attribute])*
        impl<'a, P> fmt::Debug for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: fmt::Debug>,
        {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_tuple(stringify!($forward_iterator))
                    .field(&self.0)
                    .finish()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P: Pattern<'a>> Iterator for $forward_iterator<'a, P> {
            type Item = $iterty;

            #[inline]
            fn next(&mut self) -> Option<$iterty> {
                self.0.next()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Clone for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                $forward_iterator(self.0.clone())
            }
        }

        $(#[$reverse_iterator_attribute])*
        $(#[$common_stability_attribute])*
        pub struct $reverse_iterator<'a, P: Pattern<'a>>($internal_iterator<'a, P>);

        $(#[$common_stability_attribute])*
        impl<'a, P> fmt::Debug for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: fmt::Debug>,
        {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_tuple(stringify!($reverse_iterator))
                    .field(&self.0)
                    .finish()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Iterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
        {
            type Item = $iterty;

            #[inline]
            fn next(&mut self) -> Option<$iterty> {
                self.0.next_back()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Clone for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                $reverse_iterator(self.0.clone())
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, P: Pattern<'a>> FusedIterator for $forward_iterator<'a, P> {}

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, P> FusedIterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
        {}

        generate_pattern_iterators!($($t)* with $(#[$common_stability_attribute])*,
                                                $forward_iterator,
                                                $reverse_iterator, $iterty);
    };
    {
        double ended; with $(#[$common_stability_attribute:meta])*,
                           $forward_iterator:ident,
                           $reverse_iterator:ident, $iterty:ty
    } => {
        $(#[$common_stability_attribute])*
        impl<'a, P> DoubleEndedIterator for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>,
        {
            #[inline]
            fn next_back(&mut self) -> Option<$iterty> {
                self.0.next_back()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> DoubleEndedIterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>,
        {
            #[inline]
            fn next_back(&mut self) -> Option<$iterty> {
                self.0.next()
            }
        }
    };
    {
        single ended; with $(#[$common_stability_attribute:meta])*,
                           $forward_iterator:ident,
                           $reverse_iterator:ident, $iterty:ty
    } => {}
}

derive_pattern_clone! {
    clone SplitInternal
    with |s| SplitInternal { matcher: s.matcher.clone(), ..*s }
}

struct SplitInternal<'a, P: Pattern<'a>> {
    start: usize,
    end: usize,
    matcher: P::Searcher,
    allow_trailing_empty: bool,
    finished: bool,
}

impl<'a, P> fmt::Debug for SplitInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInternal")
            .field("start", &self.start)
            .field("end", &self.end)
            .field("matcher", &self.matcher)
            .field("allow_trailing_empty", &self.allow_trailing_empty)
            .field("finished", &self.finished)
            .finish()
    }
}

impl<'a, P: Pattern<'a>> SplitInternal<'a, P> {
    #[inline]
    fn get_end(&mut self) -> Option<&'a str> {
        if !self.finished && (self.allow_trailing_empty || self.end - self.start > 0) {
            self.finished = true;
            // SAFETY: `self.start` and `self.end` always lie on unicode boundaries.
            unsafe {
                let string = self.matcher.haystack().get_unchecked(self.start..self.end);
                Some(string)
            }
        } else {
            None
        }
    }

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        if self.finished {
            return None;
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match() {
            // SAFETY: `Searcher` guarantees that `a` and `b` lie on unicode boundaries.
            Some((a, b)) => unsafe {
                let elt = haystack.get_unchecked(self.start..a);
                self.start = b;
                Some(elt)
            },
            None => self.get_end(),
        }
    }

    #[inline]
    fn next_inclusive(&mut self) -> Option<&'a str> {
        if self.finished {
            return None;
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match() {
            // SAFETY: `Searcher` guarantees that `b` lies on unicode boundary,
            // and self.start is either the start of the original string,
            // or `b` was assigned to it, so it also lies on unicode boundary.
            Some((_, b)) => unsafe {
                let elt = haystack.get_unchecked(self.start..b);
                self.start = b;
                Some(elt)
            },
            None => self.get_end(),
        }
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        if self.finished {
            return None;
        }

        if !self.allow_trailing_empty {
            self.allow_trailing_empty = true;
            match self.next_back() {
                Some(elt) if !elt.is_empty() => return Some(elt),
                _ => {
                    if self.finished {
                        return None;
                    }
                }
            }
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match_back() {
            // SAFETY: `Searcher` guarantees that `a` and `b` lie on unicode boundaries.
            Some((a, b)) => unsafe {
                let elt = haystack.get_unchecked(b..self.end);
                self.end = a;
                Some(elt)
            },
            // SAFETY: `self.start` and `self.end` always lie on unicode boundaries.
            None => unsafe {
                self.finished = true;
                Some(haystack.get_unchecked(self.start..self.end))
            },
        }
    }

    #[inline]
    fn next_back_inclusive(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        if self.finished {
            return None;
        }

        if !self.allow_trailing_empty {
            self.allow_trailing_empty = true;
            match self.next_back_inclusive() {
                Some(elt) if !elt.is_empty() => return Some(elt),
                _ => {
                    if self.finished {
                        return None;
                    }
                }
            }
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match_back() {
            // SAFETY: `Searcher` guarantees that `b` lies on unicode boundary,
            // and self.end is either the end of the original string,
            // or `b` was assigned to it, so it also lies on unicode boundary.
            Some((_, b)) => unsafe {
                let elt = haystack.get_unchecked(b..self.end);
                self.end = b;
                Some(elt)
            },
            // SAFETY: self.start is either the start of the original string,
            // or start of a substring that represents the part of the string that hasn't
            // iterated yet. Either way, it is guaranteed to lie on unicode boundary.
            // self.end is either the end of the original string,
            // or `b` was assigned to it, so it also lies on unicode boundary.
            None => unsafe {
                self.finished = true;
                Some(haystack.get_unchecked(self.start..self.end))
            },
        }
    }
}

generate_pattern_iterators! {
    forward:
        /// Created with the method [`split`].
        ///
        /// [`split`]: str::split
        struct Split;
    reverse:
        /// Created with the method [`rsplit`].
        ///
        /// [`rsplit`]: str::rsplit
        struct RSplit;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitInternal yielding (&'a str);
    delegate double ended;
}

generate_pattern_iterators! {
    forward:
        /// Created with the method [`split_terminator`].
        ///
        /// [`split_terminator`]: str::split_terminator
        struct SplitTerminator;
    reverse:
        /// Created with the method [`rsplit_terminator`].
        ///
        /// [`rsplit_terminator`]: str::rsplit_terminator
        struct RSplitTerminator;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitInternal yielding (&'a str);
    delegate double ended;
}

derive_pattern_clone! {
    clone SplitNInternal
    with |s| SplitNInternal { iter: s.iter.clone(), ..*s }
}

struct SplitNInternal<'a, P: Pattern<'a>> {
    iter: SplitInternal<'a, P>,
    /// The number of splits remaining
    count: usize,
}

impl<'a, P> fmt::Debug for SplitNInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitNInternal")
            .field("iter", &self.iter)
            .field("count", &self.count)
            .finish()
    }
}

impl<'a, P: Pattern<'a>> SplitNInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        match self.count {
            0 => None,
            1 => {
                self.count = 0;
                self.iter.get_end()
            }
            _ => {
                self.count -= 1;
                self.iter.next()
            }
        }
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        match self.count {
            0 => None,
            1 => {
                self.count = 0;
                self.iter.get_end()
            }
            _ => {
                self.count -= 1;
                self.iter.next_back()
            }
        }
    }
}

generate_pattern_iterators! {
    forward:
        /// Created with the method [`splitn`].
        ///
        /// [`splitn`]: str::splitn
        struct SplitN;
    reverse:
        /// Created with the method [`rsplitn`].
        ///
        /// [`rsplitn`]: str::rsplitn
        struct RSplitN;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitNInternal yielding (&'a str);
    delegate single ended;
}

derive_pattern_clone! {
    clone MatchIndicesInternal
    with |s| MatchIndicesInternal(s.0.clone())
}

struct MatchIndicesInternal<'a, P: Pattern<'a>>(P::Searcher);

impl<'a, P> fmt::Debug for MatchIndicesInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("MatchIndicesInternal").field(&self.0).finish()
    }
}

impl<'a, P: Pattern<'a>> MatchIndicesInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<(usize, &'a str)> {
        self.0
            .next_match()
            // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
            .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
    }

    #[inline]
    fn next_back(&mut self) -> Option<(usize, &'a str)>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        self.0
            .next_match_back()
            // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
            .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
    }
}

generate_pattern_iterators! {
    forward:
        /// Created with the method [`match_indices`].
        ///
        /// [`match_indices`]: str::match_indices
        struct MatchIndices;
    reverse:
        /// Created with the method [`rmatch_indices`].
        ///
        /// [`rmatch_indices`]: str::rmatch_indices
        struct RMatchIndices;
    stability:
        #[stable(feature = "str_match_indices", since = "1.5.0")]
    internal:
        MatchIndicesInternal yielding ((usize, &'a str));
    delegate double ended;
}

derive_pattern_clone! {
    clone MatchesInternal
    with |s| MatchesInternal(s.0.clone())
}

struct MatchesInternal<'a, P: Pattern<'a>>(P::Searcher);

impl<'a, P> fmt::Debug for MatchesInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("MatchesInternal").field(&self.0).finish()
    }
}

impl<'a, P: Pattern<'a>> MatchesInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
        self.0.next_match().map(|(a, b)| unsafe {
            // Indices are known to be on utf8 boundaries
            self.0.haystack().get_unchecked(a..b)
        })
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        // SAFETY: `Searcher` guarantees that `start` and `end` lie on unicode boundaries.
        self.0.next_match_back().map(|(a, b)| unsafe {
            // Indices are known to be on utf8 boundaries
            self.0.haystack().get_unchecked(a..b)
        })
    }
}

generate_pattern_iterators! {
    forward:
        /// Created with the method [`matches`].
        ///
        /// [`matches`]: str::matches
        struct Matches;
    reverse:
        /// Created with the method [`rmatches`].
        ///
        /// [`rmatches`]: str::rmatches
        struct RMatches;
    stability:
        #[stable(feature = "str_matches", since = "1.2.0")]
    internal:
        MatchesInternal yielding (&'a str);
    delegate double ended;
}

/// An iterator over the lines of a string, as string slices.
///
/// This struct is created with the [`lines`] method on [`str`].
/// See its documentation for more.
///
/// [`lines`]: str::lines
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone, Debug)]
pub struct Lines<'a>(Map<SplitTerminator<'a, char>, LinesAnyMap>);

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for Lines<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for Lines<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Lines<'_> {}

/// Created with the method [`lines_any`].
///
/// [`lines_any`]: str::lines_any
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_deprecated(since = "1.4.0", reason = "use lines()/Lines instead now")]
#[derive(Clone, Debug)]
#[allow(deprecated)]
pub struct LinesAny<'a>(Lines<'a>);

impl_fn_for_zst! {
    /// A nameable, cloneable fn type
    #[derive(Clone)]
    struct LinesAnyMap impl<'a> Fn = |line: &'a str| -> &'a str {
        let l = line.len();
        if l > 0 && line.as_bytes()[l - 1] == b'\r' { &line[0 .. l - 1] }
        else { line }
    };
}

#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated)]
impl<'a> Iterator for LinesAny<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated)]
impl<'a> DoubleEndedIterator for LinesAny<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
#[allow(deprecated)]
impl FusedIterator for LinesAny<'_> {}

/*
Section: UTF-8 validation
*/

// use truncation to fit u64 into usize
const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;

/// Returns `true` if any byte in the word `x` is nonascii (>= 128).
#[inline]
fn contains_nonascii(x: usize) -> bool {
    (x & NONASCII_MASK) != 0
}

/// Walks through `v` checking that it's a valid UTF-8 sequence,
/// returning `Ok(())` in that case, or, if it is invalid, `Err(err)`.
#[inline(always)]
fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
    let mut index = 0;
    let len = v.len();

    let usize_bytes = mem::size_of::<usize>();
    let ascii_block_size = 2 * usize_bytes;
    let blocks_end = if len >= ascii_block_size { len - ascii_block_size + 1 } else { 0 };
    let align = v.as_ptr().align_offset(usize_bytes);

    while index < len {
        let old_offset = index;
        macro_rules! err {
            ($error_len: expr) => {
                return Err(Utf8Error { valid_up_to: old_offset, error_len: $error_len });
            };
        }

        macro_rules! next {
            () => {{
                index += 1;
                // we needed data, but there was none: error!
                if index >= len {
                    err!(None)
                }
                v[index]
            }};
        }

        let first = v[index];
        if first >= 128 {
            let w = UTF8_CHAR_WIDTH[first as usize];
            // 2-byte encoding is for codepoints  \u{0080} to  \u{07ff}
            //        first  C2 80        last DF BF
            // 3-byte encoding is for codepoints  \u{0800} to  \u{ffff}
            //        first  E0 A0 80     last EF BF BF
            //   excluding surrogates codepoints  \u{d800} to  \u{dfff}
            //               ED A0 80 to       ED BF BF
            // 4-byte encoding is for codepoints \u{1000}0 to \u{10ff}ff
            //        first  F0 90 80 80  last F4 8F BF BF
            //
            // Use the UTF-8 syntax from the RFC
            //
            // https://tools.ietf.org/html/rfc3629
            // UTF8-1      = %x00-7F
            // UTF8-2      = %xC2-DF UTF8-tail
            // UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
            //               %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
            // UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
            //               %xF4 %x80-8F 2( UTF8-tail )
            match w {
                2 => {
                    if next!() & !CONT_MASK != TAG_CONT_U8 {
                        err!(Some(1))
                    }
                }
                3 => {
                    match (first, next!()) {
                        (0xE0, 0xA0..=0xBF)
                        | (0xE1..=0xEC, 0x80..=0xBF)
                        | (0xED, 0x80..=0x9F)
                        | (0xEE..=0xEF, 0x80..=0xBF) => {}
                        _ => err!(Some(1)),
                    }
                    if next!() & !CONT_MASK != TAG_CONT_U8 {
                        err!(Some(2))
                    }
                }
                4 => {
                    match (first, next!()) {
                        (0xF0, 0x90..=0xBF) | (0xF1..=0xF3, 0x80..=0xBF) | (0xF4, 0x80..=0x8F) => {}
                        _ => err!(Some(1)),
                    }
                    if next!() & !CONT_MASK != TAG_CONT_U8 {
                        err!(Some(2))
                    }
                    if next!() & !CONT_MASK != TAG_CONT_U8 {
                        err!(Some(3))
                    }
                }
                _ => err!(Some(1)),
            }
            index += 1;
        } else {
            // Ascii case, try to skip forward quickly.
            // When the pointer is aligned, read 2 words of data per iteration
            // until we find a word containing a non-ascii byte.
            if align != usize::MAX && align.wrapping_sub(index) % usize_bytes == 0 {
                let ptr = v.as_ptr();
                while index < blocks_end {
                    // SAFETY: since `align - index` and `ascii_block_size` are
                    // multiples of `usize_bytes`, `block = ptr.add(index)` is
                    // always aligned with a `usize` so it's safe to dereference
                    // both `block` and `block.offset(1)`.
                    unsafe {
                        let block = ptr.add(index) as *const usize;
                        // break if there is a nonascii byte
                        let zu = contains_nonascii(*block);
                        let zv = contains_nonascii(*block.offset(1));
                        if zu | zv {
                            break;
                        }
                    }
                    index += ascii_block_size;
                }
                // step from the point where the wordwise loop stopped
                while index < len && v[index] < 128 {
                    index += 1;
                }
            } else {
                index += 1;
            }
        }
    }

    Ok(())
}

// https://tools.ietf.org/html/rfc3629
static UTF8_CHAR_WIDTH: [u8; 256] = [
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x1F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x3F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x5F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x7F
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, // 0x9F
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, // 0xBF
    0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
    2, // 0xDF
    3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // 0xEF
    4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xFF
];

/// Given a first byte, determines how many bytes are in this UTF-8 character.
#[unstable(feature = "str_internals", issue = "none")]
#[inline]
pub fn utf8_char_width(b: u8) -> usize {
    UTF8_CHAR_WIDTH[b as usize] as usize
}

/// Mask of the value bits of a continuation byte.
const CONT_MASK: u8 = 0b0011_1111;
/// Value of the tag bits (tag mask is !CONT_MASK) of a continuation byte.
const TAG_CONT_U8: u8 = 0b1000_0000;

/*
Section: Trait implementations
*/

mod traits {
    use crate::cmp::Ordering;
    use crate::ops;
    use crate::ptr;
    use crate::slice::SliceIndex;

    /// Implements ordering of strings.
    ///
    /// Strings are ordered  lexicographically by their byte values. This orders Unicode code
    /// points based on their positions in the code charts. This is not necessarily the same as
    /// "alphabetical" order, which varies by language and locale. Sorting strings according to
    /// culturally-accepted standards requires locale-specific data that is outside the scope of
    /// the `str` type.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl Ord for str {
        #[inline]
        fn cmp(&self, other: &str) -> Ordering {
            self.as_bytes().cmp(other.as_bytes())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl PartialEq for str {
        #[inline]
        fn eq(&self, other: &str) -> bool {
            self.as_bytes() == other.as_bytes()
        }
        #[inline]
        fn ne(&self, other: &str) -> bool {
            !(*self).eq(other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Eq for str {}

    /// Implements comparison operations on strings.
    ///
    /// Strings are compared lexicographically by their byte values. This compares Unicode code
    /// points based on their positions in the code charts. This is not necessarily the same as
    /// "alphabetical" order, which varies by language and locale. Comparing strings according to
    /// culturally-accepted standards requires locale-specific data that is outside the scope of
    /// the `str` type.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl PartialOrd for str {
        #[inline]
        fn partial_cmp(&self, other: &str) -> Option<Ordering> {
            Some(self.cmp(other))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ops::Index<I> for str
    where
        I: SliceIndex<str>,
    {
        type Output = I::Output;

        #[inline]
        fn index(&self, index: I) -> &I::Output {
            index.index(self)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ops::IndexMut<I> for str
    where
        I: SliceIndex<str>,
    {
        #[inline]
        fn index_mut(&mut self, index: I) -> &mut I::Output {
            index.index_mut(self)
        }
    }

    #[inline(never)]
    #[cold]
    #[track_caller]
    fn str_index_overflow_fail() -> ! {
        panic!("attempted to index str up to maximum usize");
    }

    /// Implements substring slicing with syntax `&self[..]` or `&mut self[..]`.
    ///
    /// Returns a slice of the whole string, i.e., returns `&self` or `&mut
    /// self`. Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`. Unlike
    /// other indexing operations, this can never panic.
    ///
    /// This operation is `O(1)`.
    ///
    /// Prior to 1.20.0, these indexing operations were still supported by
    /// direct implementation of `Index` and `IndexMut`.
    ///
    /// Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`.
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    unsafe impl SliceIndex<str> for ops::RangeFull {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            Some(slice)
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            Some(slice)
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            slice
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            slice
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            slice
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            slice
        }
    }

    /// Implements substring slicing with syntax `&self[begin .. end]` or `&mut
    /// self[begin .. end]`.
    ///
    /// Returns a slice of the given string from the byte range
    /// [`begin`, `end`).
    ///
    /// This operation is `O(1)`.
    ///
    /// Prior to 1.20.0, these indexing operations were still supported by
    /// direct implementation of `Index` and `IndexMut`.
    ///
    /// # Panics
    ///
    /// Panics if `begin` or `end` does not point to the starting byte offset of
    /// a character (as defined by `is_char_boundary`), if `begin > end`, or if
    /// `end > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    /// assert_eq!(&s[0 .. 1], "L");
    ///
    /// assert_eq!(&s[1 .. 9], "öwe 老");
    ///
    /// // these will panic:
    /// // byte 2 lies within `ö`:
    /// // &s[2 ..3];
    ///
    /// // byte 8 lies within `老`
    /// // &s[1 .. 8];
    ///
    /// // byte 100 is outside the string
    /// // &s[3 .. 100];
    /// ```
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    unsafe impl SliceIndex<str> for ops::Range<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if self.start <= self.end
                && slice.is_char_boundary(self.start)
                && slice.is_char_boundary(self.end)
            {
                // SAFETY: just checked that `start` and `end` are on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                // We also checked char boundaries, so this is valid UTF-8.
                Some(unsafe { &*self.get_unchecked(slice) })
            } else {
                None
            }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if self.start <= self.end
                && slice.is_char_boundary(self.start)
                && slice.is_char_boundary(self.end)
            {
                // SAFETY: just checked that `start` and `end` are on a char boundary.
                // We know the pointer is unique because we got it from `slice`.
                Some(unsafe { &mut *self.get_unchecked_mut(slice) })
            } else {
                None
            }
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            let slice = slice as *const [u8];
            // SAFETY: the caller guarantees that `self` is in bounds of `slice`
            // which satisfies all the conditions for `add`.
            let ptr = unsafe { slice.as_ptr().add(self.start) };
            let len = self.end - self.start;
            ptr::slice_from_raw_parts(ptr, len) as *const str
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            let slice = slice as *mut [u8];
            // SAFETY: see comments for `get_unchecked`.
            let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
            let len = self.end - self.start;
            ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            let (start, end) = (self.start, self.end);
            match self.get(slice) {
                Some(s) => s,
                None => super::slice_error_fail(slice, start, end),
            }
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            // is_char_boundary checks that the index is in [0, .len()]
            // cannot reuse `get` as above, because of NLL trouble
            if self.start <= self.end
                && slice.is_char_boundary(self.start)
                && slice.is_char_boundary(self.end)
            {
                // SAFETY: just checked that `start` and `end` are on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                unsafe { &mut *self.get_unchecked_mut(slice) }
            } else {
                super::slice_error_fail(slice, self.start, self.end)
            }
        }
    }

    /// Implements substring slicing with syntax `&self[.. end]` or `&mut
    /// self[.. end]`.
    ///
    /// Returns a slice of the given string from the byte range [`0`, `end`).
    /// Equivalent to `&self[0 .. end]` or `&mut self[0 .. end]`.
    ///
    /// This operation is `O(1)`.
    ///
    /// Prior to 1.20.0, these indexing operations were still supported by
    /// direct implementation of `Index` and `IndexMut`.
    ///
    /// # Panics
    ///
    /// Panics if `end` does not point to the starting byte offset of a
    /// character (as defined by `is_char_boundary`), or if `end > len`.
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if slice.is_char_boundary(self.end) {
                // SAFETY: just checked that `end` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                Some(unsafe { &*self.get_unchecked(slice) })
            } else {
                None
            }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if slice.is_char_boundary(self.end) {
                // SAFETY: just checked that `end` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                Some(unsafe { &mut *self.get_unchecked_mut(slice) })
            } else {
                None
            }
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            let slice = slice as *const [u8];
            let ptr = slice.as_ptr();
            ptr::slice_from_raw_parts(ptr, self.end) as *const str
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            let slice = slice as *mut [u8];
            let ptr = slice.as_mut_ptr();
            ptr::slice_from_raw_parts_mut(ptr, self.end) as *mut str
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            let end = self.end;
            match self.get(slice) {
                Some(s) => s,
                None => super::slice_error_fail(slice, 0, end),
            }
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            if slice.is_char_boundary(self.end) {
                // SAFETY: just checked that `end` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                unsafe { &mut *self.get_unchecked_mut(slice) }
            } else {
                super::slice_error_fail(slice, 0, self.end)
            }
        }
    }

    /// Implements substring slicing with syntax `&self[begin ..]` or `&mut
    /// self[begin ..]`.
    ///
    /// Returns a slice of the given string from the byte range [`begin`,
    /// `len`). Equivalent to `&self[begin .. len]` or `&mut self[begin ..
    /// len]`.
    ///
    /// This operation is `O(1)`.
    ///
    /// Prior to 1.20.0, these indexing operations were still supported by
    /// direct implementation of `Index` and `IndexMut`.
    ///
    /// # Panics
    ///
    /// Panics if `begin` does not point to the starting byte offset of
    /// a character (as defined by `is_char_boundary`), or if `begin > len`.
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    unsafe impl SliceIndex<str> for ops::RangeFrom<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if slice.is_char_boundary(self.start) {
                // SAFETY: just checked that `start` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                Some(unsafe { &*self.get_unchecked(slice) })
            } else {
                None
            }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if slice.is_char_boundary(self.start) {
                // SAFETY: just checked that `start` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                Some(unsafe { &mut *self.get_unchecked_mut(slice) })
            } else {
                None
            }
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            let slice = slice as *const [u8];
            // SAFETY: the caller guarantees that `self` is in bounds of `slice`
            // which satisfies all the conditions for `add`.
            let ptr = unsafe { slice.as_ptr().add(self.start) };
            let len = slice.len() - self.start;
            ptr::slice_from_raw_parts(ptr, len) as *const str
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            let slice = slice as *mut [u8];
            // SAFETY: identical to `get_unchecked`.
            let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
            let len = slice.len() - self.start;
            ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            let (start, end) = (self.start, slice.len());
            match self.get(slice) {
                Some(s) => s,
                None => super::slice_error_fail(slice, start, end),
            }
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            if slice.is_char_boundary(self.start) {
                // SAFETY: just checked that `start` is on a char boundary,
                // and we are passing in a safe reference, so the return value will also be one.
                unsafe { &mut *self.get_unchecked_mut(slice) }
            } else {
                super::slice_error_fail(slice, self.start, slice.len())
            }
        }
    }

    /// Implements substring slicing with syntax `&self[begin ..= end]` or `&mut
    /// self[begin ..= end]`.
    ///
    /// Returns a slice of the given string from the byte range
    /// [`begin`, `end`]. Equivalent to `&self [begin .. end + 1]` or `&mut
    /// self[begin .. end + 1]`, except if `end` has the maximum value for
    /// `usize`.
    ///
    /// This operation is `O(1)`.
    ///
    /// # Panics
    ///
    /// Panics if `begin` does not point to the starting byte offset of
    /// a character (as defined by `is_char_boundary`), if `end` does not point
    /// to the ending byte offset of a character (`end + 1` is either a starting
    /// byte offset or equal to `len`), if `begin > end`, or if `end >= len`.
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    unsafe impl SliceIndex<str> for ops::RangeInclusive<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if *self.end() == usize::MAX {
                None
            } else {
                (*self.start()..self.end() + 1).get(slice)
            }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if *self.end() == usize::MAX {
                None
            } else {
                (*self.start()..self.end() + 1).get_mut(slice)
            }
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
            unsafe { (*self.start()..self.end() + 1).get_unchecked(slice) }
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
            unsafe { (*self.start()..self.end() + 1).get_unchecked_mut(slice) }
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            if *self.end() == usize::MAX {
                str_index_overflow_fail();
            }
            (*self.start()..self.end() + 1).index(slice)
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            if *self.end() == usize::MAX {
                str_index_overflow_fail();
            }
            (*self.start()..self.end() + 1).index_mut(slice)
        }
    }

    /// Implements substring slicing with syntax `&self[..= end]` or `&mut
    /// self[..= end]`.
    ///
    /// Returns a slice of the given string from the byte range [0, `end`].
    /// Equivalent to `&self [0 .. end + 1]`, except if `end` has the maximum
    /// value for `usize`.
    ///
    /// This operation is `O(1)`.
    ///
    /// # Panics
    ///
    /// Panics if `end` does not point to the ending byte offset of a character
    /// (`end + 1` is either a starting byte offset as defined by
    /// `is_char_boundary`, or equal to `len`), or if `end >= len`.
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    unsafe impl SliceIndex<str> for ops::RangeToInclusive<usize> {
        type Output = str;
        #[inline]
        fn get(self, slice: &str) -> Option<&Self::Output> {
            if self.end == usize::MAX { None } else { (..self.end + 1).get(slice) }
        }
        #[inline]
        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
            if self.end == usize::MAX { None } else { (..self.end + 1).get_mut(slice) }
        }
        #[inline]
        unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
            // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
            unsafe { (..self.end + 1).get_unchecked(slice) }
        }
        #[inline]
        unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
            // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
            unsafe { (..self.end + 1).get_unchecked_mut(slice) }
        }
        #[inline]
        fn index(self, slice: &str) -> &Self::Output {
            if self.end == usize::MAX {
                str_index_overflow_fail();
            }
            (..self.end + 1).index(slice)
        }
        #[inline]
        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
            if self.end == usize::MAX {
                str_index_overflow_fail();
            }
            (..self.end + 1).index_mut(slice)
        }
    }
}

// truncate `&str` to length at most equal to `max`
// return `true` if it were truncated, and the new str.
fn truncate_to_char_boundary(s: &str, mut max: usize) -> (bool, &str) {
    if max >= s.len() {
        (false, s)
    } else {
        while !s.is_char_boundary(max) {
            max -= 1;
        }
        (true, &s[..max])
    }
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
    const MAX_DISPLAY_LENGTH: usize = 256;
    let (truncated, s_trunc) = truncate_to_char_boundary(s, MAX_DISPLAY_LENGTH);
    let ellipsis = if truncated { "[...]" } else { "" };

    // 1. out of bounds
    if begin > s.len() || end > s.len() {
        let oob_index = if begin > s.len() { begin } else { end };
        panic!("byte index {} is out of bounds of `{}`{}", oob_index, s_trunc, ellipsis);
    }

    // 2. begin <= end
    assert!(
        begin <= end,
        "begin <= end ({} <= {}) when slicing `{}`{}",
        begin,
        end,
        s_trunc,
        ellipsis
    );

    // 3. character boundary
    let index = if !s.is_char_boundary(begin) { begin } else { end };
    // find the character
    let mut char_start = index;
    while !s.is_char_boundary(char_start) {
        char_start -= 1;
    }
    // `char_start` must be less than len and a char boundary
    let ch = s[char_start..].chars().next().unwrap();
    let char_range = char_start..char_start + ch.len_utf8();
    panic!(
        "byte index {} is not a char boundary; it is inside {:?} (bytes {:?}) of `{}`{}",
        index, ch, char_range, s_trunc, ellipsis
    );
}

#[lang = "str"]
#[cfg(not(test))]
impl str {
    /// Returns the length of `self`.
    ///
    /// This length is in bytes, not [`char`]s or graphemes. In other words,
    /// it may not be what a human considers the length of the string.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let len = "foo".len();
    /// assert_eq!(3, len);
    ///
    /// assert_eq!("ƒoo".len(), 4); // fancy f!
    /// assert_eq!("ƒoo".chars().count(), 3);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_str_len", since = "1.32.0")]
    #[inline]
    pub const fn len(&self) -> usize {
        self.as_bytes().len()
    }

    /// Returns `true` if `self` has a length of zero bytes.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = "";
    /// assert!(s.is_empty());
    ///
    /// let s = "not empty";
    /// assert!(!s.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_str_is_empty", since = "1.32.0")]
    pub const fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Checks that `index`-th byte is the first byte in a UTF-8 code point
    /// sequence or the end of the string.
    ///
    /// The start and end of the string (when `index == self.len()`) are
    /// considered to be boundaries.
    ///
    /// Returns `false` if `index` is greater than `self.len()`.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    /// assert!(s.is_char_boundary(0));
    /// // start of `老`
    /// assert!(s.is_char_boundary(6));
    /// assert!(s.is_char_boundary(s.len()));
    ///
    /// // second byte of `ö`
    /// assert!(!s.is_char_boundary(2));
    ///
    /// // third byte of `老`
    /// assert!(!s.is_char_boundary(8));
    /// ```
    #[stable(feature = "is_char_boundary", since = "1.9.0")]
    #[inline]
    pub fn is_char_boundary(&self, index: usize) -> bool {
        // 0 and len are always ok.
        // Test for 0 explicitly so that it can optimize out the check
        // easily and skip reading string data for that case.
        if index == 0 || index == self.len() {
            return true;
        }
        match self.as_bytes().get(index) {
            None => false,
            // This is bit magic equivalent to: b < 128 || b >= 192
            Some(&b) => (b as i8) >= -0x40,
        }
    }

    /// Converts a string slice to a byte slice. To convert the byte slice back
    /// into a string slice, use the [`from_utf8`] function.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let bytes = "bors".as_bytes();
    /// assert_eq!(b"bors", bytes);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "str_as_bytes", since = "1.32.0")]
    #[inline(always)]
    #[allow(unused_attributes)]
    #[allow_internal_unstable(const_fn_union)]
    pub const fn as_bytes(&self) -> &[u8] {
        #[repr(C)]
        union Slices<'a> {
            str: &'a str,
            slice: &'a [u8],
        }
        // SAFETY: const sound because we transmute two types with the same layout
        unsafe { Slices { str: self }.slice }
    }

    /// Converts a mutable string slice to a mutable byte slice.
    ///
    /// # Safety
    ///
    /// The caller must ensure that the content of the slice is valid UTF-8
    /// before the borrow ends and the underlying `str` is used.
    ///
    /// Use of a `str` whose contents are not valid UTF-8 is undefined behavior.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut s = String::from("Hello");
    /// let bytes = unsafe { s.as_bytes_mut() };
    ///
    /// assert_eq!(b"Hello", bytes);
    /// ```
    ///
    /// Mutability:
    ///
    /// ```
    /// let mut s = String::from("🗻∈🌏");
    ///
    /// unsafe {
    ///     let bytes = s.as_bytes_mut();
    ///
    ///     bytes[0] = 0xF0;
    ///     bytes[1] = 0x9F;
    ///     bytes[2] = 0x8D;
    ///     bytes[3] = 0x94;
    /// }
    ///
    /// assert_eq!("🍔∈🌏", s);
    /// ```
    #[stable(feature = "str_mut_extras", since = "1.20.0")]
    #[inline(always)]
    pub unsafe fn as_bytes_mut(&mut self) -> &mut [u8] {
        // SAFETY: the cast from `&str` to `&[u8]` is safe since `str`
        // has the same layout as `&[u8]` (only libstd can make this guarantee).
        // The pointer dereference is safe since it comes from a mutable reference which
        // is guaranteed to be valid for writes.
        unsafe { &mut *(self as *mut str as *mut [u8]) }
    }

    /// Converts a string slice to a raw pointer.
    ///
    /// As string slices are a slice of bytes, the raw pointer points to a
    /// [`u8`]. This pointer will be pointing to the first byte of the string
    /// slice.
    ///
    /// The caller must ensure that the returned pointer is never written to.
    /// If you need to mutate the contents of the string slice, use [`as_mut_ptr`].
    ///
    /// [`as_mut_ptr`]: str::as_mut_ptr
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = "Hello";
    /// let ptr = s.as_ptr();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "rustc_str_as_ptr", since = "1.32.0")]
    #[inline]
    pub const fn as_ptr(&self) -> *const u8 {
        self as *const str as *const u8
    }

    /// Converts a mutable string slice to a raw pointer.
    ///
    /// As string slices are a slice of bytes, the raw pointer points to a
    /// [`u8`]. This pointer will be pointing to the first byte of the string
    /// slice.
    ///
    /// It is your responsibility to make sure that the string slice only gets
    /// modified in a way that it remains valid UTF-8.
    #[stable(feature = "str_as_mut_ptr", since = "1.36.0")]
    #[inline]
    pub fn as_mut_ptr(&mut self) -> *mut u8 {
        self as *mut str as *mut u8
    }

    /// Returns a subslice of `str`.
    ///
    /// This is the non-panicking alternative to indexing the `str`. Returns
    /// [`None`] whenever equivalent indexing operation would panic.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = String::from("🗻∈🌏");
    ///
    /// assert_eq!(Some("🗻"), v.get(0..4));
    ///
    /// // indices not on UTF-8 sequence boundaries
    /// assert!(v.get(1..).is_none());
    /// assert!(v.get(..8).is_none());
    ///
    /// // out of bounds
    /// assert!(v.get(..42).is_none());
    /// ```
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    #[inline]
    pub fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
        i.get(self)
    }

    /// Returns a mutable subslice of `str`.
    ///
    /// This is the non-panicking alternative to indexing the `str`. Returns
    /// [`None`] whenever equivalent indexing operation would panic.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = String::from("hello");
    /// // correct length
    /// assert!(v.get_mut(0..5).is_some());
    /// // out of bounds
    /// assert!(v.get_mut(..42).is_none());
    /// assert_eq!(Some("he"), v.get_mut(0..2).map(|v| &*v));
    ///
    /// assert_eq!("hello", v);
    /// {
    ///     let s = v.get_mut(0..2);
    ///     let s = s.map(|s| {
    ///         s.make_ascii_uppercase();
    ///         &*s
    ///     });
    ///     assert_eq!(Some("HE"), s);
    /// }
    /// assert_eq!("HEllo", v);
    /// ```
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    #[inline]
    pub fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
        i.get_mut(self)
    }

    /// Returns an unchecked subslice of `str`.
    ///
    /// This is the unchecked alternative to indexing the `str`.
    ///
    /// # Safety
    ///
    /// Callers of this function are responsible that these preconditions are
    /// satisfied:
    ///
    /// * The starting index must not exceed the ending index;
    /// * Indexes must be within bounds of the original slice;
    /// * Indexes must lie on UTF-8 sequence boundaries.
    ///
    /// Failing that, the returned string slice may reference invalid memory or
    /// violate the invariants communicated by the `str` type.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = "🗻∈🌏";
    /// unsafe {
    ///     assert_eq!("🗻", v.get_unchecked(0..4));
    ///     assert_eq!("∈", v.get_unchecked(4..7));
    ///     assert_eq!("🌏", v.get_unchecked(7..11));
    /// }
    /// ```
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    #[inline]
    pub unsafe fn get_unchecked<I: SliceIndex<str>>(&self, i: I) -> &I::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &*i.get_unchecked(self) }
    }

    /// Returns a mutable, unchecked subslice of `str`.
    ///
    /// This is the unchecked alternative to indexing the `str`.
    ///
    /// # Safety
    ///
    /// Callers of this function are responsible that these preconditions are
    /// satisfied:
    ///
    /// * The starting index must not exceed the ending index;
    /// * Indexes must be within bounds of the original slice;
    /// * Indexes must lie on UTF-8 sequence boundaries.
    ///
    /// Failing that, the returned string slice may reference invalid memory or
    /// violate the invariants communicated by the `str` type.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = String::from("🗻∈🌏");
    /// unsafe {
    ///     assert_eq!("🗻", v.get_unchecked_mut(0..4));
    ///     assert_eq!("∈", v.get_unchecked_mut(4..7));
    ///     assert_eq!("🌏", v.get_unchecked_mut(7..11));
    /// }
    /// ```
    #[stable(feature = "str_checked_slicing", since = "1.20.0")]
    #[inline]
    pub unsafe fn get_unchecked_mut<I: SliceIndex<str>>(&mut self, i: I) -> &mut I::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &mut *i.get_unchecked_mut(self) }
    }

    /// Creates a string slice from another string slice, bypassing safety
    /// checks.
    ///
    /// This is generally not recommended, use with caution! For a safe
    /// alternative see [`str`] and [`Index`].
    ///
    /// [`Index`]: crate::ops::Index
    ///
    /// This new slice goes from `begin` to `end`, including `begin` but
    /// excluding `end`.
    ///
    /// To get a mutable string slice instead, see the
    /// [`slice_mut_unchecked`] method.
    ///
    /// [`slice_mut_unchecked`]: str::slice_mut_unchecked
    ///
    /// # Safety
    ///
    /// Callers of this function are responsible that three preconditions are
    /// satisfied:
    ///
    /// * `begin` must not exceed `end`.
    /// * `begin` and `end` must be byte positions within the string slice.
    /// * `begin` and `end` must lie on UTF-8 sequence boundaries.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    ///
    /// unsafe {
    ///     assert_eq!("Löwe 老虎 Léopard", s.slice_unchecked(0, 21));
    /// }
    ///
    /// let s = "Hello, world!";
    ///
    /// unsafe {
    ///     assert_eq!("world", s.slice_unchecked(7, 12));
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(since = "1.29.0", reason = "use `get_unchecked(begin..end)` instead")]
    #[inline]
    pub unsafe fn slice_unchecked(&self, begin: usize, end: usize) -> &str {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &*(begin..end).get_unchecked(self) }
    }

    /// Creates a string slice from another string slice, bypassing safety
    /// checks.
    /// This is generally not recommended, use with caution! For a safe
    /// alternative see [`str`] and [`IndexMut`].
    ///
    /// [`IndexMut`]: crate::ops::IndexMut
    ///
    /// This new slice goes from `begin` to `end`, including `begin` but
    /// excluding `end`.
    ///
    /// To get an immutable string slice instead, see the
    /// [`slice_unchecked`] method.
    ///
    /// [`slice_unchecked`]: str::slice_unchecked
    ///
    /// # Safety
    ///
    /// Callers of this function are responsible that three preconditions are
    /// satisfied:
    ///
    /// * `begin` must not exceed `end`.
    /// * `begin` and `end` must be byte positions within the string slice.
    /// * `begin` and `end` must lie on UTF-8 sequence boundaries.
    #[stable(feature = "str_slice_mut", since = "1.5.0")]
    #[rustc_deprecated(since = "1.29.0", reason = "use `get_unchecked_mut(begin..end)` instead")]
    #[inline]
    pub unsafe fn slice_mut_unchecked(&mut self, begin: usize, end: usize) -> &mut str {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &mut *(begin..end).get_unchecked_mut(self) }
    }

    /// Divide one string slice into two at an index.
    ///
    /// The argument, `mid`, should be a byte offset from the start of the
    /// string. It must also be on the boundary of a UTF-8 code point.
    ///
    /// The two slices returned go from the start of the string slice to `mid`,
    /// and from `mid` to the end of the string slice.
    ///
    /// To get mutable string slices instead, see the [`split_at_mut`]
    /// method.
    ///
    /// [`split_at_mut`]: str::split_at_mut
    ///
    /// # Panics
    ///
    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
    /// past the end of the last code point of the string slice.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = "Per Martin-Löf";
    ///
    /// let (first, last) = s.split_at(3);
    ///
    /// assert_eq!("Per", first);
    /// assert_eq!(" Martin-Löf", last);
    /// ```
    #[inline]
    #[stable(feature = "str_split_at", since = "1.4.0")]
    pub fn split_at(&self, mid: usize) -> (&str, &str) {
        // is_char_boundary checks that the index is in [0, .len()]
        if self.is_char_boundary(mid) {
            // SAFETY: just checked that `mid` is on a char boundary.
            unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) }
        } else {
            slice_error_fail(self, 0, mid)
        }
    }

    /// Divide one mutable string slice into two at an index.
    ///
    /// The argument, `mid`, should be a byte offset from the start of the
    /// string. It must also be on the boundary of a UTF-8 code point.
    ///
    /// The two slices returned go from the start of the string slice to `mid`,
    /// and from `mid` to the end of the string slice.
    ///
    /// To get immutable string slices instead, see the [`split_at`] method.
    ///
    /// [`split_at`]: str::split_at
    ///
    /// # Panics
    ///
    /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
    /// past the end of the last code point of the string slice.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut s = "Per Martin-Löf".to_string();
    /// {
    ///     let (first, last) = s.split_at_mut(3);
    ///     first.make_ascii_uppercase();
    ///     assert_eq!("PER", first);
    ///     assert_eq!(" Martin-Löf", last);
    /// }
    /// assert_eq!("PER Martin-Löf", s);
    /// ```
    #[inline]
    #[stable(feature = "str_split_at", since = "1.4.0")]
    pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
        // is_char_boundary checks that the index is in [0, .len()]
        if self.is_char_boundary(mid) {
            let len = self.len();
            let ptr = self.as_mut_ptr();
            // SAFETY: just checked that `mid` is on a char boundary.
            unsafe {
                (
                    from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr, mid)),
                    from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr.add(mid), len - mid)),
                )
            }
        } else {
            slice_error_fail(self, 0, mid)
        }
    }

    /// Returns an iterator over the [`char`]s of a string slice.
    ///
    /// As a string slice consists of valid UTF-8, we can iterate through a
    /// string slice by [`char`]. This method returns such an iterator.
    ///
    /// It's important to remember that [`char`] represents a Unicode Scalar
    /// Value, and may not match your idea of what a 'character' is. Iteration
    /// over grapheme clusters may be what you actually want. This functionality
    /// is not provided by Rust's standard library, check crates.io instead.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let word = "goodbye";
    ///
    /// let count = word.chars().count();
    /// assert_eq!(7, count);
    ///
    /// let mut chars = word.chars();
    ///
    /// assert_eq!(Some('g'), chars.next());
    /// assert_eq!(Some('o'), chars.next());
    /// assert_eq!(Some('o'), chars.next());
    /// assert_eq!(Some('d'), chars.next());
    /// assert_eq!(Some('b'), chars.next());
    /// assert_eq!(Some('y'), chars.next());
    /// assert_eq!(Some('e'), chars.next());
    ///
    /// assert_eq!(None, chars.next());
    /// ```
    ///
    /// Remember, [`char`]s may not match your human intuition about characters:
    ///
    /// ```
    /// let y = "y̆";
    ///
    /// let mut chars = y.chars();
    ///
    /// assert_eq!(Some('y'), chars.next()); // not 'y̆'
    /// assert_eq!(Some('\u{0306}'), chars.next());
    ///
    /// assert_eq!(None, chars.next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn chars(&self) -> Chars<'_> {
        Chars { iter: self.as_bytes().iter() }
    }

    /// Returns an iterator over the [`char`]s of a string slice, and their
    /// positions.
    ///
    /// As a string slice consists of valid UTF-8, we can iterate through a
    /// string slice by [`char`]. This method returns an iterator of both
    /// these [`char`]s, as well as their byte positions.
    ///
    /// The iterator yields tuples. The position is first, the [`char`] is
    /// second.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let word = "goodbye";
    ///
    /// let count = word.char_indices().count();
    /// assert_eq!(7, count);
    ///
    /// let mut char_indices = word.char_indices();
    ///
    /// assert_eq!(Some((0, 'g')), char_indices.next());
    /// assert_eq!(Some((1, 'o')), char_indices.next());
    /// assert_eq!(Some((2, 'o')), char_indices.next());
    /// assert_eq!(Some((3, 'd')), char_indices.next());
    /// assert_eq!(Some((4, 'b')), char_indices.next());
    /// assert_eq!(Some((5, 'y')), char_indices.next());
    /// assert_eq!(Some((6, 'e')), char_indices.next());
    ///
    /// assert_eq!(None, char_indices.next());
    /// ```
    ///
    /// Remember, [`char`]s may not match your human intuition about characters:
    ///
    /// ```
    /// let yes = "y̆es";
    ///
    /// let mut char_indices = yes.char_indices();
    ///
    /// assert_eq!(Some((0, 'y')), char_indices.next()); // not (0, 'y̆')
    /// assert_eq!(Some((1, '\u{0306}')), char_indices.next());
    ///
    /// // note the 3 here - the last character took up two bytes
    /// assert_eq!(Some((3, 'e')), char_indices.next());
    /// assert_eq!(Some((4, 's')), char_indices.next());
    ///
    /// assert_eq!(None, char_indices.next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn char_indices(&self) -> CharIndices<'_> {
        CharIndices { front_offset: 0, iter: self.chars() }
    }

    /// An iterator over the bytes of a string slice.
    ///
    /// As a string slice consists of a sequence of bytes, we can iterate
    /// through a string slice by byte. This method returns such an iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut bytes = "bors".bytes();
    ///
    /// assert_eq!(Some(b'b'), bytes.next());
    /// assert_eq!(Some(b'o'), bytes.next());
    /// assert_eq!(Some(b'r'), bytes.next());
    /// assert_eq!(Some(b's'), bytes.next());
    ///
    /// assert_eq!(None, bytes.next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn bytes(&self) -> Bytes<'_> {
        Bytes(self.as_bytes().iter().copied())
    }

    /// Splits a string slice by whitespace.
    ///
    /// The iterator returned will return string slices that are sub-slices of
    /// the original string slice, separated by any amount of whitespace.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`. If you only want to split on ASCII whitespace
    /// instead, use [`split_ascii_whitespace`].
    ///
    /// [`split_ascii_whitespace`]: str::split_ascii_whitespace
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut iter = "A few words".split_whitespace();
    ///
    /// assert_eq!(Some("A"), iter.next());
    /// assert_eq!(Some("few"), iter.next());
    /// assert_eq!(Some("words"), iter.next());
    ///
    /// assert_eq!(None, iter.next());
    /// ```
    ///
    /// All kinds of whitespace are considered:
    ///
    /// ```
    /// let mut iter = " Mary   had\ta\u{2009}little  \n\t lamb".split_whitespace();
    /// assert_eq!(Some("Mary"), iter.next());
    /// assert_eq!(Some("had"), iter.next());
    /// assert_eq!(Some("a"), iter.next());
    /// assert_eq!(Some("little"), iter.next());
    /// assert_eq!(Some("lamb"), iter.next());
    ///
    /// assert_eq!(None, iter.next());
    /// ```
    #[stable(feature = "split_whitespace", since = "1.1.0")]
    #[inline]
    pub fn split_whitespace(&self) -> SplitWhitespace<'_> {
        SplitWhitespace { inner: self.split(IsWhitespace).filter(IsNotEmpty) }
    }

    /// Splits a string slice by ASCII whitespace.
    ///
    /// The iterator returned will return string slices that are sub-slices of
    /// the original string slice, separated by any amount of ASCII whitespace.
    ///
    /// To split by Unicode `Whitespace` instead, use [`split_whitespace`].
    ///
    /// [`split_whitespace`]: str::split_whitespace
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut iter = "A few words".split_ascii_whitespace();
    ///
    /// assert_eq!(Some("A"), iter.next());
    /// assert_eq!(Some("few"), iter.next());
    /// assert_eq!(Some("words"), iter.next());
    ///
    /// assert_eq!(None, iter.next());
    /// ```
    ///
    /// All kinds of ASCII whitespace are considered:
    ///
    /// ```
    /// let mut iter = " Mary   had\ta little  \n\t lamb".split_ascii_whitespace();
    /// assert_eq!(Some("Mary"), iter.next());
    /// assert_eq!(Some("had"), iter.next());
    /// assert_eq!(Some("a"), iter.next());
    /// assert_eq!(Some("little"), iter.next());
    /// assert_eq!(Some("lamb"), iter.next());
    ///
    /// assert_eq!(None, iter.next());
    /// ```
    #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
    #[inline]
    pub fn split_ascii_whitespace(&self) -> SplitAsciiWhitespace<'_> {
        let inner =
            self.as_bytes().split(IsAsciiWhitespace).filter(BytesIsNotEmpty).map(UnsafeBytesToStr);
        SplitAsciiWhitespace { inner }
    }

    /// An iterator over the lines of a string, as string slices.
    ///
    /// Lines are ended with either a newline (`\n`) or a carriage return with
    /// a line feed (`\r\n`).
    ///
    /// The final line ending is optional.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let text = "foo\r\nbar\n\nbaz\n";
    /// let mut lines = text.lines();
    ///
    /// assert_eq!(Some("foo"), lines.next());
    /// assert_eq!(Some("bar"), lines.next());
    /// assert_eq!(Some(""), lines.next());
    /// assert_eq!(Some("baz"), lines.next());
    ///
    /// assert_eq!(None, lines.next());
    /// ```
    ///
    /// The final line ending isn't required:
    ///
    /// ```
    /// let text = "foo\nbar\n\r\nbaz";
    /// let mut lines = text.lines();
    ///
    /// assert_eq!(Some("foo"), lines.next());
    /// assert_eq!(Some("bar"), lines.next());
    /// assert_eq!(Some(""), lines.next());
    /// assert_eq!(Some("baz"), lines.next());
    ///
    /// assert_eq!(None, lines.next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn lines(&self) -> Lines<'_> {
        Lines(self.split_terminator('\n').map(LinesAnyMap))
    }

    /// An iterator over the lines of a string.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(since = "1.4.0", reason = "use lines() instead now")]
    #[inline]
    #[allow(deprecated)]
    pub fn lines_any(&self) -> LinesAny<'_> {
        LinesAny(self.lines())
    }

    /// Returns an iterator of `u16` over the string encoded as UTF-16.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let text = "Zażółć gęślą jaźń";
    ///
    /// let utf8_len = text.len();
    /// let utf16_len = text.encode_utf16().count();
    ///
    /// assert!(utf16_len <= utf8_len);
    /// ```
    #[stable(feature = "encode_utf16", since = "1.8.0")]
    pub fn encode_utf16(&self) -> EncodeUtf16<'_> {
        EncodeUtf16 { chars: self.chars(), extra: 0 }
    }

    /// Returns `true` if the given pattern matches a sub-slice of
    /// this string slice.
    ///
    /// Returns `false` if it does not.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let bananas = "bananas";
    ///
    /// assert!(bananas.contains("nana"));
    /// assert!(!bananas.contains("apples"));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn contains<'a, P: Pattern<'a>>(&'a self, pat: P) -> bool {
        pat.is_contained_in(self)
    }

    /// Returns `true` if the given pattern matches a prefix of this
    /// string slice.
    ///
    /// Returns `false` if it does not.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let bananas = "bananas";
    ///
    /// assert!(bananas.starts_with("bana"));
    /// assert!(!bananas.starts_with("nana"));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn starts_with<'a, P: Pattern<'a>>(&'a self, pat: P) -> bool {
        pat.is_prefix_of(self)
    }

    /// Returns `true` if the given pattern matches a suffix of this
    /// string slice.
    ///
    /// Returns `false` if it does not.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let bananas = "bananas";
    ///
    /// assert!(bananas.ends_with("anas"));
    /// assert!(!bananas.ends_with("nana"));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn ends_with<'a, P>(&'a self, pat: P) -> bool
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        pat.is_suffix_of(self)
    }

    /// Returns the byte index of the first character of this string slice that
    /// matches the pattern.
    ///
    /// Returns [`None`] if the pattern doesn't match.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard Gepardi";
    ///
    /// assert_eq!(s.find('L'), Some(0));
    /// assert_eq!(s.find('é'), Some(14));
    /// assert_eq!(s.find("pard"), Some(17));
    /// ```
    ///
    /// More complex patterns using point-free style and closures:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    ///
    /// assert_eq!(s.find(char::is_whitespace), Some(5));
    /// assert_eq!(s.find(char::is_lowercase), Some(1));
    /// assert_eq!(s.find(|c: char| c.is_whitespace() || c.is_lowercase()), Some(1));
    /// assert_eq!(s.find(|c: char| (c < 'o') && (c > 'a')), Some(4));
    /// ```
    ///
    /// Not finding the pattern:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    /// let x: &[_] = &['1', '2'];
    ///
    /// assert_eq!(s.find(x), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn find<'a, P: Pattern<'a>>(&'a self, pat: P) -> Option<usize> {
        pat.into_searcher(self).next_match().map(|(i, _)| i)
    }

    /// Returns the byte index for the first character of the rightmost match of the pattern in
    /// this string slice.
    ///
    /// Returns [`None`] if the pattern doesn't match.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard Gepardi";
    ///
    /// assert_eq!(s.rfind('L'), Some(13));
    /// assert_eq!(s.rfind('é'), Some(14));
    /// assert_eq!(s.rfind("pard"), Some(24));
    /// ```
    ///
    /// More complex patterns with closures:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    ///
    /// assert_eq!(s.rfind(char::is_whitespace), Some(12));
    /// assert_eq!(s.rfind(char::is_lowercase), Some(20));
    /// ```
    ///
    /// Not finding the pattern:
    ///
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    /// let x: &[_] = &['1', '2'];
    ///
    /// assert_eq!(s.rfind(x), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rfind<'a, P>(&'a self, pat: P) -> Option<usize>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        pat.into_searcher(self).next_match_back().map(|(i, _)| i)
    }

    /// An iterator over substrings of this string slice, separated by
    /// characters matched by a pattern.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will be a [`DoubleEndedIterator`] if the pattern
    /// allows a reverse search and forward/reverse search yields the same
    /// elements. This is true for, e.g., [`char`], but not for `&str`.
    ///
    /// If the pattern allows a reverse search but its results might differ
    /// from a forward search, the [`rsplit`] method can be used.
    ///
    /// [`rsplit`]: str::rsplit
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let v: Vec<&str> = "Mary had a little lamb".split(' ').collect();
    /// assert_eq!(v, ["Mary", "had", "a", "little", "lamb"]);
    ///
    /// let v: Vec<&str> = "".split('X').collect();
    /// assert_eq!(v, [""]);
    ///
    /// let v: Vec<&str> = "lionXXtigerXleopard".split('X').collect();
    /// assert_eq!(v, ["lion", "", "tiger", "leopard"]);
    ///
    /// let v: Vec<&str> = "lion::tiger::leopard".split("::").collect();
    /// assert_eq!(v, ["lion", "tiger", "leopard"]);
    ///
    /// let v: Vec<&str> = "abc1def2ghi".split(char::is_numeric).collect();
    /// assert_eq!(v, ["abc", "def", "ghi"]);
    ///
    /// let v: Vec<&str> = "lionXtigerXleopard".split(char::is_uppercase).collect();
    /// assert_eq!(v, ["lion", "tiger", "leopard"]);
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// let v: Vec<&str> = "abc1defXghi".split(|c| c == '1' || c == 'X').collect();
    /// assert_eq!(v, ["abc", "def", "ghi"]);
    /// ```
    ///
    /// If a string contains multiple contiguous separators, you will end up
    /// with empty strings in the output:
    ///
    /// ```
    /// let x = "||||a||b|c".to_string();
    /// let d: Vec<_> = x.split('|').collect();
    ///
    /// assert_eq!(d, &["", "", "", "", "a", "", "b", "c"]);
    /// ```
    ///
    /// Contiguous separators are separated by the empty string.
    ///
    /// ```
    /// let x = "(///)".to_string();
    /// let d: Vec<_> = x.split('/').collect();
    ///
    /// assert_eq!(d, &["(", "", "", ")"]);
    /// ```
    ///
    /// Separators at the start or end of a string are neighbored
    /// by empty strings.
    ///
    /// ```
    /// let d: Vec<_> = "010".split("0").collect();
    /// assert_eq!(d, &["", "1", ""]);
    /// ```
    ///
    /// When the empty string is used as a separator, it separates
    /// every character in the string, along with the beginning
    /// and end of the string.
    ///
    /// ```
    /// let f: Vec<_> = "rust".split("").collect();
    /// assert_eq!(f, &["", "r", "u", "s", "t", ""]);
    /// ```
    ///
    /// Contiguous separators can lead to possibly surprising behavior
    /// when whitespace is used as the separator. This code is correct:
    ///
    /// ```
    /// let x = "    a  b c".to_string();
    /// let d: Vec<_> = x.split(' ').collect();
    ///
    /// assert_eq!(d, &["", "", "", "", "a", "", "b", "c"]);
    /// ```
    ///
    /// It does _not_ give you:
    ///
    /// ```,ignore
    /// assert_eq!(d, &["a", "b", "c"]);
    /// ```
    ///
    /// Use [`split_whitespace`] for this behavior.
    ///
    /// [`split_whitespace`]: str::split_whitespace
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split<'a, P: Pattern<'a>>(&'a self, pat: P) -> Split<'a, P> {
        Split(SplitInternal {
            start: 0,
            end: self.len(),
            matcher: pat.into_searcher(self),
            allow_trailing_empty: true,
            finished: false,
        })
    }

    /// An iterator over substrings of this string slice, separated by
    /// characters matched by a pattern. Differs from the iterator produced by
    /// `split` in that `split_inclusive` leaves the matched part as the
    /// terminator of the substring.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_inclusive)]
    /// let v: Vec<&str> = "Mary had a little lamb\nlittle lamb\nlittle lamb."
    ///     .split_inclusive('\n').collect();
    /// assert_eq!(v, ["Mary had a little lamb\n", "little lamb\n", "little lamb."]);
    /// ```
    ///
    /// If the last element of the string is matched,
    /// that element will be considered the terminator of the preceding substring.
    /// That substring will be the last item returned by the iterator.
    ///
    /// ```
    /// #![feature(split_inclusive)]
    /// let v: Vec<&str> = "Mary had a little lamb\nlittle lamb\nlittle lamb.\n"
    ///     .split_inclusive('\n').collect();
    /// assert_eq!(v, ["Mary had a little lamb\n", "little lamb\n", "little lamb.\n"]);
    /// ```
    #[unstable(feature = "split_inclusive", issue = "72360")]
    #[inline]
    pub fn split_inclusive<'a, P: Pattern<'a>>(&'a self, pat: P) -> SplitInclusive<'a, P> {
        SplitInclusive(SplitInternal {
            start: 0,
            end: self.len(),
            matcher: pat.into_searcher(self),
            allow_trailing_empty: false,
            finished: false,
        })
    }

    /// An iterator over substrings of the given string slice, separated by
    /// characters matched by a pattern and yielded in reverse order.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator requires that the pattern supports a reverse
    /// search, and it will be a [`DoubleEndedIterator`] if a forward/reverse
    /// search yields the same elements.
    ///
    /// For iterating from the front, the [`split`] method can be used.
    ///
    /// [`split`]: str::split
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let v: Vec<&str> = "Mary had a little lamb".rsplit(' ').collect();
    /// assert_eq!(v, ["lamb", "little", "a", "had", "Mary"]);
    ///
    /// let v: Vec<&str> = "".rsplit('X').collect();
    /// assert_eq!(v, [""]);
    ///
    /// let v: Vec<&str> = "lionXXtigerXleopard".rsplit('X').collect();
    /// assert_eq!(v, ["leopard", "tiger", "", "lion"]);
    ///
    /// let v: Vec<&str> = "lion::tiger::leopard".rsplit("::").collect();
    /// assert_eq!(v, ["leopard", "tiger", "lion"]);
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// let v: Vec<&str> = "abc1defXghi".rsplit(|c| c == '1' || c == 'X').collect();
    /// assert_eq!(v, ["ghi", "def", "abc"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rsplit<'a, P>(&'a self, pat: P) -> RSplit<'a, P>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        RSplit(self.split(pat).0)
    }

    /// An iterator over substrings of the given string slice, separated by
    /// characters matched by a pattern.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// Equivalent to [`split`], except that the trailing substring
    /// is skipped if empty.
    ///
    /// [`split`]: str::split
    ///
    /// This method can be used for string data that is _terminated_,
    /// rather than _separated_ by a pattern.
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will be a [`DoubleEndedIterator`] if the pattern
    /// allows a reverse search and forward/reverse search yields the same
    /// elements. This is true for, e.g., [`char`], but not for `&str`.
    ///
    /// If the pattern allows a reverse search but its results might differ
    /// from a forward search, the [`rsplit_terminator`] method can be used.
    ///
    /// [`rsplit_terminator`]: str::rsplit_terminator
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v: Vec<&str> = "A.B.".split_terminator('.').collect();
    /// assert_eq!(v, ["A", "B"]);
    ///
    /// let v: Vec<&str> = "A..B..".split_terminator(".").collect();
    /// assert_eq!(v, ["A", "", "B", ""]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split_terminator<'a, P: Pattern<'a>>(&'a self, pat: P) -> SplitTerminator<'a, P> {
        SplitTerminator(SplitInternal { allow_trailing_empty: false, ..self.split(pat).0 })
    }

    /// An iterator over substrings of `self`, separated by characters
    /// matched by a pattern and yielded in reverse order.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// Equivalent to [`split`], except that the trailing substring is
    /// skipped if empty.
    ///
    /// [`split`]: str::split
    ///
    /// This method can be used for string data that is _terminated_,
    /// rather than _separated_ by a pattern.
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator requires that the pattern supports a
    /// reverse search, and it will be double ended if a forward/reverse
    /// search yields the same elements.
    ///
    /// For iterating from the front, the [`split_terminator`] method can be
    /// used.
    ///
    /// [`split_terminator`]: str::split_terminator
    ///
    /// # Examples
    ///
    /// ```
    /// let v: Vec<&str> = "A.B.".rsplit_terminator('.').collect();
    /// assert_eq!(v, ["B", "A"]);
    ///
    /// let v: Vec<&str> = "A..B..".rsplit_terminator(".").collect();
    /// assert_eq!(v, ["", "B", "", "A"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rsplit_terminator<'a, P>(&'a self, pat: P) -> RSplitTerminator<'a, P>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        RSplitTerminator(self.split_terminator(pat).0)
    }

    /// An iterator over substrings of the given string slice, separated by a
    /// pattern, restricted to returning at most `n` items.
    ///
    /// If `n` substrings are returned, the last substring (the `n`th substring)
    /// will contain the remainder of the string.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will not be double ended, because it is
    /// not efficient to support.
    ///
    /// If the pattern allows a reverse search, the [`rsplitn`] method can be
    /// used.
    ///
    /// [`rsplitn`]: str::rsplitn
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let v: Vec<&str> = "Mary had a little lambda".splitn(3, ' ').collect();
    /// assert_eq!(v, ["Mary", "had", "a little lambda"]);
    ///
    /// let v: Vec<&str> = "lionXXtigerXleopard".splitn(3, "X").collect();
    /// assert_eq!(v, ["lion", "", "tigerXleopard"]);
    ///
    /// let v: Vec<&str> = "abcXdef".splitn(1, 'X').collect();
    /// assert_eq!(v, ["abcXdef"]);
    ///
    /// let v: Vec<&str> = "".splitn(1, 'X').collect();
    /// assert_eq!(v, [""]);
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// let v: Vec<&str> = "abc1defXghi".splitn(2, |c| c == '1' || c == 'X').collect();
    /// assert_eq!(v, ["abc", "defXghi"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn splitn<'a, P: Pattern<'a>>(&'a self, n: usize, pat: P) -> SplitN<'a, P> {
        SplitN(SplitNInternal { iter: self.split(pat).0, count: n })
    }

    /// An iterator over substrings of this string slice, separated by a
    /// pattern, starting from the end of the string, restricted to returning
    /// at most `n` items.
    ///
    /// If `n` substrings are returned, the last substring (the `n`th substring)
    /// will contain the remainder of the string.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will not be double ended, because it is not
    /// efficient to support.
    ///
    /// For splitting from the front, the [`splitn`] method can be used.
    ///
    /// [`splitn`]: str::splitn
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// let v: Vec<&str> = "Mary had a little lamb".rsplitn(3, ' ').collect();
    /// assert_eq!(v, ["lamb", "little", "Mary had a"]);
    ///
    /// let v: Vec<&str> = "lionXXtigerXleopard".rsplitn(3, 'X').collect();
    /// assert_eq!(v, ["leopard", "tiger", "lionX"]);
    ///
    /// let v: Vec<&str> = "lion::tiger::leopard".rsplitn(2, "::").collect();
    /// assert_eq!(v, ["leopard", "lion::tiger"]);
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// let v: Vec<&str> = "abc1defXghi".rsplitn(2, |c| c == '1' || c == 'X').collect();
    /// assert_eq!(v, ["ghi", "abc1def"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rsplitn<'a, P>(&'a self, n: usize, pat: P) -> RSplitN<'a, P>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        RSplitN(self.splitn(n, pat).0)
    }

    /// Splits the string on the first occurrence of the specified delimiter and
    /// returns prefix before delimiter and suffix after delimiter.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_once)]
    ///
    /// assert_eq!("cfg".split_once('='), None);
    /// assert_eq!("cfg=foo".split_once('='), Some(("cfg", "foo")));
    /// assert_eq!("cfg=foo=bar".split_once('='), Some(("cfg", "foo=bar")));
    /// ```
    #[unstable(feature = "str_split_once", reason = "newly added", issue = "74773")]
    #[inline]
    pub fn split_once<'a, P: Pattern<'a>>(&'a self, delimiter: P) -> Option<(&'a str, &'a str)> {
        let (start, end) = delimiter.into_searcher(self).next_match()?;
        Some((&self[..start], &self[end..]))
    }

    /// Splits the string on the last occurrence of the specified delimiter and
    /// returns prefix before delimiter and suffix after delimiter.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_once)]
    ///
    /// assert_eq!("cfg".rsplit_once('='), None);
    /// assert_eq!("cfg=foo".rsplit_once('='), Some(("cfg", "foo")));
    /// assert_eq!("cfg=foo=bar".rsplit_once('='), Some(("cfg=foo", "bar")));
    /// ```
    #[unstable(feature = "str_split_once", reason = "newly added", issue = "74773")]
    #[inline]
    pub fn rsplit_once<'a, P>(&'a self, delimiter: P) -> Option<(&'a str, &'a str)>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        let (start, end) = delimiter.into_searcher(self).next_match_back()?;
        Some((&self[..start], &self[end..]))
    }

    /// An iterator over the disjoint matches of a pattern within the given string
    /// slice.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will be a [`DoubleEndedIterator`] if the pattern
    /// allows a reverse search and forward/reverse search yields the same
    /// elements. This is true for, e.g., [`char`], but not for `&str`.
    ///
    /// If the pattern allows a reverse search but its results might differ
    /// from a forward search, the [`rmatches`] method can be used.
    ///
    /// [`rmatches`]: str::matches
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v: Vec<&str> = "abcXXXabcYYYabc".matches("abc").collect();
    /// assert_eq!(v, ["abc", "abc", "abc"]);
    ///
    /// let v: Vec<&str> = "1abc2abc3".matches(char::is_numeric).collect();
    /// assert_eq!(v, ["1", "2", "3"]);
    /// ```
    #[stable(feature = "str_matches", since = "1.2.0")]
    #[inline]
    pub fn matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> Matches<'a, P> {
        Matches(MatchesInternal(pat.into_searcher(self)))
    }

    /// An iterator over the disjoint matches of a pattern within this string slice,
    /// yielded in reverse order.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator requires that the pattern supports a reverse
    /// search, and it will be a [`DoubleEndedIterator`] if a forward/reverse
    /// search yields the same elements.
    ///
    /// For iterating from the front, the [`matches`] method can be used.
    ///
    /// [`matches`]: str::matches
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v: Vec<&str> = "abcXXXabcYYYabc".rmatches("abc").collect();
    /// assert_eq!(v, ["abc", "abc", "abc"]);
    ///
    /// let v: Vec<&str> = "1abc2abc3".rmatches(char::is_numeric).collect();
    /// assert_eq!(v, ["3", "2", "1"]);
    /// ```
    #[stable(feature = "str_matches", since = "1.2.0")]
    #[inline]
    pub fn rmatches<'a, P>(&'a self, pat: P) -> RMatches<'a, P>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        RMatches(self.matches(pat).0)
    }

    /// An iterator over the disjoint matches of a pattern within this string
    /// slice as well as the index that the match starts at.
    ///
    /// For matches of `pat` within `self` that overlap, only the indices
    /// corresponding to the first match are returned.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator will be a [`DoubleEndedIterator`] if the pattern
    /// allows a reverse search and forward/reverse search yields the same
    /// elements. This is true for, e.g., [`char`], but not for `&str`.
    ///
    /// If the pattern allows a reverse search but its results might differ
    /// from a forward search, the [`rmatch_indices`] method can be used.
    ///
    /// [`rmatch_indices`]: str::match_indices
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v: Vec<_> = "abcXXXabcYYYabc".match_indices("abc").collect();
    /// assert_eq!(v, [(0, "abc"), (6, "abc"), (12, "abc")]);
    ///
    /// let v: Vec<_> = "1abcabc2".match_indices("abc").collect();
    /// assert_eq!(v, [(1, "abc"), (4, "abc")]);
    ///
    /// let v: Vec<_> = "ababa".match_indices("aba").collect();
    /// assert_eq!(v, [(0, "aba")]); // only the first `aba`
    /// ```
    #[stable(feature = "str_match_indices", since = "1.5.0")]
    #[inline]
    pub fn match_indices<'a, P: Pattern<'a>>(&'a self, pat: P) -> MatchIndices<'a, P> {
        MatchIndices(MatchIndicesInternal(pat.into_searcher(self)))
    }

    /// An iterator over the disjoint matches of a pattern within `self`,
    /// yielded in reverse order along with the index of the match.
    ///
    /// For matches of `pat` within `self` that overlap, only the indices
    /// corresponding to the last match are returned.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Iterator behavior
    ///
    /// The returned iterator requires that the pattern supports a reverse
    /// search, and it will be a [`DoubleEndedIterator`] if a forward/reverse
    /// search yields the same elements.
    ///
    /// For iterating from the front, the [`match_indices`] method can be used.
    ///
    /// [`match_indices`]: str::match_indices
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v: Vec<_> = "abcXXXabcYYYabc".rmatch_indices("abc").collect();
    /// assert_eq!(v, [(12, "abc"), (6, "abc"), (0, "abc")]);
    ///
    /// let v: Vec<_> = "1abcabc2".rmatch_indices("abc").collect();
    /// assert_eq!(v, [(4, "abc"), (1, "abc")]);
    ///
    /// let v: Vec<_> = "ababa".rmatch_indices("aba").collect();
    /// assert_eq!(v, [(2, "aba")]); // only the last `aba`
    /// ```
    #[stable(feature = "str_match_indices", since = "1.5.0")]
    #[inline]
    pub fn rmatch_indices<'a, P>(&'a self, pat: P) -> RMatchIndices<'a, P>
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        RMatchIndices(self.match_indices(pat).0)
    }

    /// Returns a string slice with leading and trailing whitespace removed.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = " Hello\tworld\t";
    ///
    /// assert_eq!("Hello\tworld", s.trim());
    /// ```
    #[must_use = "this returns the trimmed string as a slice, \
                  without modifying the original"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn trim(&self) -> &str {
        self.trim_matches(|c: char| c.is_whitespace())
    }

    /// Returns a string slice with leading whitespace removed.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`.
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. `start` in this context means the first
    /// position of that byte string; for a left-to-right language like English or
    /// Russian, this will be left side, and for right-to-left languages like
    /// Arabic or Hebrew, this will be the right side.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = " Hello\tworld\t";
    /// assert_eq!("Hello\tworld\t", s.trim_start());
    /// ```
    ///
    /// Directionality:
    ///
    /// ```
    /// let s = "  English  ";
    /// assert!(Some('E') == s.trim_start().chars().next());
    ///
    /// let s = "  עברית  ";
    /// assert!(Some('ע') == s.trim_start().chars().next());
    /// ```
    #[must_use = "this returns the trimmed string as a new slice, \
                  without modifying the original"]
    #[stable(feature = "trim_direction", since = "1.30.0")]
    pub fn trim_start(&self) -> &str {
        self.trim_start_matches(|c: char| c.is_whitespace())
    }

    /// Returns a string slice with trailing whitespace removed.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`.
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. `end` in this context means the last
    /// position of that byte string; for a left-to-right language like English or
    /// Russian, this will be right side, and for right-to-left languages like
    /// Arabic or Hebrew, this will be the left side.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = " Hello\tworld\t";
    /// assert_eq!(" Hello\tworld", s.trim_end());
    /// ```
    ///
    /// Directionality:
    ///
    /// ```
    /// let s = "  English  ";
    /// assert!(Some('h') == s.trim_end().chars().rev().next());
    ///
    /// let s = "  עברית  ";
    /// assert!(Some('ת') == s.trim_end().chars().rev().next());
    /// ```
    #[must_use = "this returns the trimmed string as a new slice, \
                  without modifying the original"]
    #[stable(feature = "trim_direction", since = "1.30.0")]
    pub fn trim_end(&self) -> &str {
        self.trim_end_matches(|c: char| c.is_whitespace())
    }

    /// Returns a string slice with leading whitespace removed.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`.
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. 'Left' in this context means the first
    /// position of that byte string; for a language like Arabic or Hebrew
    /// which are 'right to left' rather than 'left to right', this will be
    /// the _right_ side, not the left.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = " Hello\tworld\t";
    ///
    /// assert_eq!("Hello\tworld\t", s.trim_left());
    /// ```
    ///
    /// Directionality:
    ///
    /// ```
    /// let s = "  English";
    /// assert!(Some('E') == s.trim_left().chars().next());
    ///
    /// let s = "  עברית";
    /// assert!(Some('ע') == s.trim_left().chars().next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(
        since = "1.33.0",
        reason = "superseded by `trim_start`",
        suggestion = "trim_start"
    )]
    pub fn trim_left(&self) -> &str {
        self.trim_start()
    }

    /// Returns a string slice with trailing whitespace removed.
    ///
    /// 'Whitespace' is defined according to the terms of the Unicode Derived
    /// Core Property `White_Space`.
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. 'Right' in this context means the last
    /// position of that byte string; for a language like Arabic or Hebrew
    /// which are 'right to left' rather than 'left to right', this will be
    /// the _left_ side, not the right.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let s = " Hello\tworld\t";
    ///
    /// assert_eq!(" Hello\tworld", s.trim_right());
    /// ```
    ///
    /// Directionality:
    ///
    /// ```
    /// let s = "English  ";
    /// assert!(Some('h') == s.trim_right().chars().rev().next());
    ///
    /// let s = "עברית  ";
    /// assert!(Some('ת') == s.trim_right().chars().rev().next());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(
        since = "1.33.0",
        reason = "superseded by `trim_end`",
        suggestion = "trim_end"
    )]
    pub fn trim_right(&self) -> &str {
        self.trim_end()
    }

    /// Returns a string slice with all prefixes and suffixes that match a
    /// pattern repeatedly removed.
    ///
    /// The [pattern] can be a [`char`], a slice of [`char`]s, or a function
    /// or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// assert_eq!("11foo1bar11".trim_matches('1'), "foo1bar");
    /// assert_eq!("123foo1bar123".trim_matches(char::is_numeric), "foo1bar");
    ///
    /// let x: &[_] = &['1', '2'];
    /// assert_eq!("12foo1bar12".trim_matches(x), "foo1bar");
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// assert_eq!("1foo1barXX".trim_matches(|c| c == '1' || c == 'X'), "foo1bar");
    /// ```
    #[must_use = "this returns the trimmed string as a new slice, \
                  without modifying the original"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn trim_matches<'a, P>(&'a self, pat: P) -> &'a str
    where
        P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>,
    {
        let mut i = 0;
        let mut j = 0;
        let mut matcher = pat.into_searcher(self);
        if let Some((a, b)) = matcher.next_reject() {
            i = a;
            j = b; // Remember earliest known match, correct it below if
            // last match is different
        }
        if let Some((_, b)) = matcher.next_reject_back() {
            j = b;
        }
        // SAFETY: `Searcher` is known to return valid indices.
        unsafe { self.get_unchecked(i..j) }
    }

    /// Returns a string slice with all prefixes that match a pattern
    /// repeatedly removed.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. `start` in this context means the first
    /// position of that byte string; for a left-to-right language like English or
    /// Russian, this will be left side, and for right-to-left languages like
    /// Arabic or Hebrew, this will be the right side.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// assert_eq!("11foo1bar11".trim_start_matches('1'), "foo1bar11");
    /// assert_eq!("123foo1bar123".trim_start_matches(char::is_numeric), "foo1bar123");
    ///
    /// let x: &[_] = &['1', '2'];
    /// assert_eq!("12foo1bar12".trim_start_matches(x), "foo1bar12");
    /// ```
    #[must_use = "this returns the trimmed string as a new slice, \
                  without modifying the original"]
    #[stable(feature = "trim_direction", since = "1.30.0")]
    pub fn trim_start_matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> &'a str {
        let mut i = self.len();
        let mut matcher = pat.into_searcher(self);
        if let Some((a, _)) = matcher.next_reject() {
            i = a;
        }
        // SAFETY: `Searcher` is known to return valid indices.
        unsafe { self.get_unchecked(i..self.len()) }
    }

    /// Returns a string slice with the prefix removed.
    ///
    /// If the string starts with the pattern `prefix`, `Some` is returned with the substring where
    /// the prefix is removed. Unlike `trim_start_matches`, this method removes the prefix exactly
    /// once.
    ///
    /// If the string does not start with `prefix`, `None` is returned.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!("foo:bar".strip_prefix("foo:"), Some("bar"));
    /// assert_eq!("foo:bar".strip_prefix("bar"), None);
    /// assert_eq!("foofoo".strip_prefix("foo"), Some("foo"));
    /// ```
    #[must_use = "this returns the remaining substring as a new slice, \
                  without modifying the original"]
    #[stable(feature = "str_strip", since = "1.45.0")]
    pub fn strip_prefix<'a, P: Pattern<'a>>(&'a self, prefix: P) -> Option<&'a str> {
        prefix.strip_prefix_of(self)
    }

    /// Returns a string slice with the suffix removed.
    ///
    /// If the string ends with the pattern `suffix`, `Some` is returned with the substring where
    /// the suffix is removed. Unlike `trim_end_matches`, this method removes the suffix exactly
    /// once.
    ///
    /// If the string does not end with `suffix`, `None` is returned.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Examples
    ///
    /// ```
    /// assert_eq!("bar:foo".strip_suffix(":foo"), Some("bar"));
    /// assert_eq!("bar:foo".strip_suffix("bar"), None);
    /// assert_eq!("foofoo".strip_suffix("foo"), Some("foo"));
    /// ```
    #[must_use = "this returns the remaining substring as a new slice, \
                  without modifying the original"]
    #[stable(feature = "str_strip", since = "1.45.0")]
    pub fn strip_suffix<'a, P>(&'a self, suffix: P) -> Option<&'a str>
    where
        P: Pattern<'a>,
        <P as Pattern<'a>>::Searcher: ReverseSearcher<'a>,
    {
        suffix.strip_suffix_of(self)
    }

    /// Returns a string slice with all suffixes that match a pattern
    /// repeatedly removed.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. `end` in this context means the last
    /// position of that byte string; for a left-to-right language like English or
    /// Russian, this will be right side, and for right-to-left languages like
    /// Arabic or Hebrew, this will be the left side.
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// assert_eq!("11foo1bar11".trim_end_matches('1'), "11foo1bar");
    /// assert_eq!("123foo1bar123".trim_end_matches(char::is_numeric), "123foo1bar");
    ///
    /// let x: &[_] = &['1', '2'];
    /// assert_eq!("12foo1bar12".trim_end_matches(x), "12foo1bar");
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// assert_eq!("1fooX".trim_end_matches(|c| c == '1' || c == 'X'), "1foo");
    /// ```
    #[must_use = "this returns the trimmed string as a new slice, \
                  without modifying the original"]
    #[stable(feature = "trim_direction", since = "1.30.0")]
    pub fn trim_end_matches<'a, P>(&'a self, pat: P) -> &'a str
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        let mut j = 0;
        let mut matcher = pat.into_searcher(self);
        if let Some((_, b)) = matcher.next_reject_back() {
            j = b;
        }
        // SAFETY: `Searcher` is known to return valid indices.
        unsafe { self.get_unchecked(0..j) }
    }

    /// Returns a string slice with all prefixes that match a pattern
    /// repeatedly removed.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. 'Left' in this context means the first
    /// position of that byte string; for a language like Arabic or Hebrew
    /// which are 'right to left' rather than 'left to right', this will be
    /// the _right_ side, not the left.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// assert_eq!("11foo1bar11".trim_left_matches('1'), "foo1bar11");
    /// assert_eq!("123foo1bar123".trim_left_matches(char::is_numeric), "foo1bar123");
    ///
    /// let x: &[_] = &['1', '2'];
    /// assert_eq!("12foo1bar12".trim_left_matches(x), "foo1bar12");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(
        since = "1.33.0",
        reason = "superseded by `trim_start_matches`",
        suggestion = "trim_start_matches"
    )]
    pub fn trim_left_matches<'a, P: Pattern<'a>>(&'a self, pat: P) -> &'a str {
        self.trim_start_matches(pat)
    }

    /// Returns a string slice with all suffixes that match a pattern
    /// repeatedly removed.
    ///
    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
    /// function or closure that determines if a character matches.
    ///
    /// [pattern]: self::pattern
    ///
    /// # Text directionality
    ///
    /// A string is a sequence of bytes. 'Right' in this context means the last
    /// position of that byte string; for a language like Arabic or Hebrew
    /// which are 'right to left' rather than 'left to right', this will be
    /// the _left_ side, not the right.
    ///
    /// # Examples
    ///
    /// Simple patterns:
    ///
    /// ```
    /// assert_eq!("11foo1bar11".trim_right_matches('1'), "11foo1bar");
    /// assert_eq!("123foo1bar123".trim_right_matches(char::is_numeric), "123foo1bar");
    ///
    /// let x: &[_] = &['1', '2'];
    /// assert_eq!("12foo1bar12".trim_right_matches(x), "12foo1bar");
    /// ```
    ///
    /// A more complex pattern, using a closure:
    ///
    /// ```
    /// assert_eq!("1fooX".trim_right_matches(|c| c == '1' || c == 'X'), "1foo");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(
        since = "1.33.0",
        reason = "superseded by `trim_end_matches`",
        suggestion = "trim_end_matches"
    )]
    pub fn trim_right_matches<'a, P>(&'a self, pat: P) -> &'a str
    where
        P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
    {
        self.trim_end_matches(pat)
    }

    /// Parses this string slice into another type.
    ///
    /// Because `parse` is so general, it can cause problems with type
    /// inference. As such, `parse` is one of the few times you'll see
    /// the syntax affectionately known as the 'turbofish': `::<>`. This
    /// helps the inference algorithm understand specifically which type
    /// you're trying to parse into.
    ///
    /// `parse` can parse any type that implements the [`FromStr`] trait.

    ///
    /// # Errors
    ///
    /// Will return [`Err`] if it's not possible to parse this string slice into
    /// the desired type.
    ///
    /// [`Err`]: FromStr::Err
    ///
    /// # Examples
    ///
    /// Basic usage
    ///
    /// ```
    /// let four: u32 = "4".parse().unwrap();
    ///
    /// assert_eq!(4, four);
    /// ```
    ///
    /// Using the 'turbofish' instead of annotating `four`:
    ///
    /// ```
    /// let four = "4".parse::<u32>();
    ///
    /// assert_eq!(Ok(4), four);
    /// ```
    ///
    /// Failing to parse:
    ///
    /// ```
    /// let nope = "j".parse::<u32>();
    ///
    /// assert!(nope.is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn parse<F: FromStr>(&self) -> Result<F, F::Err> {
        FromStr::from_str(self)
    }

    /// Checks if all characters in this string are within the ASCII range.
    ///
    /// # Examples
    ///
    /// ```
    /// let ascii = "hello!\n";
    /// let non_ascii = "Grüße, Jürgen ❤";
    ///
    /// assert!(ascii.is_ascii());
    /// assert!(!non_ascii.is_ascii());
    /// ```
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn is_ascii(&self) -> bool {
        // We can treat each byte as character here: all multibyte characters
        // start with a byte that is not in the ascii range, so we will stop
        // there already.
        self.as_bytes().is_ascii()
    }

    /// Checks that two strings are an ASCII case-insensitive match.
    ///
    /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,
    /// but without allocating and copying temporaries.
    ///
    /// # Examples
    ///
    /// ```
    /// assert!("Ferris".eq_ignore_ascii_case("FERRIS"));
    /// assert!("Ferrös".eq_ignore_ascii_case("FERRöS"));
    /// assert!(!"Ferrös".eq_ignore_ascii_case("FERRÖS"));
    /// ```
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn eq_ignore_ascii_case(&self, other: &str) -> bool {
        self.as_bytes().eq_ignore_ascii_case(other.as_bytes())
    }

    /// Converts this string to its ASCII upper case equivalent in-place.
    ///
    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
    /// but non-ASCII letters are unchanged.
    ///
    /// To return a new uppercased value without modifying the existing one, use
    /// [`to_ascii_uppercase`].
    ///
    /// [`to_ascii_uppercase`]: #method.to_ascii_uppercase
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("Grüße, Jürgen ❤");
    ///
    /// s.make_ascii_uppercase();
    ///
    /// assert_eq!("GRüßE, JüRGEN ❤", s);
    /// ```
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    pub fn make_ascii_uppercase(&mut self) {
        // SAFETY: safe because we transmute two types with the same layout.
        let me = unsafe { self.as_bytes_mut() };
        me.make_ascii_uppercase()
    }

    /// Converts this string to its ASCII lower case equivalent in-place.
    ///
    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
    /// but non-ASCII letters are unchanged.
    ///
    /// To return a new lowercased value without modifying the existing one, use
    /// [`to_ascii_lowercase`].
    ///
    /// [`to_ascii_lowercase`]: #method.to_ascii_lowercase
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("GRÜßE, JÜRGEN ❤");
    ///
    /// s.make_ascii_lowercase();
    ///
    /// assert_eq!("grÜße, jÜrgen ❤", s);
    /// ```
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    pub fn make_ascii_lowercase(&mut self) {
        // SAFETY: safe because we transmute two types with the same layout.
        let me = unsafe { self.as_bytes_mut() };
        me.make_ascii_lowercase()
    }

    /// Return an iterator that escapes each char in `self` with [`char::escape_debug`].
    ///
    /// Note: only extended grapheme codepoints that begin the string will be
    /// escaped.
    ///
    /// # Examples
    ///
    /// As an iterator:
    ///
    /// ```
    /// for c in "❤\n!".escape_debug() {
    ///     print!("{}", c);
    /// }
    /// println!();
    /// ```
    ///
    /// Using `println!` directly:
    ///
    /// ```
    /// println!("{}", "❤\n!".escape_debug());
    /// ```
    ///
    ///
    /// Both are equivalent to:
    ///
    /// ```
    /// println!("❤\\n!");
    /// ```
    ///
    /// Using `to_string`:
    ///
    /// ```
    /// assert_eq!("❤\n!".escape_debug().to_string(), "❤\\n!");
    /// ```
    #[stable(feature = "str_escape", since = "1.34.0")]
    pub fn escape_debug(&self) -> EscapeDebug<'_> {
        let mut chars = self.chars();
        EscapeDebug {
            inner: chars
                .next()
                .map(|first| first.escape_debug_ext(true))
                .into_iter()
                .flatten()
                .chain(chars.flat_map(CharEscapeDebugContinue)),
        }
    }

    /// Return an iterator that escapes each char in `self` with [`char::escape_default`].
    ///
    /// # Examples
    ///
    /// As an iterator:
    ///
    /// ```
    /// for c in "❤\n!".escape_default() {
    ///     print!("{}", c);
    /// }
    /// println!();
    /// ```
    ///
    /// Using `println!` directly:
    ///
    /// ```
    /// println!("{}", "❤\n!".escape_default());
    /// ```
    ///
    ///
    /// Both are equivalent to:
    ///
    /// ```
    /// println!("\\u{{2764}}\\n!");
    /// ```
    ///
    /// Using `to_string`:
    ///
    /// ```
    /// assert_eq!("❤\n!".escape_default().to_string(), "\\u{2764}\\n!");
    /// ```
    #[stable(feature = "str_escape", since = "1.34.0")]
    pub fn escape_default(&self) -> EscapeDefault<'_> {
        EscapeDefault { inner: self.chars().flat_map(CharEscapeDefault) }
    }

    /// Return an iterator that escapes each char in `self` with [`char::escape_unicode`].
    ///
    /// # Examples
    ///
    /// As an iterator:
    ///
    /// ```
    /// for c in "❤\n!".escape_unicode() {
    ///     print!("{}", c);
    /// }
    /// println!();
    /// ```
    ///
    /// Using `println!` directly:
    ///
    /// ```
    /// println!("{}", "❤\n!".escape_unicode());
    /// ```
    ///
    ///
    /// Both are equivalent to:
    ///
    /// ```
    /// println!("\\u{{2764}}\\u{{a}}\\u{{21}}");
    /// ```
    ///
    /// Using `to_string`:
    ///
    /// ```
    /// assert_eq!("❤\n!".escape_unicode().to_string(), "\\u{2764}\\u{a}\\u{21}");
    /// ```
    #[stable(feature = "str_escape", since = "1.34.0")]
    pub fn escape_unicode(&self) -> EscapeUnicode<'_> {
        EscapeUnicode { inner: self.chars().flat_map(CharEscapeUnicode) }
    }
}

impl_fn_for_zst! {
    #[derive(Clone)]
    struct CharEscapeDebugContinue impl Fn = |c: char| -> char::EscapeDebug {
        c.escape_debug_ext(false)
    };

    #[derive(Clone)]
    struct CharEscapeUnicode impl Fn = |c: char| -> char::EscapeUnicode {
        c.escape_unicode()
    };
    #[derive(Clone)]
    struct CharEscapeDefault impl Fn = |c: char| -> char::EscapeDefault {
        c.escape_default()
    };
}

#[stable(feature = "rust1", since = "1.0.0")]
impl AsRef<[u8]> for str {
    #[inline]
    fn as_ref(&self) -> &[u8] {
        self.as_bytes()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Default for &str {
    /// Creates an empty str
    fn default() -> Self {
        ""
    }
}

#[stable(feature = "default_mut_str", since = "1.28.0")]
impl Default for &mut str {
    /// Creates an empty mutable str
    fn default() -> Self {
        // SAFETY: The empty string is valid UTF-8.
        unsafe { from_utf8_unchecked_mut(&mut []) }
    }
}

/// An iterator over the non-whitespace substrings of a string,
/// separated by any amount of whitespace.
///
/// This struct is created by the [`split_whitespace`] method on [`str`].
/// See its documentation for more.
///
/// [`split_whitespace`]: str::split_whitespace
#[stable(feature = "split_whitespace", since = "1.1.0")]
#[derive(Clone, Debug)]
pub struct SplitWhitespace<'a> {
    inner: Filter<Split<'a, IsWhitespace>, IsNotEmpty>,
}

/// An iterator over the non-ASCII-whitespace substrings of a string,
/// separated by any amount of ASCII whitespace.
///
/// This struct is created by the [`split_ascii_whitespace`] method on [`str`].
/// See its documentation for more.
///
/// [`split_ascii_whitespace`]: str::split_ascii_whitespace
#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct SplitAsciiWhitespace<'a> {
    inner: Map<Filter<SliceSplit<'a, u8, IsAsciiWhitespace>, BytesIsNotEmpty>, UnsafeBytesToStr>,
}

/// An iterator over the substrings of a string,
/// terminated by a substring matching to a predicate function
/// Unlike `Split`, it contains the matched part as a terminator
/// of the subslice.
///
/// This struct is created by the [`split_inclusive`] method on [`str`].
/// See its documentation for more.
///
/// [`split_inclusive`]: str::split_inclusive
#[unstable(feature = "split_inclusive", issue = "72360")]
pub struct SplitInclusive<'a, P: Pattern<'a>>(SplitInternal<'a, P>);

impl_fn_for_zst! {
    #[derive(Clone)]
    struct IsWhitespace impl Fn = |c: char| -> bool {
        c.is_whitespace()
    };

    #[derive(Clone)]
    struct IsAsciiWhitespace impl Fn = |byte: &u8| -> bool {
        byte.is_ascii_whitespace()
    };

    #[derive(Clone)]
    struct IsNotEmpty impl<'a, 'b> Fn = |s: &'a &'b str| -> bool {
        !s.is_empty()
    };

    #[derive(Clone)]
    struct BytesIsNotEmpty impl<'a, 'b> Fn = |s: &'a &'b [u8]| -> bool {
        !s.is_empty()
    };

    #[derive(Clone)]
    struct UnsafeBytesToStr impl<'a> Fn = |bytes: &'a [u8]| -> &'a str {
        // SAFETY: not safe
        unsafe { from_utf8_unchecked(bytes) }
    };
}

#[stable(feature = "split_whitespace", since = "1.1.0")]
impl<'a> Iterator for SplitWhitespace<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.inner.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "split_whitespace", since = "1.1.0")]
impl<'a> DoubleEndedIterator for SplitWhitespace<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.inner.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for SplitWhitespace<'_> {}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl<'a> Iterator for SplitAsciiWhitespace<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.inner.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl<'a> DoubleEndedIterator for SplitAsciiWhitespace<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.inner.next_back()
    }
}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl FusedIterator for SplitAsciiWhitespace<'_> {}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, P: Pattern<'a>> Iterator for SplitInclusive<'a, P> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next_inclusive()
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, P: Pattern<'a, Searcher: fmt::Debug>> fmt::Debug for SplitInclusive<'a, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInclusive").field("0", &self.0).finish()
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, P: Pattern<'a, Searcher: Clone>> Clone for SplitInclusive<'a, P> {
    fn clone(&self) -> Self {
        SplitInclusive(self.0.clone())
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, P: Pattern<'a, Searcher: ReverseSearcher<'a>>> DoubleEndedIterator
    for SplitInclusive<'a, P>
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back_inclusive()
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, P: Pattern<'a>> FusedIterator for SplitInclusive<'a, P> {}

/// An iterator of [`u16`] over the string encoded as UTF-16.
///
/// This struct is created by the [`encode_utf16`] method on [`str`].
/// See its documentation for more.
///
/// [`encode_utf16`]: str::encode_utf16
#[derive(Clone)]
#[stable(feature = "encode_utf16", since = "1.8.0")]
pub struct EncodeUtf16<'a> {
    chars: Chars<'a>,
    extra: u16,
}

#[stable(feature = "collection_debug", since = "1.17.0")]
impl fmt::Debug for EncodeUtf16<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("EncodeUtf16 { .. }")
    }
}

#[stable(feature = "encode_utf16", since = "1.8.0")]
impl<'a> Iterator for EncodeUtf16<'a> {
    type Item = u16;

    #[inline]
    fn next(&mut self) -> Option<u16> {
        if self.extra != 0 {
            let tmp = self.extra;
            self.extra = 0;
            return Some(tmp);
        }

        let mut buf = [0; 2];
        self.chars.next().map(|ch| {
            let n = ch.encode_utf16(&mut buf).len();
            if n == 2 {
                self.extra = buf[1];
            }
            buf[0]
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (low, high) = self.chars.size_hint();
        // every char gets either one u16 or two u16,
        // so this iterator is between 1 or 2 times as
        // long as the underlying iterator.
        (low, high.and_then(|n| n.checked_mul(2)))
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for EncodeUtf16<'_> {}

/// The return type of [`str::escape_debug`].
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeDebug<'a> {
    inner: Chain<
        Flatten<option::IntoIter<char::EscapeDebug>>,
        FlatMap<Chars<'a>, char::EscapeDebug, CharEscapeDebugContinue>,
    >,
}

/// The return type of [`str::escape_default`].
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeDefault<'a> {
    inner: FlatMap<Chars<'a>, char::EscapeDefault, CharEscapeDefault>,
}

/// The return type of [`str::escape_unicode`].
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeUnicode<'a> {
    inner: FlatMap<Chars<'a>, char::EscapeUnicode, CharEscapeUnicode>,
}

macro_rules! escape_types_impls {
    ($( $Name: ident ),+) => {$(
        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> fmt::Display for $Name<'a> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                self.clone().try_for_each(|c| f.write_char(c))
            }
        }

        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> Iterator for $Name<'a> {
            type Item = char;

            #[inline]
            fn next(&mut self) -> Option<char> { self.inner.next() }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }

            #[inline]
            fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
                Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
            {
                self.inner.try_fold(init, fold)
            }

            #[inline]
            fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
                where Fold: FnMut(Acc, Self::Item) -> Acc,
            {
                self.inner.fold(init, fold)
            }
        }

        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> FusedIterator for $Name<'a> {}
    )+}
}

escape_types_impls!(EscapeDebug, EscapeDefault, EscapeUnicode);