RangeInclusive internal iteration performance improvement.

src/libcore/iter/range.rs

@@ -1,6 +1,6 @@
 use convert::TryFrom;
 use mem;
-use ops::{self, Add, Sub};
+use ops::{self, Add, Sub, Try};
 use usize;
 
 use super::{FusedIterator, TrustedLen};
@@ -368,11 +368,11 @@ impl<A: Step> Iterator for ops::RangeInclusive<A> {
                 Some(Less) => {
                     self.is_empty = Some(false);
                     self.start = plus_n.add_one();
-                    return Some(plus_n)
+                    return Some(plus_n);
                 }
                 Some(Equal) => {
                     self.is_empty = Some(true);
-                    return Some(plus_n)
+                    return Some(plus_n);
                 }
                 _ => {}
             }
@@ -382,6 +382,34 @@ impl<A: Step> Iterator for ops::RangeInclusive<A> {
         None
     }
 
+    #[inline]
+    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
+    {
+        self.compute_is_empty();
+
+        if self.is_empty() {
+            return Try::from_ok(init);
+        }
+
+        let mut accum = init;
+
+        while self.start < self.end {
+            let n = self.start.add_one();
+            let n = mem::replace(&mut self.start, n);
+            accum = f(accum, n)?;
+        }
+
+        self.is_empty = Some(true);
+
+        if self.start == self.end {
+            accum = f(accum, self.start.clone())?;
+        }
+
+        Try::from_ok(accum)
+    }
+
     #[inline]
     fn last(mut self) -> Option<A> {
         self.next_back()
@@ -415,6 +443,33 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
             self.end.clone()
         })
     }
+
+    #[inline]
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
+        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
+    {
+        self.compute_is_empty();
+
+        if self.is_empty() {
+            return Try::from_ok(init);
+        }
+
+        let mut accum = init;
+
+        while self.start < self.end {
+            let n = self.end.sub_one();
+            let n = mem::replace(&mut self.end, n);
+            accum = f(accum, n)?;
+        }
+
+        self.is_empty = Some(true);
+
+        if self.start == self.end {
+            accum = f(accum, self.start.clone())?;
+        }
+
+        Try::from_ok(accum)
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]

src/libcore/ops/range.rs

@@ -334,12 +334,14 @@ pub struct RangeInclusive<Idx> {
 trait RangeInclusiveEquality: Sized {
     fn canonicalized_is_empty(range: &RangeInclusive<Self>) -> bool;
 }
+
 impl<T> RangeInclusiveEquality for T {
     #[inline]
     default fn canonicalized_is_empty(range: &RangeInclusive<Self>) -> bool {
         range.is_empty.unwrap_or_default()
     }
 }
+
 impl<T: PartialOrd> RangeInclusiveEquality for T {
     #[inline]
     fn canonicalized_is_empty(range: &RangeInclusive<Self>) -> bool {

src/libcore/tests/iter.rs

@@ -1738,19 +1738,37 @@ fn test_range_inclusive_folds() {
     assert_eq!((1..=10).sum::<i32>(), 55);
     assert_eq!((1..=10).rev().sum::<i32>(), 55);
 
-    let mut it = 40..=50;
+    let mut it = 44..=50;
     assert_eq!(it.try_fold(0, i8::checked_add), None);
-    assert_eq!(it, 44..=50);
+    assert_eq!(it, 47..=50);
+    assert_eq!(it.try_fold(0, i8::checked_add), None);
+    assert_eq!(it, 50..=50);
+    assert_eq!(it.try_fold(0, i8::checked_add), Some(50));
+    assert!(it.is_empty());
+    assert_eq!(it.try_fold(0, i8::checked_add), Some(0));
+    assert!(it.is_empty());
+
+    let mut it = 40..=47;
+    assert_eq!(it.try_rfold(0, i8::checked_add), None);
+    assert_eq!(it, 40..=44);
     assert_eq!(it.try_rfold(0, i8::checked_add), None);
-    assert_eq!(it, 44..=47);
+    assert_eq!(it, 40..=41);
+    assert_eq!(it.try_rfold(0, i8::checked_add), Some(81));
+    assert!(it.is_empty());
+    assert_eq!(it.try_rfold(0, i8::checked_add), Some(0));
+    assert!(it.is_empty());
 
     let mut it = 10..=20;
     assert_eq!(it.try_fold(0, |a,b| Some(a+b)), Some(165));
     assert!(it.is_empty());
+    assert_eq!(it.try_fold(0, |a,b| Some(a+b)), Some(0));
+    assert!(it.is_empty());
 
     let mut it = 10..=20;
     assert_eq!(it.try_rfold(0, |a,b| Some(a+b)), Some(165));
     assert!(it.is_empty());
+    assert_eq!(it.try_rfold(0, |a,b| Some(a+b)), Some(0));
+    assert!(it.is_empty());
 }
 
 #[test]