Commits on Jul 2, 2019

1. 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

   

   khuey committed Jul 2, 2019
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