Add a FusedIterator trait.

This trait can be used to avoid the overhead of a fuse wrapper when an iterator
is already well-behaved.

Conforming to: RFC 1581
Closes: #35602

src/liballoc/boxed.rs

@@ -61,6 +61,7 @@ use core::borrow;
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{self, Hash};
+use core::iter::FusedIterator;
 use core::marker::{self, Unsize};
 use core::mem;
 use core::ops::{CoerceUnsized, Deref, DerefMut};
@@ -529,6 +530,9 @@ impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {}
 
+#[unstable(feature = "fused", issue = "35602")]
+impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
+
 
 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 /// closure objects. The idea is that where one would normally store a

src/liballoc/lib.rs

@@ -91,7 +91,7 @@
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(unsize)]
 
-#![cfg_attr(not(test), feature(raw, fn_traits, placement_new_protocol))]
+#![cfg_attr(not(test), feature(fused, raw, fn_traits, placement_new_protocol))]
 #![cfg_attr(test, feature(test, box_heap))]
 
 // Allow testing this library