Rollup of 10 pull requests

compiler/rustc_index/src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(allow_internal_unstable)]
-#![feature(bool_to_option)]
 #![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(extend_one)]

compiler/rustc_metadata/src/lib.rs

@@ -1,5 +1,4 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
-#![feature(bool_to_option)]
 #![feature(core_intrinsics)]
 #![feature(crate_visibility_modifier)]
 #![feature(drain_filter)]

compiler/rustc_parse/src/lib.rs

@@ -1,6 +1,5 @@
 //! The main parser interface.
 
-#![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(bindings_after_at)]
 #![feature(iter_order_by)]

library/alloc/src/collections/btree/append.rs

@@ -67,7 +67,7 @@ impl<K, V> Root<K, V> {
 
                 // Push key-value pair and new right subtree.
                 let tree_height = open_node.height() - 1;
-                let mut right_tree = Root::new_leaf();
+                let mut right_tree = Root::new();
                 for _ in 0..tree_height {
                     right_tree.push_internal_level();
                 }

library/alloc/src/collections/btree/map.rs

@@ -9,7 +9,7 @@ use core::ops::{Index, RangeBounds};
 use core::ptr;
 
 use super::borrow::DormantMutRef;
-use super::node::{self, marker, ForceResult::*, Handle, NodeRef};
+use super::node::{self, marker, ForceResult::*, Handle, NodeRef, Root};
 use super::search::{self, SearchResult::*};
 use super::unwrap_unchecked;
 
@@ -128,7 +128,7 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BTreeMap<K, V> {
-    root: Option<node::Root<K, V>>,
+    root: Option<Root<K, V>>,
     length: usize,
 }
 
@@ -145,15 +145,15 @@ unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for BTreeMap<K, V> {
 impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
     fn clone(&self) -> BTreeMap<K, V> {
         fn clone_subtree<'a, K: Clone, V: Clone>(
-            node: node::NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>,
+            node: NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>,
         ) -> BTreeMap<K, V>
         where
             K: 'a,
             V: 'a,
         {
             match node.force() {
                 Leaf(leaf) => {
-                    let mut out_tree = BTreeMap { root: Some(node::Root::new_leaf()), length: 0 };
+                    let mut out_tree = BTreeMap { root: Some(Root::new()), length: 0 };
 
                     {
                         let root = out_tree.root.as_mut().unwrap(); // unwrap succeeds because we just wrapped
@@ -198,7 +198,7 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                                 (root, length)
                             };
 
-                            out_node.push(k, v, subroot.unwrap_or_else(node::Root::new_leaf));
+                            out_node.push(k, v, subroot.unwrap_or_else(Root::new));
                             out_tree.length += 1 + sublength;
                         }
                     }
@@ -1558,7 +1558,7 @@ pub(super) struct DrainFilterInner<'a, K: 'a, V: 'a> {
     length: &'a mut usize,
     /// Burried reference to the root field in the borrowed map.
     /// Wrapped in `Option` to allow drop handler to `take` it.
-    dormant_root: Option<DormantMutRef<'a, node::Root<K, V>>>,
+    dormant_root: Option<DormantMutRef<'a, Root<K, V>>>,
     /// Contains a leaf edge preceding the next element to be returned, or the last leaf edge.
     /// Empty if the map has no root, if iteration went beyond the last leaf edge,
     /// or if a panic occurred in the predicate.
@@ -2160,8 +2160,8 @@ impl<K, V> BTreeMap<K, V> {
 
     /// If the root node is the empty (non-allocated) root node, allocate our
     /// own node. Is an associated function to avoid borrowing the entire BTreeMap.
-    fn ensure_is_owned(root: &mut Option<node::Root<K, V>>) -> &mut node::Root<K, V> {
-        root.get_or_insert_with(node::Root::new_leaf)
+    fn ensure_is_owned(root: &mut Option<Root<K, V>>) -> &mut Root<K, V> {
+        root.get_or_insert_with(Root::new)
     }
 }
 

library/alloc/src/collections/btree/map/tests.rs

@@ -6,13 +6,14 @@ use crate::fmt::Debug;
 use crate::rc::Rc;
 use crate::string::{String, ToString};
 use crate::vec::Vec;
+use std::cmp::Ordering;
 use std::convert::TryFrom;
 use std::iter::{self, FromIterator};
 use std::mem;
 use std::ops::Bound::{self, Excluded, Included, Unbounded};
 use std::ops::RangeBounds;
 use std::panic::{catch_unwind, AssertUnwindSafe};
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 
 mod ord_chaos;
 use ord_chaos::{Cyclic3, Governed, Governor};
@@ -56,24 +57,23 @@ impl<K, V> BTreeMap<K, V> {
             assert!(root_node.ascend().is_err());
             root_node.assert_back_pointers();
 
-            // Check consistenty of `length` and some of the navigation.
+            // Check consistency of `length` with what navigation code encounters.
             assert_eq!(self.length, root_node.calc_length());
-            assert_eq!(self.length, self.keys().count());
 
             // Lastly, check the invariant causing the least harm.
             root_node.assert_min_len(if root_node.height() > 0 { 1 } else { 0 });
         } else {
-            // Check consistenty of `length` and some of the navigation.
             assert_eq!(self.length, 0);
-            assert_eq!(self.length, self.keys().count());
         }
+
+        // Check that `assert_strictly_ascending` will encounter all keys.
+        assert_eq!(self.length, self.keys().count());
     }
 
     // Panics if the map is corrupted or if the keys are not in strictly
     // ascending order, in the current opinion of the `Ord` implementation.
-    // If the `Ord` implementation does not honor transitivity, this method
-    // does not guarantee that all the keys are unique, just that adjacent
-    // keys are unique.
+    // If the `Ord` implementation violates transitivity, this method does not
+    // guarantee that all keys are unique, just that adjacent keys are unique.
     fn check(&self)
     where
         K: Debug + Ord,
@@ -879,6 +879,7 @@ mod test_drain_filter {
         map.check();
     }
 
+    // Explicitly consumes the iterator, where most test cases drop it instantly.
     #[test]
     fn consumed_keeping_all() {
         let pairs = (0..3).map(|i| (i, i));
@@ -887,6 +888,7 @@ mod test_drain_filter {
         map.check();
     }
 
+    // Explicitly consumes the iterator, where most test cases drop it instantly.
     #[test]
     fn consumed_removing_all() {
         let pairs = (0..3).map(|i| (i, i));
@@ -896,15 +898,7 @@ mod test_drain_filter {
         map.check();
     }
 
-    #[test]
-    fn dropped_removing_all() {
-        let pairs = (0..3).map(|i| (i, i));
-        let mut map: BTreeMap<_, _> = pairs.collect();
-        map.drain_filter(|_, _| true);
-        assert!(map.is_empty());
-        map.check();
-    }
-
+    // Explicitly consumes the iterator and modifies values through it.
     #[test]
     fn mutating_and_keeping() {
         let pairs = (0..3).map(|i| (i, i));
@@ -921,6 +915,7 @@ mod test_drain_filter {
         map.check();
     }
 
+    // Explicitly consumes the iterator and modifies values through it.
     #[test]
     fn mutating_and_removing() {
         let pairs = (0..3).map(|i| (i, i));
@@ -1094,7 +1089,7 @@ mod test_drain_filter {
         struct D;
         impl Drop for D {
             fn drop(&mut self) {
-                if DROPS.fetch_add(1, Ordering::SeqCst) == 1 {
+                if DROPS.fetch_add(1, SeqCst) == 1 {
                     panic!("panic in `drop`");
                 }
             }
@@ -1105,14 +1100,14 @@ mod test_drain_filter {
 
         catch_unwind(move || {
             drop(map.drain_filter(|i, _| {
-                PREDS.fetch_add(1usize << i, Ordering::SeqCst);
+                PREDS.fetch_add(1usize << i, SeqCst);
                 true
             }))
         })
         .unwrap_err();
 
-        assert_eq!(PREDS.load(Ordering::SeqCst), 0x011);
-        assert_eq!(DROPS.load(Ordering::SeqCst), 3);
+        assert_eq!(PREDS.load(SeqCst), 0x011);
+        assert_eq!(DROPS.load(SeqCst), 3);
     }
 
     #[test]
@@ -1123,7 +1118,7 @@ mod test_drain_filter {
         struct D;
         impl Drop for D {
             fn drop(&mut self) {
-                DROPS.fetch_add(1, Ordering::SeqCst);
+                DROPS.fetch_add(1, SeqCst);
             }
         }
 
@@ -1132,7 +1127,7 @@ mod test_drain_filter {
 
         catch_unwind(AssertUnwindSafe(|| {
             drop(map.drain_filter(|i, _| {
-                PREDS.fetch_add(1usize << i, Ordering::SeqCst);
+                PREDS.fetch_add(1usize << i, SeqCst);
                 match i {
                     0 => true,
                     _ => panic!(),
@@ -1141,8 +1136,8 @@ mod test_drain_filter {
         }))
         .unwrap_err();
 
-        assert_eq!(PREDS.load(Ordering::SeqCst), 0x011);
-        assert_eq!(DROPS.load(Ordering::SeqCst), 1);
+        assert_eq!(PREDS.load(SeqCst), 0x011);
+        assert_eq!(DROPS.load(SeqCst), 1);
         assert_eq!(map.len(), 2);
         assert_eq!(map.first_entry().unwrap().key(), &4);
         assert_eq!(map.last_entry().unwrap().key(), &8);
@@ -1158,7 +1153,7 @@ mod test_drain_filter {
         struct D;
         impl Drop for D {
             fn drop(&mut self) {
-                DROPS.fetch_add(1, Ordering::SeqCst);
+                DROPS.fetch_add(1, SeqCst);
             }
         }
 
@@ -1167,7 +1162,7 @@ mod test_drain_filter {
 
         {
             let mut it = map.drain_filter(|i, _| {
-                PREDS.fetch_add(1usize << i, Ordering::SeqCst);
+                PREDS.fetch_add(1usize << i, SeqCst);
                 match i {
                     0 => true,
                     _ => panic!(),
@@ -1180,8 +1175,8 @@ mod test_drain_filter {
             assert!(matches!(result, Ok(None)));
         }
 
-        assert_eq!(PREDS.load(Ordering::SeqCst), 0x011);
-        assert_eq!(DROPS.load(Ordering::SeqCst), 1);
+        assert_eq!(PREDS.load(SeqCst), 0x011);
+        assert_eq!(DROPS.load(SeqCst), 1);
         assert_eq!(map.len(), 2);
         assert_eq!(map.first_entry().unwrap().key(), &4);
         assert_eq!(map.last_entry().unwrap().key(), &8);
@@ -1315,8 +1310,6 @@ fn test_zst() {
 // undefined.
 #[test]
 fn test_bad_zst() {
-    use std::cmp::Ordering;
-
     #[derive(Clone, Copy, Debug)]
     struct Bad;
 
@@ -1763,7 +1756,7 @@ fn test_append_drop_leak() {
 
     impl Drop for D {
         fn drop(&mut self) {
-            if DROPS.fetch_add(1, Ordering::SeqCst) == 0 {
+            if DROPS.fetch_add(1, SeqCst) == 0 {
                 panic!("panic in `drop`");
             }
         }
@@ -1779,7 +1772,7 @@ fn test_append_drop_leak() {
 
     catch_unwind(move || left.append(&mut right)).unwrap_err();
 
-    assert_eq!(DROPS.load(Ordering::SeqCst), 4); // Rust issue #47949 ate one little piggy
+    assert_eq!(DROPS.load(SeqCst), 4); // Rust issue #47949 ate one little piggy
 }
 
 #[test]
@@ -1894,7 +1887,7 @@ fn test_into_iter_drop_leak_height_0() {
 
     impl Drop for D {
         fn drop(&mut self) {
-            if DROPS.fetch_add(1, Ordering::SeqCst) == 3 {
+            if DROPS.fetch_add(1, SeqCst) == 3 {
                 panic!("panic in `drop`");
             }
         }
@@ -1909,7 +1902,7 @@ fn test_into_iter_drop_leak_height_0() {
 
     catch_unwind(move || drop(map.into_iter())).unwrap_err();
 
-    assert_eq!(DROPS.load(Ordering::SeqCst), 5);
+    assert_eq!(DROPS.load(SeqCst), 5);
 }
 
 #[test]
@@ -1921,18 +1914,18 @@ fn test_into_iter_drop_leak_height_1() {
     struct D;
     impl Drop for D {
         fn drop(&mut self) {
-            if DROPS.fetch_add(1, Ordering::SeqCst) == PANIC_POINT.load(Ordering::SeqCst) {
+            if DROPS.fetch_add(1, SeqCst) == PANIC_POINT.load(SeqCst) {
                 panic!("panic in `drop`");
             }
         }
     }
 
     for panic_point in vec![0, 1, size - 2, size - 1] {
-        DROPS.store(0, Ordering::SeqCst);
-        PANIC_POINT.store(panic_point, Ordering::SeqCst);
+        DROPS.store(0, SeqCst);
+        PANIC_POINT.store(panic_point, SeqCst);
         let map: BTreeMap<_, _> = (0..size).map(|i| (i, D)).collect();
         catch_unwind(move || drop(map.into_iter())).unwrap_err();
-        assert_eq!(DROPS.load(Ordering::SeqCst), size);
+        assert_eq!(DROPS.load(SeqCst), size);
     }
 }
 

library/alloc/src/collections/btree/map/tests/ord_chaos.rs

@@ -2,6 +2,7 @@ use std::cell::Cell;
 use std::cmp::Ordering::{self, *};
 use std::ptr;
 
+// Minimal type with an `Ord` implementation violating transitivity.
 #[derive(Debug)]
 pub enum Cyclic3 {
     A,
@@ -34,6 +35,7 @@ impl PartialEq for Cyclic3 {
 
 impl Eq for Cyclic3 {}
 
+// Controls the ordering of values wrapped by `Governed`.
 #[derive(Debug)]
 pub struct Governor {
     flipped: Cell<bool>,
@@ -49,6 +51,9 @@ impl Governor {
     }
 }
 
+// Type with an `Ord` implementation that forms a total order at any moment
+// (assuming that `T` respects total order), but can suddenly be made to invert
+// that total order.
 #[derive(Debug)]
 pub struct Governed<'a, T>(pub T, pub &'a Governor);
 

library/alloc/src/collections/btree/node.rs

@@ -134,13 +134,13 @@ pub type Root<K, V> = NodeRef<marker::Owned, K, V, marker::LeafOrInternal>;
 
 impl<K, V> Root<K, V> {
     /// Returns a new owned tree, with its own root node that is initially empty.
-    pub fn new_leaf() -> Self {
-        NodeRef::new().forget_type()
+    pub fn new() -> Self {
+        NodeRef::new_leaf().forget_type()
     }
 }
 
 impl<K, V> NodeRef<marker::Owned, K, V, marker::Leaf> {
-    fn new() -> Self {
+    fn new_leaf() -> Self {
         Self::from_new_leaf(Box::new(unsafe { LeafNode::new() }))
     }
 

library/alloc/src/collections/btree/node/tests.rs

@@ -74,12 +74,12 @@ fn test_splitpoint() {
 
 #[test]
 fn test_partial_cmp_eq() {
-    let mut root1 = NodeRef::new();
+    let mut root1 = NodeRef::new_leaf();
     let mut leaf1 = root1.borrow_mut();
     leaf1.push(1, ());
     let mut root1 = root1.forget_type();
     root1.push_internal_level();
-    let root2 = Root::new_leaf();
+    let root2 = Root::new();
     root1.reborrow().assert_back_pointers();
     root2.reborrow().assert_back_pointers();
 

library/alloc/src/collections/btree/search.rs

@@ -50,7 +50,7 @@ where
 {
     match search_linear(&node, key) {
         (idx, true) => Found(unsafe { Handle::new_kv(node, idx) }),
-        (idx, false) => SearchResult::GoDown(unsafe { Handle::new_edge(node, idx) }),
+        (idx, false) => GoDown(unsafe { Handle::new_edge(node, idx) }),
     }
 }
 

library/alloc/src/collections/btree/set/tests.rs

@@ -1,9 +1,10 @@
 use super::super::DeterministicRng;
 use super::*;
 use crate::vec::Vec;
+use std::cmp::Ordering;
 use std::iter::FromIterator;
 use std::panic::{catch_unwind, AssertUnwindSafe};
-use std::sync::atomic::{AtomicU32, Ordering};
+use std::sync::atomic::{AtomicU32, Ordering::SeqCst};
 
 #[test]
 fn test_clone_eq() {
@@ -355,7 +356,7 @@ fn test_drain_filter_drop_panic_leak() {
     struct D(i32);
     impl Drop for D {
         fn drop(&mut self) {
-            if DROPS.fetch_add(1, Ordering::SeqCst) == 1 {
+            if DROPS.fetch_add(1, SeqCst) == 1 {
                 panic!("panic in `drop`");
             }
         }
@@ -368,14 +369,14 @@ fn test_drain_filter_drop_panic_leak() {
 
     catch_unwind(move || {
         drop(set.drain_filter(|d| {
-            PREDS.fetch_add(1u32 << d.0, Ordering::SeqCst);
+            PREDS.fetch_add(1u32 << d.0, SeqCst);
             true
         }))
     })
     .ok();
 
-    assert_eq!(PREDS.load(Ordering::SeqCst), 0x011);
-    assert_eq!(DROPS.load(Ordering::SeqCst), 3);
+    assert_eq!(PREDS.load(SeqCst), 0x011);
+    assert_eq!(DROPS.load(SeqCst), 3);
 }
 
 #[test]
@@ -387,7 +388,7 @@ fn test_drain_filter_pred_panic_leak() {
     struct D(i32);
     impl Drop for D {
         fn drop(&mut self) {
-            DROPS.fetch_add(1, Ordering::SeqCst);
+            DROPS.fetch_add(1, SeqCst);
         }
     }
 
@@ -398,7 +399,7 @@ fn test_drain_filter_pred_panic_leak() {
 
     catch_unwind(AssertUnwindSafe(|| {
         drop(set.drain_filter(|d| {
-            PREDS.fetch_add(1u32 << d.0, Ordering::SeqCst);
+            PREDS.fetch_add(1u32 << d.0, SeqCst);
             match d.0 {
                 0 => true,
                 _ => panic!(),
@@ -407,8 +408,8 @@ fn test_drain_filter_pred_panic_leak() {
     }))
     .ok();
 
-    assert_eq!(PREDS.load(Ordering::SeqCst), 0x011);
-    assert_eq!(DROPS.load(Ordering::SeqCst), 1);
+    assert_eq!(PREDS.load(SeqCst), 0x011);
+    assert_eq!(DROPS.load(SeqCst), 1);
     assert_eq!(set.len(), 2);
     assert_eq!(set.first().unwrap().0, 4);
     assert_eq!(set.last().unwrap().0, 8);
@@ -498,8 +499,6 @@ fn test_extend_ref() {
 
 #[test]
 fn test_recovery() {
-    use std::cmp::Ordering;
-
     #[derive(Debug)]
     struct Foo(&'static str, i32);
 

library/core/src/alloc/layout.rs

@@ -50,7 +50,7 @@ impl Layout {
     ///    must not overflow (i.e., the rounded value must be less than
     ///    or equal to `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if !align.is_power_of_two() {
@@ -96,15 +96,15 @@ impl Layout {
 
     /// The minimum size in bytes for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn size(&self) -> usize {
         self.size_
     }
 
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()

library/core/src/bool.rs

@@ -23,12 +23,10 @@ impl bool {
     /// # Examples
     ///
     /// ```
-    /// #![feature(bool_to_option)]
-    ///
     /// assert_eq!(false.then(|| 0), None);
     /// assert_eq!(true.then(|| 0), Some(0));
     /// ```
-    #[unstable(feature = "bool_to_option", issue = "64260")]
+    #[stable(feature = "lazy_bool_to_option", since = "1.50.0")]
     #[inline]
     pub fn then<T, F: FnOnce() -> T>(self, f: F) -> Option<T> {
         if self { Some(f()) } else { None }

library/core/src/cell.rs

@@ -1027,7 +1027,6 @@ impl<T: Default> RefCell<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(refcell_take)]
     /// use std::cell::RefCell;
     ///
     /// let c = RefCell::new(5);
@@ -1036,7 +1035,7 @@ impl<T: Default> RefCell<T> {
     /// assert_eq!(five, 5);
     /// assert_eq!(c.into_inner(), 0);
     /// ```
-    #[unstable(feature = "refcell_take", issue = "71395")]
+    #[stable(feature = "refcell_take", since = "1.50.0")]
     pub fn take(&self) -> T {
         self.replace(Default::default())
     }

library/core/src/iter/adapters/chain.rs

@@ -1,6 +1,5 @@
 use crate::iter::{DoubleEndedIterator, FusedIterator, Iterator, TrustedLen};
-use crate::ops::Try;
-use crate::usize;
+use crate::{ops::Try, usize};
 
 /// An iterator that links two iterators together, in a chain.
 ///

library/core/src/iter/adapters/cloned.rs

@@ -0,0 +1,139 @@
+use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
+use crate::iter::{FusedIterator, TrustedLen};
+use crate::ops::Try;
+
+/// An iterator that clones the elements of an underlying iterator.
+///
+/// This `struct` is created by the [`cloned`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`cloned`]: Iterator::cloned
+/// [`Iterator`]: trait.Iterator.html
+#[stable(feature = "iter_cloned", since = "1.1.0")]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[derive(Clone, Debug)]
+pub struct Cloned<I> {
+    it: I,
+}
+
+impl<I> Cloned<I> {
+    pub(in crate::iter) fn new(it: I) -> Cloned<I> {
+        Cloned { it }
+    }
+}
+
+fn clone_try_fold<T: Clone, Acc, R>(mut f: impl FnMut(Acc, T) -> R) -> impl FnMut(Acc, &T) -> R {
+    move |acc, elt| f(acc, elt.clone())
+}
+
+#[stable(feature = "iter_cloned", since = "1.1.0")]
+impl<'a, I, T: 'a> Iterator for Cloned<I>
+where
+    I: Iterator<Item = &'a T>,
+    T: Clone,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        self.it.next().cloned()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.it.size_hint()
+    }
+
+    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.it.try_fold(init, clone_try_fold(f))
+    }
+
+    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.it.map(T::clone).fold(init, f)
+    }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> T
+    where
+        Self: TrustedRandomAccess,
+    {
+        // SAFETY: the caller must uphold the contract for
+        // `Iterator::__iterator_get_unchecked`.
+        unsafe { try_get_unchecked(&mut self.it, idx).clone() }
+    }
+}
+
+#[stable(feature = "iter_cloned", since = "1.1.0")]
+impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I>
+where
+    I: DoubleEndedIterator<Item = &'a T>,
+    T: Clone,
+{
+    fn next_back(&mut self) -> Option<T> {
+        self.it.next_back().cloned()
+    }
+
+    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.it.try_rfold(init, clone_try_fold(f))
+    }
+
+    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.it.map(T::clone).rfold(init, f)
+    }
+}
+
+#[stable(feature = "iter_cloned", since = "1.1.0")]
+impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
+where
+    I: ExactSizeIterator<Item = &'a T>,
+    T: Clone,
+{
+    fn len(&self) -> usize {
+        self.it.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.it.is_empty()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<'a, I, T: 'a> FusedIterator for Cloned<I>
+where
+    I: FusedIterator<Item = &'a T>,
+    T: Clone,
+{
+}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I> TrustedRandomAccess for Cloned<I>
+where
+    I: TrustedRandomAccess,
+{
+    #[inline]
+    fn may_have_side_effect() -> bool {
+        true
+    }
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, I, T: 'a> TrustedLen for Cloned<I>
+where
+    I: TrustedLen<Item = &'a T>,
+    T: Clone,
+{
+}

library/core/src/iter/adapters/copied.rs

@@ -0,0 +1,155 @@
+use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
+use crate::iter::{FusedIterator, TrustedLen};
+use crate::ops::Try;
+
+/// An iterator that copies the elements of an underlying iterator.
+///
+/// This `struct` is created by the [`copied`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`copied`]: Iterator::copied
+/// [`Iterator`]: trait.Iterator.html
+#[stable(feature = "iter_copied", since = "1.36.0")]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[derive(Clone, Debug)]
+pub struct Copied<I> {
+    it: I,
+}
+
+impl<I> Copied<I> {
+    pub(in crate::iter) fn new(it: I) -> Copied<I> {
+        Copied { it }
+    }
+}
+
+fn copy_fold<T: Copy, Acc>(mut f: impl FnMut(Acc, T) -> Acc) -> impl FnMut(Acc, &T) -> Acc {
+    move |acc, &elt| f(acc, elt)
+}
+
+fn copy_try_fold<T: Copy, Acc, R>(mut f: impl FnMut(Acc, T) -> R) -> impl FnMut(Acc, &T) -> R {
+    move |acc, &elt| f(acc, elt)
+}
+
+#[stable(feature = "iter_copied", since = "1.36.0")]
+impl<'a, I, T: 'a> Iterator for Copied<I>
+where
+    I: Iterator<Item = &'a T>,
+    T: Copy,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        self.it.next().copied()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.it.size_hint()
+    }
+
+    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.it.try_fold(init, copy_try_fold(f))
+    }
+
+    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.it.fold(init, copy_fold(f))
+    }
+
+    fn nth(&mut self, n: usize) -> Option<T> {
+        self.it.nth(n).copied()
+    }
+
+    fn last(self) -> Option<T> {
+        self.it.last().copied()
+    }
+
+    fn count(self) -> usize {
+        self.it.count()
+    }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> T
+    where
+        Self: TrustedRandomAccess,
+    {
+        // SAFETY: the caller must uphold the contract for
+        // `Iterator::__iterator_get_unchecked`.
+        *unsafe { try_get_unchecked(&mut self.it, idx) }
+    }
+}
+
+#[stable(feature = "iter_copied", since = "1.36.0")]
+impl<'a, I, T: 'a> DoubleEndedIterator for Copied<I>
+where
+    I: DoubleEndedIterator<Item = &'a T>,
+    T: Copy,
+{
+    fn next_back(&mut self) -> Option<T> {
+        self.it.next_back().copied()
+    }
+
+    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.it.try_rfold(init, copy_try_fold(f))
+    }
+
+    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.it.rfold(init, copy_fold(f))
+    }
+}
+
+#[stable(feature = "iter_copied", since = "1.36.0")]
+impl<'a, I, T: 'a> ExactSizeIterator for Copied<I>
+where
+    I: ExactSizeIterator<Item = &'a T>,
+    T: Copy,
+{
+    fn len(&self) -> usize {
+        self.it.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.it.is_empty()
+    }
+}
+
+#[stable(feature = "iter_copied", since = "1.36.0")]
+impl<'a, I, T: 'a> FusedIterator for Copied<I>
+where
+    I: FusedIterator<Item = &'a T>,
+    T: Copy,
+{
+}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I> TrustedRandomAccess for Copied<I>
+where
+    I: TrustedRandomAccess,
+{
+    #[inline]
+    fn may_have_side_effect() -> bool {
+        I::may_have_side_effect()
+    }
+}
+
+#[stable(feature = "iter_copied", since = "1.36.0")]
+unsafe impl<'a, I, T: 'a> TrustedLen for Copied<I>
+where
+    I: TrustedLen<Item = &'a T>,
+    T: Copy,
+{
+}

library/core/src/iter/adapters/cycle.rs

@@ -0,0 +1,87 @@
+use crate::{iter::FusedIterator, ops::Try};
+
+/// An iterator that repeats endlessly.
+///
+/// This `struct` is created by the [`cycle`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`cycle`]: Iterator::cycle
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Cycle<I> {
+    orig: I,
+    iter: I,
+}
+
+impl<I: Clone> Cycle<I> {
+    pub(in crate::iter) fn new(iter: I) -> Cycle<I> {
+        Cycle { orig: iter.clone(), iter }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> Iterator for Cycle<I>
+where
+    I: Clone + Iterator,
+{
+    type Item = <I as Iterator>::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<<I as Iterator>::Item> {
+        match self.iter.next() {
+            None => {
+                self.iter = self.orig.clone();
+                self.iter.next()
+            }
+            y => y,
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // the cycle iterator is either empty or infinite
+        match self.orig.size_hint() {
+            sz @ (0, Some(0)) => sz,
+            (0, _) => (0, None),
+            _ => (usize::MAX, None),
+        }
+    }
+
+    #[inline]
+    fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
+    where
+        F: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        // fully iterate the current iterator. this is necessary because
+        // `self.iter` may be empty even when `self.orig` isn't
+        acc = self.iter.try_fold(acc, &mut f)?;
+        self.iter = self.orig.clone();
+
+        // complete a full cycle, keeping track of whether the cycled
+        // iterator is empty or not. we need to return early in case
+        // of an empty iterator to prevent an infinite loop
+        let mut is_empty = true;
+        acc = self.iter.try_fold(acc, |acc, x| {
+            is_empty = false;
+            f(acc, x)
+        })?;
+
+        if is_empty {
+            return try { acc };
+        }
+
+        loop {
+            self.iter = self.orig.clone();
+            acc = self.iter.try_fold(acc, &mut f)?;
+        }
+    }
+
+    // No `fold` override, because `fold` doesn't make much sense for `Cycle`,
+    // and we can't do anything better than the default.
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}

library/core/src/iter/adapters/enumerate.rs

@@ -0,0 +1,238 @@
+use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
+use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
+use crate::ops::{Add, AddAssign, Try};
+
+/// An iterator that yields the current count and the element during iteration.
+///
+/// This `struct` is created by the [`enumerate`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`enumerate`]: Iterator::enumerate
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Enumerate<I> {
+    iter: I,
+    count: usize,
+}
+impl<I> Enumerate<I> {
+    pub(in crate::iter) fn new(iter: I) -> Enumerate<I> {
+        Enumerate { iter, count: 0 }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> Iterator for Enumerate<I>
+where
+    I: Iterator,
+{
+    type Item = (usize, <I as Iterator>::Item);
+
+    /// # Overflow Behavior
+    ///
+    /// The method does no guarding against overflows, so enumerating more than
+    /// `usize::MAX` elements either produces the wrong result or panics. If
+    /// debug assertions are enabled, a panic is guaranteed.
+    ///
+    /// # Panics
+    ///
+    /// Might panic if the index of the element overflows a `usize`.
+    #[inline]
+    fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
+        let a = self.iter.next()?;
+        let i = self.count;
+        // Possible undefined overflow.
+        AddAssign::add_assign(&mut self.count, 1);
+        Some((i, a))
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
+        let a = self.iter.nth(n)?;
+        // Possible undefined overflow.
+        let i = Add::add(self.count, n);
+        self.count = Add::add(i, 1);
+        Some((i, a))
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+
+    #[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>,
+    {
+        #[inline]
+        fn enumerate<'a, T, Acc, R>(
+            count: &'a mut usize,
+            mut fold: impl FnMut(Acc, (usize, T)) -> R + 'a,
+        ) -> impl FnMut(Acc, T) -> R + 'a {
+            move |acc, item| {
+                let acc = fold(acc, (*count, item));
+                // Possible undefined overflow.
+                AddAssign::add_assign(count, 1);
+                acc
+            }
+        }
+
+        self.iter.try_fold(init, enumerate(&mut self.count, fold))
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn enumerate<T, Acc>(
+            mut count: usize,
+            mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc {
+            move |acc, item| {
+                let acc = fold(acc, (count, item));
+                // Possible undefined overflow.
+                AddAssign::add_assign(&mut count, 1);
+                acc
+            }
+        }
+
+        self.iter.fold(init, enumerate(self.count, fold))
+    }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
+    where
+        Self: TrustedRandomAccess,
+    {
+        // SAFETY: the caller must uphold the contract for
+        // `Iterator::__iterator_get_unchecked`.
+        let value = unsafe { try_get_unchecked(&mut self.iter, idx) };
+        (Add::add(self.count, idx), value)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> DoubleEndedIterator for Enumerate<I>
+where
+    I: ExactSizeIterator + DoubleEndedIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
+        let a = self.iter.next_back()?;
+        let len = self.iter.len();
+        // Can safely add, `ExactSizeIterator` promises that the number of
+        // elements fits into a `usize`.
+        Some((self.count + len, a))
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
+        let a = self.iter.nth_back(n)?;
+        let len = self.iter.len();
+        // Can safely add, `ExactSizeIterator` promises that the number of
+        // elements fits into a `usize`.
+        Some((self.count + len, a))
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        // Can safely add and subtract the count, as `ExactSizeIterator` promises
+        // that the number of elements fits into a `usize`.
+        fn enumerate<T, Acc, R>(
+            mut count: usize,
+            mut fold: impl FnMut(Acc, (usize, T)) -> R,
+        ) -> impl FnMut(Acc, T) -> R {
+            move |acc, item| {
+                count -= 1;
+                fold(acc, (count, item))
+            }
+        }
+
+        let count = self.count + self.iter.len();
+        self.iter.try_rfold(init, enumerate(count, fold))
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        // Can safely add and subtract the count, as `ExactSizeIterator` promises
+        // that the number of elements fits into a `usize`.
+        fn enumerate<T, Acc>(
+            mut count: usize,
+            mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc {
+            move |acc, item| {
+                count -= 1;
+                fold(acc, (count, item))
+            }
+        }
+
+        let count = self.count + self.iter.len();
+        self.iter.rfold(init, enumerate(count, fold))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> ExactSizeIterator for Enumerate<I>
+where
+    I: ExactSizeIterator,
+{
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I> TrustedRandomAccess for Enumerate<I>
+where
+    I: TrustedRandomAccess,
+{
+    fn may_have_side_effect() -> bool {
+        I::may_have_side_effect()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I> TrustedLen for Enumerate<I> where I: TrustedLen {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I: Iterator> SourceIter for Enumerate<I>
+where
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable> InPlaceIterable for Enumerate<I> {}

library/core/src/iter/adapters/filter.rs

@@ -0,0 +1,152 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::ops::Try;
+
+/// An iterator that filters the elements of `iter` with `predicate`.
+///
+/// This `struct` is created by the [`filter`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`filter`]: Iterator::filter
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct Filter<I, P> {
+    iter: I,
+    predicate: P,
+}
+impl<I, P> Filter<I, P> {
+    pub(in crate::iter) fn new(iter: I, predicate: P) -> Filter<I, P> {
+        Filter { iter, predicate }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, P> fmt::Debug for Filter<I, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Filter").field("iter", &self.iter).finish()
+    }
+}
+
+fn filter_fold<T, Acc>(
+    mut predicate: impl FnMut(&T) -> bool,
+    mut fold: impl FnMut(Acc, T) -> Acc,
+) -> impl FnMut(Acc, T) -> Acc {
+    move |acc, item| if predicate(&item) { fold(acc, item) } else { acc }
+}
+
+fn filter_try_fold<'a, T, Acc, R: Try<Ok = Acc>>(
+    predicate: &'a mut impl FnMut(&T) -> bool,
+    mut fold: impl FnMut(Acc, T) -> R + 'a,
+) -> impl FnMut(Acc, T) -> R + 'a {
+    move |acc, item| if predicate(&item) { fold(acc, item) } else { try { acc } }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator, P> Iterator for Filter<I, P>
+where
+    P: FnMut(&I::Item) -> bool,
+{
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<I::Item> {
+        self.iter.find(&mut self.predicate)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+
+    // this special case allows the compiler to make `.filter(_).count()`
+    // branchless. Barring perfect branch prediction (which is unattainable in
+    // the general case), this will be much faster in >90% of cases (containing
+    // virtually all real workloads) and only a tiny bit slower in the rest.
+    //
+    // Having this specialization thus allows us to write `.filter(p).count()`
+    // where we would otherwise write `.map(|x| p(x) as usize).sum()`, which is
+    // less readable and also less backwards-compatible to Rust before 1.10.
+    //
+    // Using the branchless version will also simplify the LLVM byte code, thus
+    // leaving more budget for LLVM optimizations.
+    #[inline]
+    fn count(self) -> usize {
+        #[inline]
+        fn to_usize<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut(T) -> usize {
+            move |x| predicate(&x) as usize
+        }
+
+        self.iter.map(to_usize(self.predicate)).sum()
+    }
+
+    #[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.iter.try_fold(init, filter_try_fold(&mut self.predicate, fold))
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.fold(init, filter_fold(self.predicate, fold))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
+where
+    P: FnMut(&I::Item) -> bool,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<I::Item> {
+        self.iter.rfind(&mut self.predicate)
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        self.iter.try_rfold(init, filter_try_fold(&mut self.predicate, fold))
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.rfold(init, filter_fold(self.predicate, fold))
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I: FusedIterator, P> FusedIterator for Filter<I, P> where P: FnMut(&I::Item) -> bool {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, P, I: Iterator> SourceIter for Filter<I, P>
+where
+    P: FnMut(&I::Item) -> bool,
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable, P> InPlaceIterable for Filter<I, P> where P: FnMut(&I::Item) -> bool {}

library/core/src/iter/adapters/filter_map.rs

@@ -0,0 +1,150 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::ops::{ControlFlow, Try};
+
+/// An iterator that uses `f` to both filter and map elements from `iter`.
+///
+/// This `struct` is created by the [`filter_map`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`filter_map`]: Iterator::filter_map
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct FilterMap<I, F> {
+    iter: I,
+    f: F,
+}
+impl<I, F> FilterMap<I, F> {
+    pub(in crate::iter) fn new(iter: I, f: F) -> FilterMap<I, F> {
+        FilterMap { iter, f }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, F> fmt::Debug for FilterMap<I, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FilterMap").field("iter", &self.iter).finish()
+    }
+}
+
+fn filter_map_fold<T, B, Acc>(
+    mut f: impl FnMut(T) -> Option<B>,
+    mut fold: impl FnMut(Acc, B) -> Acc,
+) -> impl FnMut(Acc, T) -> Acc {
+    move |acc, item| match f(item) {
+        Some(x) => fold(acc, x),
+        None => acc,
+    }
+}
+
+fn filter_map_try_fold<'a, T, B, Acc, R: Try<Ok = Acc>>(
+    f: &'a mut impl FnMut(T) -> Option<B>,
+    mut fold: impl FnMut(Acc, B) -> R + 'a,
+) -> impl FnMut(Acc, T) -> R + 'a {
+    move |acc, item| match f(item) {
+        Some(x) => fold(acc, x),
+        None => try { acc },
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
+where
+    F: FnMut(I::Item) -> Option<B>,
+{
+    type Item = B;
+
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        self.iter.find_map(&mut self.f)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+
+    #[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.iter.try_fold(init, filter_map_try_fold(&mut self.f, fold))
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.fold(init, filter_map_fold(self.f, fold))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
+where
+    F: FnMut(I::Item) -> Option<B>,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<B> {
+        #[inline]
+        fn find<T, B>(
+            f: &mut impl FnMut(T) -> Option<B>,
+        ) -> impl FnMut((), T) -> ControlFlow<B> + '_ {
+            move |(), x| match f(x) {
+                Some(x) => ControlFlow::Break(x),
+                None => ControlFlow::CONTINUE,
+            }
+        }
+
+        self.iter.try_rfold((), find(&mut self.f)).break_value()
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        self.iter.try_rfold(init, filter_map_try_fold(&mut self.f, fold))
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.rfold(init, filter_map_fold(self.f, fold))
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<B, I: FusedIterator, F> FusedIterator for FilterMap<I, F> where F: FnMut(I::Item) -> Option<B> {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, B, I: Iterator, F> SourceIter for FilterMap<I, F>
+where
+    F: FnMut(I::Item) -> Option<B>,
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<B, I: InPlaceIterable, F> InPlaceIterable for FilterMap<I, F> where
+    F: FnMut(I::Item) -> Option<B>
+{
+}

library/core/src/iter/adapters/flatten.rs

@@ -1,9 +1,7 @@
 use crate::fmt;
+use crate::iter::{DoubleEndedIterator, Fuse, FusedIterator, Iterator, Map};
 use crate::ops::Try;
 
-use super::super::{DoubleEndedIterator, Fuse, FusedIterator, Iterator};
-use super::Map;
-
 /// An iterator that maps each element to an iterator, and yields the elements
 /// of the produced iterators.
 ///
@@ -14,8 +12,9 @@ use super::Map;
 pub struct FlatMap<I, U: IntoIterator, F> {
     inner: FlattenCompat<Map<I, F>, <U as IntoIterator>::IntoIter>,
 }
+
 impl<I: Iterator, U: IntoIterator, F: FnMut(I::Item) -> U> FlatMap<I, U, F> {
-    pub(in super::super) fn new(iter: I, f: F) -> FlatMap<I, U, F> {
+    pub(in crate::iter) fn new(iter: I, f: F) -> FlatMap<I, U, F> {
         FlatMap { inner: FlattenCompat::new(iter.map(f)) }
     }
 }

library/core/src/iter/adapters/fuse.rs

@@ -1,9 +1,6 @@
-use super::InPlaceIterable;
 use crate::intrinsics;
-use crate::iter::adapters::zip::try_get_unchecked;
-use crate::iter::adapters::SourceIter;
-use crate::iter::TrustedRandomAccess;
-use crate::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator};
+use crate::iter::adapters::{zip::try_get_unchecked, InPlaceIterable, SourceIter};
+use crate::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedRandomAccess};
 use crate::ops::Try;
 
 /// An iterator that yields `None` forever after the underlying iterator

library/core/src/iter/adapters/inspect.rs

@@ -0,0 +1,167 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::ops::Try;
+
+/// An iterator that calls a function with a reference to each element before
+/// yielding it.
+///
+/// This `struct` is created by the [`inspect`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`inspect`]: Iterator::inspect
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct Inspect<I, F> {
+    iter: I,
+    f: F,
+}
+impl<I, F> Inspect<I, F> {
+    pub(in crate::iter) fn new(iter: I, f: F) -> Inspect<I, F> {
+        Inspect { iter, f }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, F> fmt::Debug for Inspect<I, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Inspect").field("iter", &self.iter).finish()
+    }
+}
+
+impl<I: Iterator, F> Inspect<I, F>
+where
+    F: FnMut(&I::Item),
+{
+    #[inline]
+    fn do_inspect(&mut self, elt: Option<I::Item>) -> Option<I::Item> {
+        if let Some(ref a) = elt {
+            (self.f)(a);
+        }
+
+        elt
+    }
+}
+
+fn inspect_fold<T, Acc>(
+    mut f: impl FnMut(&T),
+    mut fold: impl FnMut(Acc, T) -> Acc,
+) -> impl FnMut(Acc, T) -> Acc {
+    move |acc, item| {
+        f(&item);
+        fold(acc, item)
+    }
+}
+
+fn inspect_try_fold<'a, T, Acc, R>(
+    f: &'a mut impl FnMut(&T),
+    mut fold: impl FnMut(Acc, T) -> R + 'a,
+) -> impl FnMut(Acc, T) -> R + 'a {
+    move |acc, item| {
+        f(&item);
+        fold(acc, item)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator, F> Iterator for Inspect<I, F>
+where
+    F: FnMut(&I::Item),
+{
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<I::Item> {
+        let next = self.iter.next();
+        self.do_inspect(next)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.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.iter.try_fold(init, inspect_try_fold(&mut self.f, fold))
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.fold(init, inspect_fold(self.f, fold))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
+where
+    F: FnMut(&I::Item),
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<I::Item> {
+        let next = self.iter.next_back();
+        self.do_inspect(next)
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        self.iter.try_rfold(init, inspect_try_fold(&mut self.f, fold))
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.rfold(init, inspect_fold(self.f, fold))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F>
+where
+    F: FnMut(&I::Item),
+{
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I: FusedIterator, F> FusedIterator for Inspect<I, F> where F: FnMut(&I::Item) {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I: Iterator, F> SourceIter for Inspect<I, F>
+where
+    F: FnMut(&I::Item),
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable, F> InPlaceIterable for Inspect<I, F> where F: FnMut(&I::Item) {}

library/core/src/iter/adapters/map.rs

@@ -0,0 +1,213 @@
+use crate::fmt;
+use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
+use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
+use crate::ops::Try;
+
+/// An iterator that maps the values of `iter` with `f`.
+///
+/// This `struct` is created by the [`map`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`map`]: Iterator::map
+/// [`Iterator`]: trait.Iterator.html
+///
+/// # Notes about side effects
+///
+/// The [`map`] iterator implements [`DoubleEndedIterator`], meaning that
+/// you can also [`map`] backwards:
+///
+/// ```rust
+/// let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
+///
+/// assert_eq!(v, [4, 3, 2]);
+/// ```
+///
+/// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
+///
+/// But if your closure has state, iterating backwards may act in a way you do
+/// not expect. Let's go through an example. First, in the forward direction:
+///
+/// ```rust
+/// let mut c = 0;
+///
+/// for pair in vec!['a', 'b', 'c'].into_iter()
+///                                .map(|letter| { c += 1; (letter, c) }) {
+///     println!("{:?}", pair);
+/// }
+/// ```
+///
+/// This will print "('a', 1), ('b', 2), ('c', 3)".
+///
+/// Now consider this twist where we add a call to `rev`. This version will
+/// print `('c', 1), ('b', 2), ('a', 3)`. Note that the letters are reversed,
+/// but the values of the counter still go in order. This is because `map()` is
+/// still being called lazily on each item, but we are popping items off the
+/// back of the vector now, instead of shifting them from the front.
+///
+/// ```rust
+/// let mut c = 0;
+///
+/// for pair in vec!['a', 'b', 'c'].into_iter()
+///                                .map(|letter| { c += 1; (letter, c) })
+///                                .rev() {
+///     println!("{:?}", pair);
+/// }
+/// ```
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct Map<I, F> {
+    iter: I,
+    f: F,
+}
+impl<I, F> Map<I, F> {
+    pub(in crate::iter) fn new(iter: I, f: F) -> Map<I, F> {
+        Map { iter, f }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Map").field("iter", &self.iter).finish()
+    }
+}
+
+fn map_fold<T, B, Acc>(
+    mut f: impl FnMut(T) -> B,
+    mut g: impl FnMut(Acc, B) -> Acc,
+) -> impl FnMut(Acc, T) -> Acc {
+    move |acc, elt| g(acc, f(elt))
+}
+
+fn map_try_fold<'a, T, B, Acc, R>(
+    f: &'a mut impl FnMut(T) -> B,
+    mut g: impl FnMut(Acc, B) -> R + 'a,
+) -> impl FnMut(Acc, T) -> R + 'a {
+    move |acc, elt| g(acc, f(elt))
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I: Iterator, F> Iterator for Map<I, F>
+where
+    F: FnMut(I::Item) -> B,
+{
+    type Item = B;
+
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        self.iter.next().map(&mut self.f)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
+    where
+        Self: Sized,
+        G: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        self.iter.try_fold(init, map_try_fold(&mut self.f, g))
+    }
+
+    fn fold<Acc, G>(self, init: Acc, g: G) -> Acc
+    where
+        G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.fold(init, map_fold(self.f, g))
+    }
+
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
+    where
+        Self: TrustedRandomAccess,
+    {
+        // SAFETY: the caller must uphold the contract for
+        // `Iterator::__iterator_get_unchecked`.
+        unsafe { (self.f)(try_get_unchecked(&mut self.iter, idx)) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F>
+where
+    F: FnMut(I::Item) -> B,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<B> {
+        self.iter.next_back().map(&mut self.f)
+    }
+
+    fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
+    where
+        Self: Sized,
+        G: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        self.iter.try_rfold(init, map_try_fold(&mut self.f, g))
+    }
+
+    fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc
+    where
+        G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.rfold(init, map_fold(self.f, g))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
+where
+    F: FnMut(I::Item) -> B,
+{
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<B, I: FusedIterator, F> FusedIterator for Map<I, F> where F: FnMut(I::Item) -> B {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<B, I, F> TrustedLen for Map<I, F>
+where
+    I: TrustedLen,
+    F: FnMut(I::Item) -> B,
+{
+}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I, F> TrustedRandomAccess for Map<I, F>
+where
+    I: TrustedRandomAccess,
+{
+    #[inline]
+    fn may_have_side_effect() -> bool {
+        true
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, B, I: Iterator, F> SourceIter for Map<I, F>
+where
+    F: FnMut(I::Item) -> B,
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<B, I: InPlaceIterable, F> InPlaceIterable for Map<I, F> where F: FnMut(I::Item) -> B {}

library/core/src/iter/adapters/map_while.rs

@@ -0,0 +1,101 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, InPlaceIterable};
+use crate::ops::{ControlFlow, Try};
+
+/// An iterator that only accepts elements while `predicate` returns `Some(_)`.
+///
+/// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`map_while`]: Iterator::map_while
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
+#[derive(Clone)]
+pub struct MapWhile<I, P> {
+    iter: I,
+    predicate: P,
+}
+
+impl<I, P> MapWhile<I, P> {
+    pub(in crate::iter) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
+        MapWhile { iter, predicate }
+    }
+}
+
+#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
+impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MapWhile").field("iter", &self.iter).finish()
+    }
+}
+
+#[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
+impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
+where
+    P: FnMut(I::Item) -> Option<B>,
+{
+    type Item = B;
+
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        let x = self.iter.next()?;
+        (self.predicate)(x)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+
+    #[inline]
+    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        let Self { iter, predicate } = self;
+        iter.try_fold(init, |acc, x| match predicate(x) {
+            Some(item) => ControlFlow::from_try(fold(acc, item)),
+            None => ControlFlow::Break(try { acc }),
+        })
+        .into_try()
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
+            move |acc, x| Ok(f(acc, x))
+        }
+
+        self.try_fold(init, ok(fold)).unwrap()
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, B, I: Iterator, P> SourceIter for MapWhile<I, P>
+where
+    P: FnMut(I::Item) -> Option<B>,
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<B, I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> where
+    P: FnMut(I::Item) -> Option<B>
+{
+}

library/core/src/iter/adapters/peekable.rs

@@ -0,0 +1,301 @@
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
+use crate::ops::Try;
+
+/// An iterator with a `peek()` that returns an optional reference to the next
+/// element.
+///
+/// This `struct` is created by the [`peekable`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`peekable`]: Iterator::peekable
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Peekable<I: Iterator> {
+    iter: I,
+    /// Remember a peeked value, even if it was None.
+    peeked: Option<Option<I::Item>>,
+}
+
+impl<I: Iterator> Peekable<I> {
+    pub(in crate::iter) fn new(iter: I) -> Peekable<I> {
+        Peekable { iter, peeked: None }
+    }
+}
+
+// Peekable must remember if a None has been seen in the `.peek()` method.
+// It ensures that `.peek(); .peek();` or `.peek(); .next();` only advances the
+// underlying iterator at most once. This does not by itself make the iterator
+// fused.
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator> Iterator for Peekable<I> {
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<I::Item> {
+        match self.peeked.take() {
+            Some(v) => v,
+            None => self.iter.next(),
+        }
+    }
+
+    #[inline]
+    #[rustc_inherit_overflow_checks]
+    fn count(mut self) -> usize {
+        match self.peeked.take() {
+            Some(None) => 0,
+            Some(Some(_)) => 1 + self.iter.count(),
+            None => self.iter.count(),
+        }
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<I::Item> {
+        match self.peeked.take() {
+            Some(None) => None,
+            Some(v @ Some(_)) if n == 0 => v,
+            Some(Some(_)) => self.iter.nth(n - 1),
+            None => self.iter.nth(n),
+        }
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<I::Item> {
+        let peek_opt = match self.peeked.take() {
+            Some(None) => return None,
+            Some(v) => v,
+            None => None,
+        };
+        self.iter.last().or(peek_opt)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let peek_len = match self.peeked {
+            Some(None) => return (0, Some(0)),
+            Some(Some(_)) => 1,
+            None => 0,
+        };
+        let (lo, hi) = self.iter.size_hint();
+        let lo = lo.saturating_add(peek_len);
+        let hi = match hi {
+            Some(x) => x.checked_add(peek_len),
+            None => None,
+        };
+        (lo, hi)
+    }
+
+    #[inline]
+    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        let acc = match self.peeked.take() {
+            Some(None) => return try { init },
+            Some(Some(v)) => f(init, v)?,
+            None => init,
+        };
+        self.iter.try_fold(acc, f)
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let acc = match self.peeked {
+            Some(None) => return init,
+            Some(Some(v)) => fold(init, v),
+            None => init,
+        };
+        self.iter.fold(acc, fold)
+    }
+}
+
+#[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for Peekable<I>
+where
+    I: DoubleEndedIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match self.peeked.as_mut() {
+            Some(v @ Some(_)) => self.iter.next_back().or_else(|| v.take()),
+            Some(None) => None,
+            None => self.iter.next_back(),
+        }
+    }
+
+    #[inline]
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        match self.peeked.take() {
+            Some(None) => try { init },
+            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
+                Ok(acc) => f(acc, v),
+                Err(e) => {
+                    self.peeked = Some(Some(v));
+                    Try::from_error(e)
+                }
+            },
+            None => self.iter.try_rfold(init, f),
+        }
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        match self.peeked {
+            Some(None) => init,
+            Some(Some(v)) => {
+                let acc = self.iter.rfold(init, &mut fold);
+                fold(acc, v)
+            }
+            None => self.iter.rfold(init, fold),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I: FusedIterator> FusedIterator for Peekable<I> {}
+
+impl<I: Iterator> Peekable<I> {
+    /// Returns a reference to the next() value without advancing the iterator.
+    ///
+    /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
+    /// But if the iteration is over, `None` is returned.
+    ///
+    /// [`next`]: Iterator::next
+    ///
+    /// Because `peek()` returns a reference, and many iterators iterate over
+    /// references, there can be a possibly confusing situation where the
+    /// return value is a double reference. You can see this effect in the
+    /// examples below.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let xs = [1, 2, 3];
+    ///
+    /// let mut iter = xs.iter().peekable();
+    ///
+    /// // peek() lets us see into the future
+    /// assert_eq!(iter.peek(), Some(&&1));
+    /// assert_eq!(iter.next(), Some(&1));
+    ///
+    /// assert_eq!(iter.next(), Some(&2));
+    ///
+    /// // The iterator does not advance even if we `peek` multiple times
+    /// assert_eq!(iter.peek(), Some(&&3));
+    /// assert_eq!(iter.peek(), Some(&&3));
+    ///
+    /// assert_eq!(iter.next(), Some(&3));
+    ///
+    /// // After the iterator is finished, so is `peek()`
+    /// assert_eq!(iter.peek(), None);
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn peek(&mut self) -> Option<&I::Item> {
+        let iter = &mut self.iter;
+        self.peeked.get_or_insert_with(|| iter.next()).as_ref()
+    }
+
+    /// Consume and return the next value of this iterator if a condition is true.
+    ///
+    /// If `func` returns `true` for the next value of this iterator, consume and return it.
+    /// Otherwise, return `None`.
+    ///
+    /// # Examples
+    /// Consume a number if it's equal to 0.
+    /// ```
+    /// #![feature(peekable_next_if)]
+    /// let mut iter = (0..5).peekable();
+    /// // The first item of the iterator is 0; consume it.
+    /// assert_eq!(iter.next_if(|&x| x == 0), Some(0));
+    /// // The next item returned is now 1, so `consume` will return `false`.
+    /// assert_eq!(iter.next_if(|&x| x == 0), None);
+    /// // `next_if` saves the value of the next item if it was not equal to `expected`.
+    /// assert_eq!(iter.next(), Some(1));
+    /// ```
+    ///
+    /// Consume any number less than 10.
+    /// ```
+    /// #![feature(peekable_next_if)]
+    /// let mut iter = (1..20).peekable();
+    /// // Consume all numbers less than 10
+    /// while iter.next_if(|&x| x < 10).is_some() {}
+    /// // The next value returned will be 10
+    /// assert_eq!(iter.next(), Some(10));
+    /// ```
+    #[unstable(feature = "peekable_next_if", issue = "72480")]
+    pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
+        match self.next() {
+            Some(matched) if func(&matched) => Some(matched),
+            other => {
+                // Since we called `self.next()`, we consumed `self.peeked`.
+                assert!(self.peeked.is_none());
+                self.peeked = Some(other);
+                None
+            }
+        }
+    }
+
+    /// Consume and return the next item if it is equal to `expected`.
+    ///
+    /// # Example
+    /// Consume a number if it's equal to 0.
+    /// ```
+    /// #![feature(peekable_next_if)]
+    /// let mut iter = (0..5).peekable();
+    /// // The first item of the iterator is 0; consume it.
+    /// assert_eq!(iter.next_if_eq(&0), Some(0));
+    /// // The next item returned is now 1, so `consume` will return `false`.
+    /// assert_eq!(iter.next_if_eq(&0), None);
+    /// // `next_if_eq` saves the value of the next item if it was not equal to `expected`.
+    /// assert_eq!(iter.next(), Some(1));
+    /// ```
+    #[unstable(feature = "peekable_next_if", issue = "72480")]
+    pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>
+    where
+        T: ?Sized,
+        I::Item: PartialEq<T>,
+    {
+        self.next_if(|next| next == expected)
+    }
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I> TrustedLen for Peekable<I> where I: TrustedLen {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I: Iterator> SourceIter for Peekable<I>
+where
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable> InPlaceIterable for Peekable<I> {}

library/core/src/iter/adapters/rev.rs

@@ -0,0 +1,137 @@
+use crate::iter::{FusedIterator, TrustedLen};
+use crate::ops::Try;
+
+/// A double-ended iterator with the direction inverted.
+///
+/// This `struct` is created by the [`rev`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`rev`]: Iterator::rev
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Rev<T> {
+    iter: T,
+}
+
+impl<T> Rev<T> {
+    pub(in crate::iter) fn new(iter: T) -> Rev<T> {
+        Rev { iter }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> Iterator for Rev<I>
+where
+    I: DoubleEndedIterator,
+{
+    type Item = <I as Iterator>::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<<I as Iterator>::Item> {
+        self.iter.next_back()
+    }
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        self.iter.advance_back_by(n)
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        self.iter.nth_back(n)
+    }
+
+    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.iter.try_rfold(init, f)
+    }
+
+    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.rfold(init, f)
+    }
+
+    #[inline]
+    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
+    where
+        P: FnMut(&Self::Item) -> bool,
+    {
+        self.iter.rfind(predicate)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> DoubleEndedIterator for Rev<I>
+where
+    I: DoubleEndedIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
+        self.iter.next()
+    }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.iter.advance_by(n)
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        self.iter.nth(n)
+    }
+
+    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Ok = B>,
+    {
+        self.iter.try_fold(init, f)
+    }
+
+    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.iter.fold(init, f)
+    }
+
+    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
+    where
+        P: FnMut(&Self::Item) -> bool,
+    {
+        self.iter.find(predicate)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> ExactSizeIterator for Rev<I>
+where
+    I: ExactSizeIterator + DoubleEndedIterator,
+{
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I> FusedIterator for Rev<I> where I: FusedIterator + DoubleEndedIterator {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I> TrustedLen for Rev<I> where I: TrustedLen + DoubleEndedIterator {}

library/core/src/iter/adapters/scan.rs

@@ -0,0 +1,111 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, InPlaceIterable};
+use crate::ops::{ControlFlow, Try};
+
+/// An iterator to maintain state while iterating another iterator.
+///
+/// This `struct` is created by the [`scan`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`scan`]: Iterator::scan
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct Scan<I, St, F> {
+    iter: I,
+    f: F,
+    state: St,
+}
+
+impl<I, St, F> Scan<I, St, F> {
+    pub(in crate::iter) fn new(iter: I, state: St, f: F) -> Scan<I, St, F> {
+        Scan { iter, state, f }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, St: fmt::Debug, F> fmt::Debug for Scan<I, St, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Scan").field("iter", &self.iter).field("state", &self.state).finish()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B, I, St, F> Iterator for Scan<I, St, F>
+where
+    I: Iterator,
+    F: FnMut(&mut St, I::Item) -> Option<B>,
+{
+    type Item = B;
+
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        let a = self.iter.next()?;
+        (self.f)(&mut self.state, a)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the scan function
+    }
+
+    #[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>,
+    {
+        fn scan<'a, T, St, B, Acc, R: Try<Ok = Acc>>(
+            state: &'a mut St,
+            f: &'a mut impl FnMut(&mut St, T) -> Option<B>,
+            mut fold: impl FnMut(Acc, B) -> R + 'a,
+        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
+            move |acc, x| match f(state, x) {
+                None => ControlFlow::Break(try { acc }),
+                Some(x) => ControlFlow::from_try(fold(acc, x)),
+            }
+        }
+
+        let state = &mut self.state;
+        let f = &mut self.f;
+        self.iter.try_fold(init, scan(state, f, fold)).into_try()
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
+            move |acc, x| Ok(f(acc, x))
+        }
+
+        self.try_fold(init, ok(fold)).unwrap()
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<St, F, B, S: Iterator, I: Iterator> SourceIter for Scan<I, St, F>
+where
+    I: SourceIter<Source = S>,
+    F: FnMut(&mut St, I::Item) -> Option<B>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<St, F, B, I: InPlaceIterable> InPlaceIterable for Scan<I, St, F> where
+    F: FnMut(&mut St, I::Item) -> Option<B>
+{
+}

library/core/src/iter/adapters/skip.rs

@@ -0,0 +1,199 @@
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::ops::{ControlFlow, Try};
+
+/// An iterator that skips over `n` elements of `iter`.
+///
+/// This `struct` is created by the [`skip`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`skip`]: Iterator::skip
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Skip<I> {
+    iter: I,
+    n: usize,
+}
+
+impl<I> Skip<I> {
+    pub(in crate::iter) fn new(iter: I, n: usize) -> Skip<I> {
+        Skip { iter, n }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> Iterator for Skip<I>
+where
+    I: Iterator,
+{
+    type Item = <I as Iterator>::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<I::Item> {
+        if self.n == 0 {
+            self.iter.next()
+        } else {
+            let old_n = self.n;
+            self.n = 0;
+            self.iter.nth(old_n)
+        }
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<I::Item> {
+        // Can't just add n + self.n due to overflow.
+        if self.n > 0 {
+            let to_skip = self.n;
+            self.n = 0;
+            // nth(n) skips n+1
+            self.iter.nth(to_skip - 1)?;
+        }
+        self.iter.nth(n)
+    }
+
+    #[inline]
+    fn count(mut self) -> usize {
+        if self.n > 0 {
+            // nth(n) skips n+1
+            if self.iter.nth(self.n - 1).is_none() {
+                return 0;
+            }
+        }
+        self.iter.count()
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<I::Item> {
+        if self.n > 0 {
+            // nth(n) skips n+1
+            self.iter.nth(self.n - 1)?;
+        }
+        self.iter.last()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.iter.size_hint();
+
+        let lower = lower.saturating_sub(self.n);
+        let upper = match upper {
+            Some(x) => Some(x.saturating_sub(self.n)),
+            None => None,
+        };
+
+        (lower, upper)
+    }
+
+    #[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>,
+    {
+        let n = self.n;
+        self.n = 0;
+        if n > 0 {
+            // nth(n) skips n+1
+            if self.iter.nth(n - 1).is_none() {
+                return try { init };
+            }
+        }
+        self.iter.try_fold(init, fold)
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        if self.n > 0 {
+            // nth(n) skips n+1
+            if self.iter.nth(self.n - 1).is_none() {
+                return init;
+            }
+        }
+        self.iter.fold(init, fold)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}
+
+#[stable(feature = "double_ended_skip_iterator", since = "1.9.0")]
+impl<I> DoubleEndedIterator for Skip<I>
+where
+    I: DoubleEndedIterator + ExactSizeIterator,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.len() > 0 { self.iter.next_back() } else { None }
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
+        let len = self.len();
+        if n < len {
+            self.iter.nth_back(n)
+        } else {
+            if len > 0 {
+                // consume the original iterator
+                self.iter.nth_back(len - 1);
+            }
+            None
+        }
+    }
+
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        fn check<T, Acc, R: Try<Ok = Acc>>(
+            mut n: usize,
+            mut fold: impl FnMut(Acc, T) -> R,
+        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> {
+            move |acc, x| {
+                n -= 1;
+                let r = fold(acc, x);
+                if n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
+            }
+        }
+
+        let n = self.len();
+        if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() }
+    }
+
+    fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn ok<Acc, T>(mut f: impl FnMut(Acc, T) -> Acc) -> impl FnMut(Acc, T) -> Result<Acc, !> {
+            move |acc, x| Ok(f(acc, x))
+        }
+
+        self.try_rfold(init, ok(fold)).unwrap()
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I> FusedIterator for Skip<I> where I: FusedIterator {}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I: Iterator> SourceIter for Skip<I>
+where
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable> InPlaceIterable for Skip<I> {}

library/core/src/iter/adapters/skip_while.rs

@@ -0,0 +1,126 @@
+use crate::fmt;
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::ops::Try;
+
+/// An iterator that rejects elements while `predicate` returns `true`.
+///
+/// This `struct` is created by the [`skip_while`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`skip_while`]: Iterator::skip_while
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct SkipWhile<I, P> {
+    iter: I,
+    flag: bool,
+    predicate: P,
+}
+
+impl<I, P> SkipWhile<I, P> {
+    pub(in crate::iter) fn new(iter: I, predicate: P) -> SkipWhile<I, P> {
+        SkipWhile { iter, flag: false, predicate }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, P> fmt::Debug for SkipWhile<I, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("SkipWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator, P> Iterator for SkipWhile<I, P>
+where
+    P: FnMut(&I::Item) -> bool,
+{
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<I::Item> {
+        fn check<'a, T>(
+            flag: &'a mut bool,
+            pred: &'a mut impl FnMut(&T) -> bool,
+        ) -> impl FnMut(&T) -> bool + 'a {
+            move |x| {
+                if *flag || !pred(x) {
+                    *flag = true;
+                    true
+                } else {
+                    false
+                }
+            }
+        }
+
+        let flag = &mut self.flag;
+        let pred = &mut self.predicate;
+        self.iter.find(check(flag, pred))
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+
+    #[inline]
+    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        if !self.flag {
+            match self.next() {
+                Some(v) => init = fold(init, v)?,
+                None => return try { init },
+            }
+        }
+        self.iter.try_fold(init, fold)
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(mut self, mut init: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        if !self.flag {
+            match self.next() {
+                Some(v) => init = fold(init, v),
+                None => return init,
+            }
+        }
+        self.iter.fold(init, fold)
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I, P> FusedIterator for SkipWhile<I, P>
+where
+    I: FusedIterator,
+    P: FnMut(&I::Item) -> bool,
+{
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, P, I: Iterator> SourceIter for SkipWhile<I, P>
+where
+    P: FnMut(&I::Item) -> bool,
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable, F> InPlaceIterable for SkipWhile<I, F> where
+    F: FnMut(&I::Item) -> bool
+{
+}

library/core/src/iter/adapters/step_by.rs

@@ -0,0 +1,235 @@
+use crate::{intrinsics, iter::from_fn, ops::Try};
+
+/// An iterator for stepping iterators by a custom amount.
+///
+/// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
+/// its documentation for more.
+///
+/// [`step_by`]: Iterator::step_by
+/// [`Iterator`]: trait.Iterator.html
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "iterator_step_by", since = "1.28.0")]
+#[derive(Clone, Debug)]
+pub struct StepBy<I> {
+    iter: I,
+    step: usize,
+    first_take: bool,
+}
+
+impl<I> StepBy<I> {
+    pub(in crate::iter) fn new(iter: I, step: usize) -> StepBy<I> {
+        assert!(step != 0);
+        StepBy { iter, step: step - 1, first_take: true }
+    }
+}
+
+#[stable(feature = "iterator_step_by", since = "1.28.0")]
+impl<I> Iterator for StepBy<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.first_take {
+            self.first_take = false;
+            self.iter.next()
+        } else {
+            self.iter.nth(self.step)
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        #[inline]
+        fn first_size(step: usize) -> impl Fn(usize) -> usize {
+            move |n| if n == 0 { 0 } else { 1 + (n - 1) / (step + 1) }
+        }
+
+        #[inline]
+        fn other_size(step: usize) -> impl Fn(usize) -> usize {
+            move |n| n / (step + 1)
+        }
+
+        let (low, high) = self.iter.size_hint();
+
+        if self.first_take {
+            let f = first_size(self.step);
+            (f(low), high.map(f))
+        } else {
+            let f = other_size(self.step);
+            (f(low), high.map(f))
+        }
+    }
+
+    #[inline]
+    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
+        if self.first_take {
+            self.first_take = false;
+            let first = self.iter.next();
+            if n == 0 {
+                return first;
+            }
+            n -= 1;
+        }
+        // n and self.step are indices, we need to add 1 to get the amount of elements
+        // When calling `.nth`, we need to subtract 1 again to convert back to an index
+        // step + 1 can't overflow because `.step_by` sets `self.step` to `step - 1`
+        let mut step = self.step + 1;
+        // n + 1 could overflow
+        // thus, if n is usize::MAX, instead of adding one, we call .nth(step)
+        if n == usize::MAX {
+            self.iter.nth(step - 1);
+        } else {
+            n += 1;
+        }
+
+        // overflow handling
+        loop {
+            let mul = n.checked_mul(step);
+            {
+                if intrinsics::likely(mul.is_some()) {
+                    return self.iter.nth(mul.unwrap() - 1);
+                }
+            }
+            let div_n = usize::MAX / n;
+            let div_step = usize::MAX / step;
+            let nth_n = div_n * n;
+            let nth_step = div_step * step;
+            let nth = if nth_n > nth_step {
+                step -= div_n;
+                nth_n
+            } else {
+                n -= div_step;
+                nth_step
+            };
+            self.iter.nth(nth - 1);
+        }
+    }
+
+    fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
+    where
+        F: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        #[inline]
+        fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth(step)
+        }
+
+        if self.first_take {
+            self.first_take = false;
+            match self.iter.next() {
+                None => return try { acc },
+                Some(x) => acc = f(acc, x)?,
+            }
+        }
+        from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
+    }
+
+    fn fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth(step)
+        }
+
+        if self.first_take {
+            self.first_take = false;
+            match self.iter.next() {
+                None => return acc,
+                Some(x) => acc = f(acc, x),
+            }
+        }
+        from_fn(nth(&mut self.iter, self.step)).fold(acc, f)
+    }
+}
+
+impl<I> StepBy<I>
+where
+    I: ExactSizeIterator,
+{
+    // The zero-based index starting from the end of the iterator of the
+    // last element. Used in the `DoubleEndedIterator` implementation.
+    fn next_back_index(&self) -> usize {
+        let rem = self.iter.len() % (self.step + 1);
+        if self.first_take {
+            if rem == 0 { self.step } else { rem - 1 }
+        } else {
+            rem
+        }
+    }
+}
+
+#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for StepBy<I>
+where
+    I: DoubleEndedIterator + ExactSizeIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.nth_back(self.next_back_index())
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        // `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
+        // is out of bounds because the length of `self.iter` does not exceed
+        // `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
+        // zero-indexed
+        let n = n.saturating_mul(self.step + 1).saturating_add(self.next_back_index());
+        self.iter.nth_back(n)
+    }
+
+    fn try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
+    where
+        F: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        #[inline]
+        fn nth_back<I: DoubleEndedIterator>(
+            iter: &mut I,
+            step: usize,
+        ) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth_back(step)
+        }
+
+        match self.next_back() {
+            None => try { init },
+            Some(x) => {
+                let acc = f(init, x)?;
+                from_fn(nth_back(&mut self.iter, self.step)).try_fold(acc, f)
+            }
+        }
+    }
+
+    #[inline]
+    fn rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
+    where
+        Self: Sized,
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn nth_back<I: DoubleEndedIterator>(
+            iter: &mut I,
+            step: usize,
+        ) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth_back(step)
+        }
+
+        match self.next_back() {
+            None => init,
+            Some(x) => {
+                let acc = f(init, x);
+                from_fn(nth_back(&mut self.iter, self.step)).fold(acc, f)
+            }
+        }
+    }
+}
+
+// StepBy can only make the iterator shorter, so the len will still fit.
+#[stable(feature = "iterator_step_by", since = "1.28.0")]
+impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}

library/core/src/iter/adapters/take.rs

@@ -0,0 +1,209 @@
+use crate::cmp;
+use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
+use crate::ops::{ControlFlow, Try};
+
+/// An iterator that only iterates over the first `n` iterations of `iter`.
+///
+/// This `struct` is created by the [`take`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`take`]: Iterator::take
+/// [`Iterator`]: trait.Iterator.html
+#[derive(Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Take<I> {
+    iter: I,
+    n: usize,
+}
+
+impl<I> Take<I> {
+    pub(in crate::iter) fn new(iter: I, n: usize) -> Take<I> {
+        Take { iter, n }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> Iterator for Take<I>
+where
+    I: Iterator,
+{
+    type Item = <I as Iterator>::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<<I as Iterator>::Item> {
+        if self.n != 0 {
+            self.n -= 1;
+            self.iter.next()
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<I::Item> {
+        if self.n > n {
+            self.n -= n + 1;
+            self.iter.nth(n)
+        } else {
+            if self.n > 0 {
+                self.iter.nth(self.n - 1);
+                self.n = 0;
+            }
+            None
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.n == 0 {
+            return (0, Some(0));
+        }
+
+        let (lower, upper) = self.iter.size_hint();
+
+        let lower = cmp::min(lower, self.n);
+
+        let upper = match upper {
+            Some(x) if x < self.n => Some(x),
+            _ => Some(self.n),
+        };
+
+        (lower, upper)
+    }
+
+    #[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>,
+    {
+        fn check<'a, T, Acc, R: Try<Ok = Acc>>(
+            n: &'a mut usize,
+            mut fold: impl FnMut(Acc, T) -> R + 'a,
+        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
+            move |acc, x| {
+                *n -= 1;
+                let r = fold(acc, x);
+                if *n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
+            }
+        }
+
+        if self.n == 0 {
+            try { init }
+        } else {
+            let n = &mut self.n;
+            self.iter.try_fold(init, check(n, fold)).into_try()
+        }
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
+            move |acc, x| Ok(f(acc, x))
+        }
+
+        self.try_fold(init, ok(fold)).unwrap()
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I: Iterator> SourceIter for Take<I>
+where
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I: InPlaceIterable> InPlaceIterable for Take<I> {}
+
+#[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for Take<I>
+where
+    I: DoubleEndedIterator + ExactSizeIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.n == 0 {
+            None
+        } else {
+            let n = self.n;
+            self.n -= 1;
+            self.iter.nth_back(self.iter.len().saturating_sub(n))
+        }
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.iter.len();
+        if self.n > n {
+            let m = len.saturating_sub(self.n) + n;
+            self.n -= n + 1;
+            self.iter.nth_back(m)
+        } else {
+            if len > 0 {
+                self.iter.nth_back(len - 1);
+            }
+            None
+        }
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Ok = Acc>,
+    {
+        if self.n == 0 {
+            try { init }
+        } else {
+            let len = self.iter.len();
+            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
+                try { init }
+            } else {
+                self.iter.try_rfold(init, fold)
+            }
+        }
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        if self.n == 0 {
+            init
+        } else {
+            let len = self.iter.len();
+            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
+                init
+            } else {
+                self.iter.rfold(init, fold)
+            }
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I> FusedIterator for Take<I> where I: FusedIterator {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}