Specialize iter::ArrayChunks::fold for TrustedRandomAccess iterators
#103446
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(rust-highfive has picked a reviewer for you, use r? to override) |
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Seems broadly okay but I would like to hear your thoughts on especially the second comment around the safety condition. |
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This also updates the existing iter::Copied::next_chunk benchmark so that the thing it benches doesn't get masked by the ArrayChunks specialization
This is fairly safe use of TRA since it consumes the iterator so no struct in an unsafe state will be left exposed to user code
The type is unsafe and now exposed to the whole crate. Document it properly and add an unsafe method so the caller can make it visible that something unsafe is happening.
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@bors r=Mark-Simulacrum rollup |
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Maybe don't rollup optimizations? |
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This is unstable API that's not used in the compiler yet, it should be fine. |
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Rollup of 7 pull requests Successful merges: - rust-lang#103446 (Specialize `iter::ArrayChunks::fold` for TrustedRandomAccess iterators) - rust-lang#103651 (Fix `rustc_parse_format` spans following escaped utf-8 multibyte chars) - rust-lang#103865 (Move `fallback_has_occurred` state tracking to `FnCtxt`) - rust-lang#103955 (Update linker-plugin-lto.md to contain up to Rust 1.65) - rust-lang#103987 (Remove `in_tail_expr` from FnCtxt) - rust-lang#104067 (fix debuginfo for windows_gnullvm_base.rs) - rust-lang#104094 (fully move `on_unimplemented` to `error_reporting`) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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The resulting assembly is similar to #103166 but the specialization kicks in under different (partially overlapping) conditions compared to that PR. They're complementary.
In principle a TRA-based specialization could be applied to all
ArrayChunksmethods, includingnext()as we do forZipbut that would have all the same hazards as the Zip specialization. Only doing it forfoldis far less hazardous. The downside is that it only helps with internal, exhaustive iteration. I.e.for _ inortry_foldwill not benefit.Note that the regular,
try_fold-based and the specializedfold()impl have observably slightly different behavior. Namely the specialized variant does not fetch the remainder elements from the underlying iterator. We do have a few other places in the standard library where beyond-the-end-of-iteration side-effects are being elided under some circumstances but not others.Inspired by https://old.reddit.com/r/rust/comments/yaft60/zerocost_iterator_abstractionsnot_so_zerocost/