Auto merge of #99929 - the8472:default-iters, r=scottmcm

Implement Default for some alloc/core iterators

Add `Default` impls to the following collection iterators:

* slice::{Iter, IterMut}
* binary_heap::IntoIter
* btree::map::{Iter, IterMut, Keys, Values, Range, IntoIter, IntoKeys, IntoValues}
* btree::set::{Iter, IntoIter, Range}
* linked_list::IntoIter
* vec::IntoIter

and these adapters:

* adapters::{Chain, Cloned, Copied, Rev, Enumerate, Flatten, Fuse, Rev}

For iterators which are generic over allocators it only implements it for the global allocator because we can't conjure an allocator from nothing or would have to turn the allocator field into an `Option` just for this change.

These changes will be insta-stable.

ACP: https://github.com/rust-lang/libs-team/issues/77

14 files changed, 371 insertions, 0 deletions

diff --git a/library/alloc/src/collections/binary_heap/mod.rs b/library/alloc/src/collections/binary_heap/mod.rs
index f1d0a305d99..d0a87e3fb9f 100644
--- a/library/alloc/src/collections/binary_heap/mod.rs
+++ b/library/alloc/src/collections/binary_heap/mod.rs
@@ -1468,6 +1468,20 @@ impl<T> ExactSizeIterator for IntoIter<T> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for IntoIter<T> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for IntoIter<T> {
+    /// Creates an empty `binary_heap::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::binary_heap;
+    /// let iter: binary_heap::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { iter: Default::default() }
+    }
+}
+
 // In addition to the SAFETY invariants of the following three unsafe traits
 // also refer to the vec::in_place_collect module documentation to get an overview
 #[unstable(issue = "none", feature = "inplace_iteration")]
diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 386cd1a1657..61db46314b7 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -362,6 +362,20 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Iter<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, K: 'a, V: 'a> Default for Iter<'a, K, V> {
+    /// Creates an empty `btree_map::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Iter<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { range: Default::default(), length: 0 }
+    }
+}
+
 /// A mutable iterator over the entries of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`iter_mut`] method on [`BTreeMap`]. See its
@@ -386,6 +400,20 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, K: 'a, V: 'a> Default for IterMut<'a, K, V> {
+    /// Creates an empty `btree_map::IterMut`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IterMut<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IterMut { range: Default::default(), length: 0, _marker: PhantomData {} }
+    }
+}
+
 /// An owning iterator over the entries of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`BTreeMap`]
@@ -421,6 +449,23 @@ impl<K: Debug, V: Debug, A: Allocator + Clone> Debug for IntoIter<K, V, A> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V, A> Default for IntoIter<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoIter<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { range: Default::default(), length: 0, alloc: Default::default() }
+    }
+}
+
 /// An iterator over the keys of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`keys`] method on [`BTreeMap`]. See its
@@ -1768,6 +1813,20 @@ impl<K, V> Clone for Keys<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Keys<'_, K, V> {
+    /// Creates an empty `btree_map::Keys`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Keys<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Keys { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, K, V> Iterator for Values<'a, K, V> {
     type Item = &'a V;
@@ -1809,6 +1868,20 @@ impl<K, V> Clone for Values<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Values<'_, K, V> {
+    /// Creates an empty `btree_map::Values`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Values<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Values { inner: Default::default() }
+    }
+}
+
 /// An iterator produced by calling `drain_filter` on BTreeMap.
 #[unstable(feature = "btree_drain_filter", issue = "70530")]
 pub struct DrainFilter<
@@ -1945,6 +2018,20 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Range<'_, K, V> {
+    /// Creates an empty `btree_map::Range`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Range<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        Range { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     type Item = &'a mut V;
@@ -2021,6 +2108,23 @@ impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoKeys<K, V, A> {
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> FusedIterator for IntoKeys<K, V, A> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V, A> Default for IntoKeys<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoKeys`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoKeys<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoKeys { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> Iterator for IntoValues<K, V, A> {
     type Item = V;
@@ -2055,6 +2159,23 @@ impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoValues<K, V, A> {
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> FusedIterator for IntoValues<K, V, A> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V, A> Default for IntoValues<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoValues`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoValues<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoValues { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
     fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
diff --git a/library/alloc/src/collections/btree/navigate.rs b/library/alloc/src/collections/btree/navigate.rs
index b890717e50b..a85a3162451 100644
--- a/library/alloc/src/collections/btree/navigate.rs
+++ b/library/alloc/src/collections/btree/navigate.rs
@@ -19,6 +19,12 @@ impl<'a, K: 'a, V: 'a> Clone for LeafRange<marker::Immut<'a>, K, V> {
     }
 }
 
+impl<B, K, V> Default for LeafRange<B, K, V> {
+    fn default() -> Self {
+        LeafRange { front: None, back: None }
+    }
+}
+
 impl<BorrowType, K, V> LeafRange<BorrowType, K, V> {
     pub fn none() -> Self {
         LeafRange { front: None, back: None }
@@ -124,6 +130,12 @@ pub struct LazyLeafRange<BorrowType, K, V> {
     back: Option<LazyLeafHandle<BorrowType, K, V>>,
 }
 
+impl<B, K, V> Default for LazyLeafRange<B, K, V> {
+    fn default() -> Self {
+        LazyLeafRange { front: None, back: None }
+    }
+}
+
 impl<'a, K: 'a, V: 'a> Clone for LazyLeafRange<marker::Immut<'a>, K, V> {
     fn clone(&self) -> Self {
         LazyLeafRange { front: self.front.clone(), back: self.back.clone() }
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 4de30d40825..a7cb3948aa1 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -1539,6 +1539,21 @@ impl<T, A: Allocator + Clone> Iterator for IntoIter<T, A> {
         self.iter.size_hint()
     }
 }
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `btree_set::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator + Clone> DoubleEndedIterator for IntoIter<T, A> {
     fn next_back(&mut self) -> Option<T> {
@@ -1555,6 +1570,23 @@ impl<T, A: Allocator + Clone> ExactSizeIterator for IntoIter<T, A> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T, A: Allocator + Clone> FusedIterator for IntoIter<T, A> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T, A> Default for IntoIter<T, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_set::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<T> Clone for Range<'_, T> {
     fn clone(&self) -> Self {
@@ -1593,6 +1625,20 @@ impl<'a, T> DoubleEndedIterator for Range<'a, T> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for Range<'_, T> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Range<'_, T> {
+    /// Creates an empty `btree_set::Range`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::Range<'_, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        Range { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator + Clone> Clone for Difference<'_, T, A> {
     fn clone(&self) -> Self {
diff --git a/library/alloc/src/collections/linked_list.rs b/library/alloc/src/collections/linked_list.rs
index f2f5dffc25d..080a4a14eda 100644
--- a/library/alloc/src/collections/linked_list.rs
+++ b/library/alloc/src/collections/linked_list.rs
@@ -1075,6 +1075,20 @@ impl<T> ExactSizeIterator for Iter<'_, T> {}
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for Iter<'_, T> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `linked_list::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::linked_list;
+    /// let iter: linked_list::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { head: None, tail: None, len: 0, marker: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> Iterator for IterMut<'a, T> {
     type Item = &'a mut T;
@@ -1129,6 +1143,13 @@ impl<T> ExactSizeIterator for IterMut<'_, T> {}
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for IterMut<'_, T> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for IterMut<'_, T> {
+    fn default() -> Self {
+        IterMut { head: None, tail: None, len: 0, marker: Default::default() }
+    }
+}
+
 /// A cursor over a `LinkedList`.
 ///
 /// A `Cursor` is like an iterator, except that it can freely seek back-and-forth.
@@ -1808,6 +1829,20 @@ impl<T> ExactSizeIterator for IntoIter<T> {}
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for IntoIter<T> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for IntoIter<T> {
+    /// Creates an empty `linked_list::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::linked_list;
+    /// let iter: linked_list::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        LinkedList::new().into_iter()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> FromIterator<T> for LinkedList<T> {
     fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index 37966007eb7..f6525eb9003 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -347,6 +347,24 @@ impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}
 #[unstable(feature = "trusted_len", issue = "37572")]
 unsafe impl<T, A: Allocator> TrustedLen for IntoIter<T, A> {}
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<T, A> Default for IntoIter<T, A>
+where
+    A: Allocator + Default,
+{
+    /// Creates an empty `vec::IntoIter`.
+    ///
+    /// ```
+    /// # use std::vec;
+    /// let iter: vec::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// assert_eq!(iter.as_slice(), &[]);
+    /// ```
+    fn default() -> Self {
+        super::Vec::new_in(Default::default()).into_iter()
+    }
+}
+
 #[doc(hidden)]
 #[unstable(issue = "none", feature = "std_internals")]
 #[rustc_unsafe_specialization_marker]
diff --git a/library/core/src/iter/adapters/chain.rs b/library/core/src/iter/adapters/chain.rs
index 60eb3a6da3a..d4b2640e81d 100644
--- a/library/core/src/iter/adapters/chain.rs
+++ b/library/core/src/iter/adapters/chain.rs
@@ -282,6 +282,28 @@ where
 {
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<A: Default, B: Default> Default for Chain<A, B> {
+    /// Creates a `Chain` from the default values for `A` and `B`.
+    ///
+    /// ```
+    /// # use core::iter::Chain;
+    /// # use core::slice;
+    /// # use std::collections::{btree_set, BTreeSet};
+    /// # use std::mem;
+    /// struct Foo<'a>(Chain<slice::Iter<'a, u8>, btree_set::Iter<'a, u8>>);
+    ///
+    /// let set = BTreeSet::<u8>::new();
+    /// let slice: &[u8] = &[];
+    /// let mut foo = Foo(slice.iter().chain(set.iter()));
+    ///
+    /// // take requires `Default`
+    /// let _: Chain<_, _> = mem::take(&mut foo.0);
+    fn default() -> Self {
+        Chain::new(Default::default(), Default::default())
+    }
+}
+
 #[inline]
 fn and_then_or_clear<T, U>(opt: &mut Option<T>, f: impl FnOnce(&mut T) -> Option<U>) -> Option<U> {
     let x = f(opt.as_mut()?);
diff --git a/library/core/src/iter/adapters/cloned.rs b/library/core/src/iter/adapters/cloned.rs
index 914ff86c1a9..bb7e1660c6e 100644
--- a/library/core/src/iter/adapters/cloned.rs
+++ b/library/core/src/iter/adapters/cloned.rs
@@ -153,3 +153,17 @@ where
         item.clone()
     }
 }
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Cloned<I> {
+    /// Creates a `Cloned` iterator from the default value of `I`
+    /// ```
+    /// # use core::slice;
+    /// # use core::iter::Cloned;
+    /// let iter: Cloned<slice::Iter<'_, u8>> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
diff --git a/library/core/src/iter/adapters/copied.rs b/library/core/src/iter/adapters/copied.rs
index 62d3afb8160..a076ab925e3 100644
--- a/library/core/src/iter/adapters/copied.rs
+++ b/library/core/src/iter/adapters/copied.rs
@@ -240,3 +240,17 @@ where
         }
     }
 }
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Copied<I> {
+    /// Creates a `Copied` iterator from the default value of `I`
+    /// ```
+    /// # use core::slice;
+    /// # use core::iter::Copied;
+    /// let iter: Copied<slice::Iter<'_, u8>> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
diff --git a/library/core/src/iter/adapters/enumerate.rs b/library/core/src/iter/adapters/enumerate.rs
index 14a12695111..8c32a35a12f 100644
--- a/library/core/src/iter/adapters/enumerate.rs
+++ b/library/core/src/iter/adapters/enumerate.rs
@@ -264,3 +264,17 @@ where
 
 #[unstable(issue = "none", feature = "inplace_iteration")]
 unsafe impl<I: InPlaceIterable> InPlaceIterable for Enumerate<I> {}
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Enumerate<I> {
+    /// Creates an `Enumerate` iterator from the default value of `I`
+    /// ```
+    /// # use core::slice;
+    /// # use std::iter::Enumerate;
+    /// let iter: Enumerate<slice::Iter<'_, u8>> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Enumerate::new(Default::default())
+    }
+}
diff --git a/library/core/src/iter/adapters/flatten.rs b/library/core/src/iter/adapters/flatten.rs
index b040a0ea901..e4020c45f71 100644
--- a/library/core/src/iter/adapters/flatten.rs
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -302,6 +302,24 @@ where
 {
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I> Default for Flatten<I>
+where
+    I: Default + Iterator<Item: IntoIterator>,
+{
+    /// Creates a `Flatten` iterator from the default value of `I`.
+    ///
+    /// ```
+    /// # use core::slice;
+    /// # use std::iter::Flatten;
+    /// let iter: Flatten<slice::Iter<'_, [u8; 4]>> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        Flatten::new(Default::default())
+    }
+}
+
 /// Real logic of both `Flatten` and `FlatMap` which simply delegate to
 /// this type.
 #[derive(Clone, Debug)]
diff --git a/library/core/src/iter/adapters/fuse.rs b/library/core/src/iter/adapters/fuse.rs
index c9314454203..de91c188eeb 100644
--- a/library/core/src/iter/adapters/fuse.rs
+++ b/library/core/src/iter/adapters/fuse.rs
@@ -181,6 +181,21 @@ where
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Fuse<I> {
+    /// Creates a `Fuse` iterator from the default value of `I`.
+    ///
+    /// ```
+    /// # use core::slice;
+    /// # use std::iter::Fuse;
+    /// let iter: Fuse<slice::Iter<'_, u8>> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Fuse { iter: Default::default() }
+    }
+}
+
 #[unstable(feature = "trusted_len", issue = "37572")]
 // SAFETY: `TrustedLen` requires that an accurate length is reported via `size_hint()`. As `Fuse`
 // is just forwarding this to the wrapped iterator `I` this property is preserved and it is safe to
diff --git a/library/core/src/iter/adapters/rev.rs b/library/core/src/iter/adapters/rev.rs
index 139fb7bbdd9..8ae6d96fde4 100644
--- a/library/core/src/iter/adapters/rev.rs
+++ b/library/core/src/iter/adapters/rev.rs
@@ -135,3 +135,17 @@ 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 {}
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Rev<I> {
+    /// Creates a `Rev` iterator from the default value of `I`
+    /// ```
+    /// # use core::slice;
+    /// # use core::iter::Rev;
+    /// let iter: Rev<slice::Iter<'_, u8>> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Rev::new(Default::default())
+    }
+}
diff --git a/library/core/src/slice/iter/macros.rs b/library/core/src/slice/iter/macros.rs
index 89b92a7d597..a800da546b4 100644
--- a/library/core/src/slice/iter/macros.rs
+++ b/library/core/src/slice/iter/macros.rs
@@ -393,6 +393,20 @@ macro_rules! iterator {
                 }
             }
         }
+
+        #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+        impl<T> Default for $name<'_, T> {
+            /// Creates an empty slice iterator.
+            ///
+            /// ```
+            #[doc = concat!("# use core::slice::", stringify!($name), ";")]
+            #[doc = concat!("let iter: ", stringify!($name<'_, u8>), " = Default::default();")]
+            /// assert_eq!(iter.len(), 0);
+            /// ```
+            fn default() -> Self {
+                (& $( $mut_ )? []).into_iter()
+            }
+        }
     }
 }