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.

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> {}

src/liballoc/lib.rs

@@ -79,6 +79,7 @@
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_parametricity)]
+#![cfg_attr(not(test), feature(exact_size_is_empty))]
 #![feature(fundamental)]
 #![feature(lang_items)]
 #![feature(needs_allocator)]

src/libcollections/binary_heap.rs

@@ -986,7 +986,11 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
+impl<'a, T> ExactSizeIterator for Iter<'a, T> {
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T> FusedIterator for Iter<'a, T> {}
@@ -1022,7 +1026,11 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> ExactSizeIterator for IntoIter<T> {}
+impl<T> ExactSizeIterator for IntoIter<T> {
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for IntoIter<T> {}
@@ -1057,7 +1065,11 @@ impl<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
 }
 
 #[stable(feature = "drain", since = "1.6.0")]
-impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}
+impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T: 'a> FusedIterator for Drain<'a, T> {}

src/libcollections/vec_deque.rs

@@ -810,7 +810,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_empty(&self) -> bool {
-        self.len() == 0
+        self.tail == self.head
     }
 
     /// Create a draining iterator that removes the specified range in the
@@ -1916,7 +1916,11 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
+impl<'a, T> ExactSizeIterator for Iter<'a, T> {
+    fn is_empty(&self) -> bool {
+        self.head == self.tail
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T> FusedIterator for Iter<'a, T> {}
@@ -1980,7 +1984,11 @@ impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
+impl<'a, T> ExactSizeIterator for IterMut<'a, T> {
+    fn is_empty(&self) -> bool {
+        self.head == self.tail
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<'a, T> FusedIterator for IterMut<'a, T> {}
@@ -2017,7 +2025,11 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> ExactSizeIterator for IntoIter<T> {}
+impl<T> ExactSizeIterator for IntoIter<T> {
+    fn is_empty(&self) -> bool {
+        self.inner.is_empty()
+    }
+}
 
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for IntoIter<T> {}

src/libcollectionstest/vec_deque.rs

@@ -1007,3 +1007,24 @@ fn assert_covariance() {
         d
     }
 }
+
+#[test]
+fn test_is_empty() {
+    let mut v = VecDeque::<i32>::new();
+    assert!(v.is_empty());
+    assert!(v.iter().is_empty());
+    assert!(v.iter_mut().is_empty());
+    v.extend(&[2, 3, 4]);
+    assert!(!v.is_empty());
+    assert!(!v.iter().is_empty());
+    assert!(!v.iter_mut().is_empty());
+    while let Some(_) = v.pop_front() {
+        assert_eq!(v.is_empty(), v.len() == 0);
+        assert_eq!(v.iter().is_empty(), v.iter().len() == 0);
+        assert_eq!(v.iter_mut().is_empty(), v.iter_mut().len() == 0);
+    }
+    assert!(v.is_empty());
+    assert!(v.iter().is_empty());
+    assert!(v.iter_mut().is_empty());
+    assert!(v.into_iter().is_empty());
+}

src/libcore/iter/traits.rs

@@ -552,7 +552,14 @@ pub trait ExactSizeIterator: Iterator {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I {}
+impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I {
+    fn len(&self) -> usize {
+        (**self).len()
+    }
+    fn is_empty(&self) -> bool {
+        (**self).is_empty()
+    }
+}
 
 /// Trait to represent types that can be created by summing up an iterator.
 ///

src/libcore/str/mod.rs

@@ -618,6 +618,11 @@ impl<'a> ExactSizeIterator for Bytes<'a> {
     fn len(&self) -> usize {
         self.0.len()
     }
+
+    #[inline]
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 #[unstable(feature = "fused", issue = "35602")]

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")]

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)]