When possible without changing semantics, implement Iterator::last in terms of DoubleEndedIterator::next_back for types in liballoc and libcore.

Provided that the iterator has finite length and does not trigger user-provided code, this is safe.

What follows is a full list of the DoubleEndedIterators in liballoc/libcore and whether this optimization is safe, and if not, why not.

src/liballoc/boxed.rs
Box: Pass through to avoid defeating optimization of the underlying DoubleIterator implementation. This has no correctness impact.

src/liballoc/collections/binary_heap.rs
Iter: Pass through to avoid defeating optimizations on slice::Iter
IntoIter: Not safe, changes Drop order
Drain: Not safe, changes Drop order

src/liballoc/collections/btree/map.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order
Keys: Safe to call next_back, invokes no user defined code.
Values: ditto
ValuesMut: ditto
Range: ditto
RangeMut: ditto

src/liballoc/collections/btree/set.rs
Iter: Safe to call next_back, invokes no user defined code.
IntoIter: Not safe, changes Drop order
Range: Safe to call next_back, invokes no user defined code.

src/liballoc/collections/linked_list.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order

src/liballoc/collections/vec_deque.rs
Iter: Safe to call next_back, invokes no user defined code.
IterMut: ditto
IntoIter: Not safe, changes Drop order
Drain: ditto

src/liballoc/string.rs
Drain: Safe because return type is a primitive (char)

src/liballoc/vec.rs
IntoIter: Not safe, changes Drop order
Drain: ditto
Splice: ditto

src/libcore/ascii.rs
EscapeDefault: Safe because return type is a primitive (u8)

src/libcore/iter/adapters/chain.rs
Chain: Not safe, invokes user defined code (Iterator impl)

src/libcore/iter/adapters/flatten.rs
FlatMap: Not safe, invokes user defined code (Iterator impl)
Flatten: ditto
FlattenCompat: ditto

src/libcore/iter/adapters/mod.rs
Rev: Not safe, invokes user defined code (Iterator impl)
Copied: ditto
Cloned: Not safe, invokes user defined code (Iterator impl and T::clone)
Map: Not safe, invokes user defined code (Iterator impl + closure)
Filter: ditto
FilterMap: ditto
Enumerate: Not safe, invokes user defined code (Iterator impl)
Skip: ditto
Fuse: ditto
Inspect: ditto

src/libcore/iter/adapters/zip.rs
Zip: Not safe, invokes user defined code (Iterator impl)

src/libcore/iter/range.rs
ops::Range: Not safe, changes Drop order, but ALREADY HAS SPECIALIZATION
ops::RangeInclusive: ditto

src/libcore/iter/sources.rs
Repeat: Not safe, calling last should iloop.
Empty: No point, iterator is at most one item long.
Once: ditto
OnceWith: ditto

src/libcore/option.rs
Item: No point, iterator is at most one item long.
Iter: ditto
IterMut: ditto
IntoIter: ditto

src/libcore/result.rs
Iter: No point, iterator is at most one item long
IterMut: ditto
IntoIter: ditto

src/libcore/slice/mod.rs
Split: Not safe, invokes user defined closure
SplitMut: ditto
RSplit: ditto
RSplitMut: ditto
Windows: Safe, already has specialization
Chunks: ditto
ChunksMut: ditto
ChunksExact: ditto
ChunksExactMut: ditto
RChunks: ditto
RChunksMut: ditto
RChunksExact: ditto
RChunksExactMut: ditto

src/libcore/str/mod.rs
Chars: Safe, already has specialization
CharIndices: ditto
Bytes: ditto
Lines: Safe to call next_back, invokes no user defined code.
LinesAny: Deprecated
Everything that is generic over P: Pattern: Not safe because Pattern invokes user defined code.
SplitWhitespace: Safe to call next_back, invokes no user defined code.
SplitAsciiWhitespace: ditto

Author: khuey

src/liballoc/boxed.rs

@@ -716,6 +716,14 @@ impl<I: Iterator + ?Sized> Iterator for Box<I> {
         (**self).nth(n)
     }
 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator + Sized> Iterator for Box<I> {
+    fn last(self) -> Option<I::Item> where I: Sized {
+        (*self).last()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
     fn next_back(&mut self) -> Option<I::Item> {

src/liballoc/collections/binary_heap.rs

@@ -1035,6 +1035,11 @@ impl<'a, T> Iterator for Iter<'a, T> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+
+    #[inline]
+    fn last(self) -> Option<&'a T> {
+        self.iter.last()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/collections/btree/map.rs

@@ -1193,6 +1193,10 @@ impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (self.length, Some(self.length))
     }
+
+    fn last(mut self) -> Option<(&'a K, &'a V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1253,6 +1257,10 @@ impl<'a, K: 'a, V: 'a> Iterator for IterMut<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (self.length, Some(self.length))
     }
+
+    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1421,6 +1429,10 @@ impl<'a, K, V> Iterator for Keys<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+
+    fn last(mut self) -> Option<&'a K> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1458,6 +1470,10 @@ impl<'a, K, V> Iterator for Values<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+
+    fn last(mut self) -> Option<&'a V> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1495,6 +1511,10 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
             unsafe { Some(self.next_unchecked()) }
         }
     }
+
+    fn last(mut self) -> Option<(&'a K, &'a V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "map_values_mut", since = "1.10.0")]
@@ -1508,6 +1528,10 @@ impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+
+    fn last(mut self) -> Option<&'a mut V> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "map_values_mut", since = "1.10.0")]
@@ -1626,6 +1650,10 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
             unsafe { Some(self.next_unchecked()) }
         }
     }
+
+    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next_back()
+    }
 }
 
 impl<'a, K, V> RangeMut<'a, K, V> {

src/liballoc/collections/btree/set.rs

@@ -1019,6 +1019,9 @@ impl<'a, T> Iterator for Iter<'a, T> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+    fn last(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
@@ -1073,6 +1076,10 @@ impl<'a, T> Iterator for Range<'a, T> {
     fn next(&mut self) -> Option<&'a T> {
         self.iter.next().map(|(k, _)| k)
     }
+
+    fn last(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "btree_range", since = "1.17.0")]

src/liballoc/collections/linked_list.rs

@@ -832,6 +832,11 @@ impl<'a, T> Iterator for Iter<'a, T> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (self.len, Some(self.len))
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -881,6 +886,11 @@ impl<'a, T> Iterator for IterMut<'a, T> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (self.len, Some(self.len))
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a mut T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/collections/vec_deque.rs

@@ -2206,6 +2206,11 @@ impl<'a, T> Iterator for Iter<'a, T> {
         self.tail = self.head - iter.len();
         final_res
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -2319,6 +2324,11 @@ impl<'a, T> Iterator for IterMut<'a, T> {
         accum = front.iter_mut().fold(accum, &mut f);
         back.iter_mut().fold(accum, &mut f)
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a mut T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/string.rs

@@ -2385,6 +2385,11 @@ impl Iterator for Drain<'_> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+
+    #[inline]
+    fn last(mut self) -> Option<char> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "drain", since = "1.6.0")]

src/libcore/ascii.rs

@@ -117,6 +117,7 @@ impl Iterator for EscapeDefault {
     type Item = u8;
     fn next(&mut self) -> Option<u8> { self.range.next().map(|i| self.data[i]) }
     fn size_hint(&self) -> (usize, Option<usize>) { self.range.size_hint() }
+    fn last(mut self) -> Option<u8> { self.next_back() }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl DoubleEndedIterator for EscapeDefault {

src/libcore/str/mod.rs

@@ -1333,6 +1333,11 @@ impl<'a> Iterator for Lines<'a> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.0.size_hint()
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a str> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -4241,6 +4246,11 @@ impl<'a> Iterator for SplitWhitespace<'a> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a str> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "split_whitespace", since = "1.1.0")]
@@ -4267,6 +4277,11 @@ impl<'a> Iterator for SplitAsciiWhitespace<'a> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a str> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]