Auto merge of #38149 - bluss:is-empty, r=alexcrichton

Forward more ExactSizeIterator methods and `is_empty` edits

- Forward ExactSizeIterator methods in more places, like `&mut I` and `Box<I>` iterator impls.
- Improve `VecDeque::is_empty` itself (see commit 4)
- All the collections iterators now have `len` or `is_empty` forwarded if doing so is a benefit. In the remaining cases, they already use a simple size hint (using something like a stored `usize` value), which is sufficient for the default implementation of len and is_empty.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index d10d7a5e58d..addb056f534 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -535,7 +535,14 @@ impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
     }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
+impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
+    fn len(&self) -> usize {
+        (**self).len()
+    }
+    fn is_empty(&self) -> bool {
+        (**self).is_empty()
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}