Implement Default for some alloc/core iterators

This way one can `mem::take()` them out of structs or #[derive(Default)] on structs containing them.

These changes will be insta-stable.

8 files changed, 78 insertions, 0 deletions

diff --git a/library/core/src/iter/adapters/chain.rs b/library/core/src/iter/adapters/chain.rs
index 60eb3a6da3a..21fc3bc6f6f 100644
--- a/library/core/src/iter/adapters/chain.rs
+++ b/library/core/src/iter/adapters/chain.rs
@@ -282,6 +282,17 @@ where
 {
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<A, B> Default for Chain<A, B>
+where
+    A: Iterator + Default,
+    B: Iterator + Default,
+{
+    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..d22a6e721f5 100644
--- a/library/core/src/iter/adapters/cloned.rs
+++ b/library/core/src/iter/adapters/cloned.rs
@@ -153,3 +153,14 @@ where
         item.clone()
     }
 }
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, T: 'a> Default for Cloned<I>
+where
+    I: Default + Iterator<Item = &'a T>,
+    T: Clone,
+{
+    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..d5e579834ee 100644
--- a/library/core/src/iter/adapters/copied.rs
+++ b/library/core/src/iter/adapters/copied.rs
@@ -240,3 +240,14 @@ where
         }
     }
 }
+
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, T: 'a> Default for Copied<I>
+where
+    I: Default + Iterator<Item = &'a T>,
+    T: Copy,
+{
+    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..0b44139a83b 100644
--- a/library/core/src/iter/adapters/enumerate.rs
+++ b/library/core/src/iter/adapters/enumerate.rs
@@ -264,3 +264,13 @@ 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 for Enumerate<I>
+where
+    I: Iterator + Default,
+{
+    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..c0a347a7c1d 100644
--- a/library/core/src/iter/adapters/flatten.rs
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -302,6 +302,17 @@ where
 {
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I> Default for Flatten<I>
+where
+    I: Iterator + Default,
+    <I as Iterator>::Item: IntoIterator,
+{
+    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..87275fa3951 100644
--- a/library/core/src/iter/adapters/fuse.rs
+++ b/library/core/src/iter/adapters/fuse.rs
@@ -181,6 +181,13 @@ where
     }
 }
 
+#[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+impl<I: Default> Default for Fuse<I> {
+    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..4ad75ec0ea2 100644
--- a/library/core/src/iter/adapters/rev.rs
+++ b/library/core/src/iter/adapters/rev.rs
@@ -135,3 +135,13 @@ 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 for Rev<I>
+where
+    I: Default + Iterator,
+{
+    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..57754182c4e 100644
--- a/library/core/src/slice/iter/macros.rs
+++ b/library/core/src/slice/iter/macros.rs
@@ -393,6 +393,13 @@ macro_rules! iterator {
                 }
             }
         }
+
+        #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+        impl<T> Default for $name<'_, T> {
+            fn default() -> Self {
+                (& $( $mut_ )? []).into_iter()
+            }
+        }
     }
 }