Add Range[Inclusive]::is_empty

During the RFC, it was discussed that figuring out whether a range is empty was subtle, and thus there should be a clear and obvious way to do it.  It can't just be ExactSizeIterator::is_empty (also unstable) because not all ranges are ExactSize -- not even Range<i32> or RangeInclusive<usize>.

src/libcore/iter/traits.rs

@@ -706,7 +706,7 @@ pub trait ExactSizeIterator: Iterator {
     /// ```
     /// #![feature(exact_size_is_empty)]
     ///
-    /// let mut one_element = 0..1;
+    /// let mut one_element = std::iter::once(0);
     /// assert!(!one_element.is_empty());
     ///
     /// assert_eq!(one_element.next(), Some(0));

src/libcore/ops/range.rs

@@ -92,7 +92,6 @@ impl<Idx: fmt::Debug> fmt::Debug for Range<Idx> {
     }
 }
 
-#[unstable(feature = "range_contains", reason = "recently added as per RFC", issue = "32311")]
 impl<Idx: PartialOrd<Idx>> Range<Idx> {
     /// Returns `true` if `item` is contained in the range.
     ///
@@ -109,9 +108,26 @@ impl<Idx: PartialOrd<Idx>> Range<Idx> {
     /// assert!(!(3..3).contains(3));
     /// assert!(!(3..2).contains(3));
     /// ```
+    #[unstable(feature = "range_contains", reason = "recently added as per RFC", issue = "32311")]
     pub fn contains(&self, item: Idx) -> bool {
         (self.start <= item) && (item < self.end)
     }
+
+    /// Returns `true` if the range contains no items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(range_is_empty)]
+    ///
+    /// assert!(!(3..5).is_empty());
+    /// assert!( (3..3).is_empty());
+    /// assert!( (3..2).is_empty());
+    /// ```
+    #[unstable(feature = "range_is_empty", reason = "recently added", issue = "123456789")]
+    pub fn is_empty(&self) -> bool {
+        !(self.start < self.end)
+    }
 }
 
 /// A range only bounded inclusively below (`start..`).
@@ -280,7 +296,6 @@ impl<Idx: fmt::Debug> fmt::Debug for RangeInclusive<Idx> {
     }
 }
 
-#[unstable(feature = "range_contains", reason = "recently added as per RFC", issue = "32311")]
 impl<Idx: PartialOrd<Idx>> RangeInclusive<Idx> {
     /// Returns `true` if `item` is contained in the range.
     ///
@@ -298,9 +313,26 @@ impl<Idx: PartialOrd<Idx>> RangeInclusive<Idx> {
     /// assert!( (3..=3).contains(3));
     /// assert!(!(3..=2).contains(3));
     /// ```
+    #[unstable(feature = "range_contains", reason = "recently added as per RFC", issue = "32311")]
     pub fn contains(&self, item: Idx) -> bool {
         self.start <= item && item <= self.end
     }
+
+    /// Returns `true` if the range contains no items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(range_is_empty,inclusive_range_syntax)]
+    ///
+    /// assert!(!(3..=5).is_empty());
+    /// assert!(!(3..=3).is_empty());
+    /// assert!( (3..=2).is_empty());
+    /// ```
+    #[unstable(feature = "range_is_empty", reason = "recently added", issue = "123456789")]
+    pub fn is_empty(&self) -> bool {
+        !(self.start <= self.end)
+    }
 }
 
 /// A range only bounded inclusively above (`..=end`).

src/libcore/tests/iter.rs

@@ -1427,9 +1427,9 @@ fn test_range_inclusive_nth() {
     assert_eq!(r, 13..=20);
     assert_eq!(r.nth(2), Some(15));
     assert_eq!(r, 16..=20);
-    assert_eq!(r.is_empty(), false);
+    assert_eq!(ExactSizeIterator::is_empty(&r), false);
     assert_eq!(r.nth(10), None);
-    assert_eq!(r.is_empty(), true);
+    assert_eq!(ExactSizeIterator::is_empty(&r), true);
     assert_eq!(r, 1..=0);  // We may not want to document/promise this detail
 }
 

src/libcore/tests/lib.rs

@@ -29,6 +29,7 @@
 #![feature(iter_rfold)]
 #![feature(nonzero)]
 #![feature(pattern)]
+#![feature(range_is_empty)]
 #![feature(raw)]
 #![feature(refcell_replace_swap)]
 #![feature(sip_hash_13)]

src/libcore/tests/ops.rs

@@ -68,3 +68,27 @@ fn test_range_inclusive() {
     assert_eq!(r.size_hint(), (0, Some(0)));
     assert_eq!(r.next(), None);
 }
+
+
+#[test]
+fn test_range_is_empty() {
+    use core::f32::*;
+
+    assert!(!(0.0 .. 10.0).is_empty());
+    assert!( (-0.0 .. 0.0).is_empty());
+    assert!( (10.0 .. 0.0).is_empty());
+
+    assert!(!(NEG_INFINITY .. INFINITY).is_empty());
+    assert!( (EPSILON .. NAN).is_empty());
+    assert!( (NAN .. EPSILON).is_empty());
+    assert!( (NAN .. NAN).is_empty());
+
+    assert!(!(0.0 ..= 10.0).is_empty());
+    assert!(!(-0.0 ..= 0.0).is_empty());
+    assert!( (10.0 ..= 0.0).is_empty());
+
+    assert!(!(NEG_INFINITY ..= INFINITY).is_empty());
+    assert!( (EPSILON ..= NAN).is_empty());
+    assert!( (NAN ..= EPSILON).is_empty());
+    assert!( (NAN ..= NAN).is_empty());
+}