We hope later to extend `core::str::Pattern` to slices too, perhaps as
part of stabilising that. We want to minimise the amount of type
inference breakage when we do that, so we don't want to stabilise
strip_prefix and strip_suffix taking a simple `&[T]`.
@KodrAus suggested the approach of introducing a new perma-unstable
trait, which reduces this future inference break risk.
I found it necessary to make two impls of this trait, as the unsize
coercion don't apply when hunting for trait implementations.
Since SlicePattern's only method returns a reference, and the whole
trait is just a wrapper for slices, I made the trait type be the
non-reference type [T] or [T;N] rather than the reference. Otherwise
the trait would have a lifetime parameter.
I marked both the no-op conversion functions `#[inline]`. I'm not
sure if that is necessary but it seemed at the very least harmless.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Document unsafety in core::slice::memchr
Contributes to #66219
Note sure if that's good enough, especially for the `align_to` call.
The docs only mention transmuting and I don't think that everything related to reference lifetimes and state validity mentioned in the [nomicon](https://doc.rust-lang.org/nomicon/transmutes.html) are relevant here.
More consistently use spaces after commas in lists in docs
This PR changes instances of lists that didn't use spaces after commas, like `vec![1,2,3]`, to `vec![1, 2, 3]` to be more consistent with idiomatic Rust style (the way these were looks strange to me, especially because there are often lists that *do* use spaces after the commas later in the same code block 😬).
I noticed one of these in an example in the stdlib docs and went looking for more, but as far as I can see, I'm only changing those spots in user-facing documentation or rustc output, and the changes make no semantic difference.
Improve documentation for slice strip_* functions
Prompted by the stabilisation tracking issue #73413 I looked at the docs for `strip_prefix` and `strip_suffix` for both `str` and `slice`, and I felt they could be slightly improved.
Thanks for your attention.
Add [T]::as_chunks(_mut)
Allows getting the slices directly, rather than just through an iterator as in `array_chunks(_mut)`. The constructors for those iterators are then written in terms of these methods, so the iterator constructors no longer have any `unsafe` of their own.
Unstable, of course. #74985
replace `#[allow_internal_unstable]` with `#[rustc_allow_const_fn_unstable]` for `const fn`s
`#[allow_internal_unstable]` is currently used to side-step feature gate and stability checks.
While it was originally only meant to be used only on macros, its use was expanded to `const fn`s.
This pr adds stricter checks for the usage of `#[allow_internal_unstable]` (only on macros) and introduces the `#[rustc_allow_const_fn_unstable]` attribute for usage on `const fn`s.
This pr does not change any of the functionality associated with the use of `#[allow_internal_unstable]` on macros or the usage of `#[rustc_allow_const_fn_unstable]` (instead of `#[allow_internal_unstable]`) on `const fn`s (see https://github.com/rust-lang/rust/issues/69399#issuecomment-712911540).
Note: The check for `#[rustc_allow_const_fn_unstable]` currently only validates that the attribute is used on a function, because I don't know how I would check if the function is a `const fn` at the place of the check. I therefore openend this as a 'draft pull request'.
Closesrust-lang/rust#69399
r? @oli-obk
Check for exhaustion in RangeInclusive::contains and slicing
When a range has finished iteration, `is_empty` returns true, so it
should also be the case that `contains` returns false.
Fixes#77941.
Doc formating consistency between slice sort and sort_unstable, and big O notation consistency
Updated documentation for slice sorting methods to be consistent between stable and unstable versions, which just ended up being minor formatting differences.
I also went through and updated any doc comments with big O notation to be consistent with #74010 by italicizing them rather than having them in a code block.
Move `slice::check_range` to `RangeBounds`
Since this method doesn't take a slice anymore (#76662), it makes more sense to define it on `RangeBounds`.
Questions:
- Should the new method be `assert_len` or `assert_length`?
The stabilisation issue, #73413, has an open item for documentation.
I looked at the docs and it is all there, but I felt it could do with
some minor wording improvement.
I looked at the `str::strip_prefix` docs for a template. (That
resulted in me slightly changing that doc too.)
I de-linkified `None` and `Some`, as I felt that rather noisy.. I
searched stdlib, and these don't seem to be usually linkified.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
This stabilizes the functionality in slice_partition_at_index,
but under the names `select_nth_unstable*`. The functions
`partition_at_index*` are left as deprecated, to be removed in
a later release.
Closes#55300
Allows getting the slices directly, rather than just through an iterator as in `array_chunks(_mut)`. The constructors for those iterators are then written in terms of these methods, so the iterator constructors no longer have any `unsafe` of their own.
Hint the maximum length permitted by invariant of slices
One of the safety invariants of references, and in particular of references to slices, is that they may not cover more than `isize::MAX` bytes. The unsafe `from_raw_parts` constructors of slices explicitly requires the caller to guarantee this fact. Violating it would also be UB with regards to the semantics of generated llvm code.
This effectively bounds the length of a (non-ZST) slice from above by a compile time constant. But when the length is loaded from a function argument it appears llvm is not aware of this requirement. The additional value range assertions allow some further elision of code branches, including overflow checks, especially in the presence of artithmetic on the indices.
This may have a performance impact, adding more code to a common method but allowing more optimization. I'm not quite sure, is the Rust side of const-prop strong enough to elide the irrelevant match branches?
Fixes: #67186
Uses assume to check the length against a constant upper bound. The
inlined result then informs the optimizer of the sound value range.
This was tried with unreachable_unchecked before which introduces a
branch. This has the advantage of not being executed in sound code but
complicates basic blocks. It resulted in ~2% increased compile time in
some worst cases.
Add a codegen test for the assumption, testing the issue from #67186
Stabilize slice_ptr_range.
This has been unstable for almost a year now. Time to stabilize?
Closes#65807.
@rustbot modify labels: +T-libs +A-raw-pointers +A-slice +needs-fcp
Refactor memchr to allow optimization
Closes#75659
The implementation already uses naive search if the slice if short enough, but the case is complicated enough to not be optimized away. This PR refactors memchr so that it exists early when the slice is short enough.
Codegen-wise, as shown in #75659, memchr was not inlined previously so the only way I could find to test this is to check if there is no memchr call. Let me know if there is a more robust solution here.
It's possible for method resolution to pick this method over a lower
priority stable method, causing compilation errors. Since this method
is permanently unstable, give it a name that is very unlikely to be used
in user code.
Make [].as_[mut_]ptr_range() (unstably) const.
Gated behind `const_ptr_offset`, as suggested by https://github.com/rust-lang/rust/issues/65807#issuecomment-697229404
This also marks `[].as_mut_ptr()` as const, because it's used by `as_mut_ptr_range`. I gated it behind the same feature, because I figured it's not worth adding a separate tracking issue for const `as_mut_ptr`.
