specialize some collection and iterator operations to run in-place #70793
Commits on Sep 3, 2020
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switch to try_fold and segregate the drop handling to keep collect::<Vec<u8>>() and similar optimizer-friendly It comes at the cost of less accurate debug_asserts and code complexity
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restore Vec::extend specialization for vec::IntoIter sources that
was lost during refactoring
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use memmove instead of generic in-place iteration for IntoIter source
this is the original SpecExtend<_, IntoIter> logic except generalizing the fast-path to include a memmove
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use From specializations on extend if extended Vec is empty
this enables in-place iteration and allocation reuse in additional cases
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specialize creating a Vec from a slice iterator where T: Copy
this was already implemented for Extend but not for FromIterator
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fix some in-place-collect edge-cases
- it's an allocation optimization, so don't attempt to do it on ZSTs - drop the tail of partially exhausted iters
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add in-place iteration for Zip
this picks the left hand side as source since it might be more natural to consume that as IntoIter source
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support in-place iteration for most adapters
`Take` is not included since users probably call it with small constants and it doesn't make sense to hold onto huge allocations in that case
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remove unecessary feature flag
# Conflicts: # library/alloc/src/lib.rs
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avoid exposing that binary heap's IntoIter is backed by vec::IntoIter…
…, use a private trait instead
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work around compiler overhead around lambdas in generics by extractin…
…g them into free functions
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avoid applying in-place collect specialization in type-length test
the test was sized to match external iteration only, since vec's in-place collect now uses internal iteration we collect into a different type now. Note that any other try-fold based operation such as count() would also have exceeded the type length limit for that iterator.
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get things to work under min_specialization by leaning more heavily o…
…n #[rustc_unsafe_specialization_marker]
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remove empty Vec extend optimization
The optimization meant that every extend code path had to emit llvm IR for from_iter and extend spec_extend, which likely impacts compile times while only improving a few edge-cases
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The InPlaceIterable debug assert checks that the write pointer did not advance beyond the read pointer. But TrustedRandomAccess never advances the read pointer, thus triggering the assert. Skip the assert if the source pointer did not change during iteration.
