cow_rc_str.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

use alloc::borrow::{Borrow, Cow};
use alloc::rc::Rc;
use alloc::string::String;
use core::{cmp, fmt, hash, marker, mem, ops, ptr, slice, str};

/// A string that is either shared (heap-allocated and reference-counted) or borrowed.
///
/// Equivalent to `enum { Borrowed(&'a str), Shared(Rc<String>) }`, but stored more compactly.
///
/// * If `borrowed_len_or_max == usize::MAX`, then `ptr` represents `NonZero<*const String>`
///   from `Rc::into_raw`.
///   The lifetime parameter `'a` is irrelevant in this case.
///
/// * Otherwise, `ptr` represents the `NonZero<*const u8>` data component of `&'a str`,
///   and `borrowed_len_or_max` its length.
pub struct CowRcStr<'a> {
    ptr: ptr::NonNull<()>,
    borrowed_len_or_max: usize,

    phantom: marker::PhantomData<Result<&'a str, Rc<String>>>,
}

fn _static_assert_same_size() {
    // "Instantiate" the generic function without calling it.
    let _ = mem::transmute::<CowRcStr<'_>, Option<CowRcStr<'_>>>;
}

impl<'a> From<Cow<'a, str>> for CowRcStr<'a> {
    #[inline]
    fn from(s: Cow<'a, str>) -> Self {
        match s {
            Cow::Borrowed(s) => CowRcStr::from(s),
            Cow::Owned(s) => CowRcStr::from(s),
        }
    }
}

impl<'a> From<&'a str> for CowRcStr<'a> {
    #[inline]
    fn from(s: &'a str) -> Self {
        let len = s.len();
        assert!(len < usize::MAX);
        CowRcStr {
            ptr: unsafe { ptr::NonNull::new_unchecked(s.as_ptr() as *mut ()) },
            borrowed_len_or_max: len,
            phantom: marker::PhantomData,
        }
    }
}

impl From<String> for CowRcStr<'_> {
    #[inline]
    fn from(s: String) -> Self {
        CowRcStr::from_rc(Rc::new(s))
    }
}

impl<'a> CowRcStr<'a> {
    #[inline]
    fn from_rc(s: Rc<String>) -> Self {
        let ptr = unsafe { ptr::NonNull::new_unchecked(Rc::into_raw(s) as *mut ()) };
        CowRcStr {
            ptr,
            borrowed_len_or_max: usize::MAX,
            phantom: marker::PhantomData,
        }
    }

    #[inline]
    fn unpack(&self) -> Result<&'a str, *const String> {
        if self.borrowed_len_or_max == usize::MAX {
            Err(self.ptr.as_ptr() as *const String)
        } else {
            unsafe {
                Ok(str::from_utf8_unchecked(slice::from_raw_parts(
                    self.ptr.as_ptr() as *const u8,
                    self.borrowed_len_or_max,
                )))
            }
        }
    }
}

impl Clone for CowRcStr<'_> {
    #[inline]
    fn clone(&self) -> Self {
        match self.unpack() {
            Err(ptr) => {
                let rc = unsafe { Rc::from_raw(ptr) };
                let new_rc = rc.clone();
                mem::forget(rc); // Don’t actually take ownership of this strong reference
                CowRcStr::from_rc(new_rc)
            }
            Ok(_) => CowRcStr { ..*self },
        }
    }
}

impl Drop for CowRcStr<'_> {
    #[inline]
    fn drop(&mut self) {
        if let Err(ptr) = self.unpack() {
            mem::drop(unsafe { Rc::from_raw(ptr) })
        }
    }
}

impl ops::Deref for CowRcStr<'_> {
    type Target = str;

    #[inline]
    fn deref(&self) -> &str {
        self.unpack().unwrap_or_else(|ptr| unsafe { &**ptr })
    }
}

// Boilerplate / trivial impls below.

impl AsRef<str> for CowRcStr<'_> {
    #[inline]
    fn as_ref(&self) -> &str {
        self
    }
}

impl Borrow<str> for CowRcStr<'_> {
    #[inline]
    fn borrow(&self) -> &str {
        self
    }
}

impl Default for CowRcStr<'_> {
    #[inline]
    fn default() -> Self {
        Self::from("")
    }
}

impl hash::Hash for CowRcStr<'_> {
    #[inline]
    fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
        str::hash(self, hasher)
    }
}

impl<T: AsRef<str>> PartialEq<T> for CowRcStr<'_> {
    #[inline]
    fn eq(&self, other: &T) -> bool {
        str::eq(self, other.as_ref())
    }
}

impl<T: AsRef<str>> PartialOrd<T> for CowRcStr<'_> {
    #[inline]
    fn partial_cmp(&self, other: &T) -> Option<cmp::Ordering> {
        str::partial_cmp(self, other.as_ref())
    }
}

impl Eq for CowRcStr<'_> {}

impl Ord for CowRcStr<'_> {
    #[inline]
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        str::cmp(self, other)
    }
}

impl fmt::Display for CowRcStr<'_> {
    #[inline]
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        str::fmt(self, formatter)
    }
}

impl fmt::Debug for CowRcStr<'_> {
    #[inline]
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        str::fmt(self, formatter)
    }
}