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.

2 files changed, 3 insertions, 0 deletions

diff --git a/src/libstd/env.rs b/src/libstd/env.rs
index baa2b5d2846..ee6a907f616 100644
--- a/src/libstd/env.rs
+++ b/src/libstd/env.rs
@@ -630,6 +630,7 @@ impl Iterator for Args {
 #[stable(feature = "env", since = "1.0.0")]
 impl ExactSizeIterator for Args {
     fn len(&self) -> usize { self.inner.len() }
+    fn is_empty(&self) -> bool { self.inner.is_empty() }
 }
 
 #[stable(feature = "env_iterators", since = "1.11.0")]
@@ -649,6 +650,7 @@ impl Iterator for ArgsOs {
 #[stable(feature = "env", since = "1.0.0")]
 impl ExactSizeIterator for ArgsOs {
     fn len(&self) -> usize { self.inner.len() }
+    fn is_empty(&self) -> bool { self.inner.is_empty() }
 }
 
 #[stable(feature = "env_iterators", since = "1.11.0")]
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 8132b139260..1f40d3fd1d3 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -250,6 +250,7 @@
 #![feature(core_float)]
 #![feature(core_intrinsics)]
 #![feature(dropck_parametricity)]
+#![feature(exact_size_is_empty)]
 #![feature(float_extras)]
 #![feature(float_from_str_radix)]
 #![feature(fn_traits)]