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

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