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Rollup merge of rust-lang#62316 - khuey:efficient_last, r=sfackler
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 This is attempt 2 of rust-lang#60130. r? @sfackler
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