std: Forward ExactSizeIterator::is_empty for Args, ArgsOs iterators

src/libstd/env.rs

@@ -630,6 +630,7 @@ fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 #[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 @@ fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 #[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")]

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)]