MIRI says `reverse` is UB, so replace it with something LLVM can vectorize
For small types with padding, the current implementation is UB because it does integer operations on uninit values.
```
error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/mod.rs:836:5
|
836 | / uint_impl! { u32, u32, i32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
837 | | "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]", "[0x12, 0x34, 0x56, 0x78]", "", "" }
| |________________________________________________________________________________________________^ using uninitialized data, but this operation requires initialized memory
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: inside `core::num::<impl u32>::rotate_left` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:211:13
= note: inside `core::slice::<impl [Foo]>::reverse` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/mod.rs:701:58
```
<https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=340739f22ca5b457e1da6f361768edc6>
But LLVM has gotten smarter since I wrote the previous implementation in 2017, so this PR removes all the manual magic and just writes it in such a way that LLVM will vectorize. This code is much simpler and has very little `unsafe`, and is actually faster to boot!
If you're curious to see the codegen: <https://rust.godbolt.org/z/Pcn13Y9E3>
Before:
```
running 7 tests
test slice::reverse_simd_f64x4 ... bench: 17,940 ns/iter (+/- 481) = 58448 MB/s
test slice::reverse_u128 ... bench: 17,758 ns/iter (+/- 205) = 59048 MB/s
test slice::reverse_u16 ... bench: 158,234 ns/iter (+/- 6,876) = 6626 MB/s
test slice::reverse_u32 ... bench: 62,047 ns/iter (+/- 1,117) = 16899 MB/s
test slice::reverse_u64 ... bench: 31,582 ns/iter (+/- 552) = 33201 MB/s
test slice::reverse_u8 ... bench: 81,253 ns/iter (+/- 1,510) = 12905 MB/s
test slice::reverse_u8x3 ... bench: 270,615 ns/iter (+/- 11,463) = 3874 MB/s
```
After:
```
running 7 tests
test slice::reverse_simd_f64x4 ... bench: 17,731 ns/iter (+/- 306) = 59137 MB/s
test slice::reverse_u128 ... bench: 17,919 ns/iter (+/- 239) = 58517 MB/s
test slice::reverse_u16 ... bench: 43,160 ns/iter (+/- 607) = 24295 MB/s
test slice::reverse_u32 ... bench: 21,065 ns/iter (+/- 371) = 49778 MB/s
test slice::reverse_u64 ... bench: 21,118 ns/iter (+/- 482) = 49653 MB/s
test slice::reverse_u8 ... bench: 76,878 ns/iter (+/- 1,688) = 13639 MB/s
test slice::reverse_u8x3 ... bench: 264,723 ns/iter (+/- 5,544) = 3961 MB/s
```
Those are the existing benches, <14a2fd640e/library/alloc/benches/slice.rs (L322-L346)>
For small types with padding, the current implementation is UB because it does integer operations on uninit values. But LLVM has gotten smarter since I wrote the previous implementation in 2017, so remove all the manual magic and just write it in such a way that LLVM will vectorize. This code is much simpler (albeit nuanced) and has very little `unsafe`, and is actually faster to boot!
Add #[must_use] to remaining core functions
I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is everything remaining from the `core` crate.
Ignored by clippy for reasons unknown:
```rust
core::alloc::Layout unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self;
core::any const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str;
```
Ignored by clippy because of `mut`:
```rust
str fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
```
<del>
Ignored by clippy presumably because a caller might want `f` called for side effects. That seems like a bad usage of `map` to me.
```rust
core::cell::Ref<'b, T> fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, T>;
core::cell::Ref<'b, T> fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>);
```
</del>
Parent issue: #89692
r? ```@joshtriplett```
Add #[must_use] to expensive computations
The unifying theme for this commit is weak, admittedly. I put together a list of "expensive" functions when I originally proposed this whole effort, but nobody's cared about that criterion. Still, it's a decent way to bite off a not-too-big chunk of work.
Given the grab bag nature of this commit, the messages I used vary quite a bit. I'm open to wording changes.
For some reason clippy flagged four `BTreeSet` methods but didn't say boo about equivalent ones on `HashSet`. I stared at them for a while but I can't figure out the difference so I added the `HashSet` ones in.
```rust
// Flagged by clippy.
alloc::collections::btree_set::BTreeSet<T> fn difference<'a>(&'a self, other: &'a BTreeSet<T>) -> Difference<'a, T>;
alloc::collections::btree_set::BTreeSet<T> fn symmetric_difference<'a>(&'a self, other: &'a BTreeSet<T>) -> SymmetricDifference<'a, T>
alloc::collections::btree_set::BTreeSet<T> fn intersection<'a>(&'a self, other: &'a BTreeSet<T>) -> Intersection<'a, T>;
alloc::collections::btree_set::BTreeSet<T> fn union<'a>(&'a self, other: &'a BTreeSet<T>) -> Union<'a, T>;
// Ignored by clippy, but not by me.
std::collections::HashSet<T, S> fn difference<'a>(&'a self, other: &'a HashSet<T, S>) -> Difference<'a, T, S>;
std::collections::HashSet<T, S> fn symmetric_difference<'a>(&'a self, other: &'a HashSet<T, S>) -> SymmetricDifference<'a, T, S>
std::collections::HashSet<T, S> fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S>;
std::collections::HashSet<T, S> fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S>;
```
Parent issue: #89692
r? ```@joshtriplett```
track_caller for slice length assertions
`clone_from_slice` was missing `#[track_caller]`, and its assert did not report a useful location.
These are small generic methods, so hopefully track_caller gets inlined into nothingness, but it may be worth running a benchmark on this.
Make RSplit<T, P>: Clone not require T: Clone
This addresses a TODO comment. The behavior of `#[derive(Clone)]` *does* result in a `T: Clone` requirement. Playground example:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a8b1a9581ff8893baf401d624a53d35b
Add a manual `Clone` implementation, mirroring `Split` and `SplitInclusive`.
`(R)?SplitN(Mut)?` don't have any `Clone` implementations, but I'll leave that for its own pull request.
This addresses a TODO comment. The behavior of #[derive(Clone)]
*does* result in a T: Clone requirement.
Add a manual Clone implementation, matching Split and SplitInclusive.
The unifying theme for this commit is weak, admittedly. I put together a
list of "expensive" functions when I originally proposed this whole
effort, but nobody's cared about that criterion. Still, it's a decent
way to bite off a not-too-big chunk of work.
Given the grab bag nature of this commit, the messages I used vary quite
a bit.
`[].split_inclusive()` currently yields a single, empty slice. That's
different from `"".split_inslusive()`, which yields no output at
all. I think that makes the slice version harder to use.
The case where I ran into this bug was when writing code for
generating a diff between two slices of bytes. I wanted to prefix
removed lines with "-" and a added lines with "+". Due to
`split_inclusive()`'s current behavior, that means that my code prints
just a "-" or "+" for empty files. I suspect most existing callers
have similar "bugs" (which would be fixed by this patch).
Closes#89716.
These methods could be misconstrued as modifying their arguments instead
of returning new values.
Where possible I made the note recommend a method that does mutate in
place.
Make `<[T]>::split_at_unchecked` and `<[T]>::split_at_mut_unchecked` public
The methods were originally added in https://github.com/rust-lang/rust/pull/75936 (30dc32b10e), but for some reason as private. Nevertheless, the methods have documentation and even a [tracking issue](https://github.com/rust-lang/rust/issues/76014).
It's very weird to have a tracking issue for private methods and these methods may be useful outside of the standard library. As such, this PR makes the methods public.
Remove ignore-tidy-undocumented-unsafe from core::slice::sort
Write down the missing safety arguments to be able to remove `ignore-tidy-undocumented-unsafe` from `core::slice::sort`.
Helps with #66219
``@rustbot`` label C-cleanup T-libs
Most of these are because alloc uses `#[lang_item]` to define methods,
but core documents primitives before those methods are available.
- Fix rustdoc-js-std test
For some reason this change made CStr not show up in the results for
`str,u8`. Since it still shows up for str, and since it wasn't a great
match for that query anyway, I think this is ok to let slide.
- Add test that all primitives can be linked to
- Enable `doc(primitive)` in `core` as well
- Add linkcheck exception specifically for Windows
Ideally this would be done automatically by the linkchecker by
replacing `\\` with forward slashes, but this PR is already a ton of
work ...
- Don't forcibly fail linkchecking if there's a broken intra-doc link on Windows
Previously, it would exit with a hard error if a missing file had `::`
in it. This changes it to report a missing file instead, which allows
adding an exception.
The libs-api team agrees to allow const_trait_impl to appear in the
standard library as long as stable code cannot be broken (they are
properly gated) this means if the compiler teams thinks it's okay, then
it's okay.
My priority on constifying would be:
1. Non-generic impls (e.g. Default) or generic impls with no
bounds
2. Generic functions with bounds (that use const impls)
3. Generic impls with bounds
4. Impls for traits with associated types
For people opening constification PRs: please cc me and/or oli-obk.
Adds extensive test for all the [r]split* iterators.
Fixes size_hint upper bound for split_inclusive* iterators which was one higher than necessary for non-empty slices.
Fixes size_hint lower bound for [r]splitn* iterators when n==0, which was one too high.
Partially stabilize `const_slice_first_last`
This stabilizes the non-`mut` methods of `const_slice_first_last` as `const`. These methods are trivial to implement and have no blockers that I am aware of.
`@rustbot` label +A-const-fn +S-waiting-on-review +T-libs-api
Implement advance_by, advance_back_by for slice::{Iter, IterMut}
Part of #77404.
Picking up where #77633 was closed.
I have addressed https://github.com/rust-lang/rust/pull/77633#issuecomment-771842599 by restoring `nth` and `nth_back`. So according to that comment this should already be r=m-ou-se, but it has been sitting for a while.
Add MIR pass to lower call to `core::slice::len` into `Len` operand
During some larger experiment with range analysis I've found that code like `let l = slice.len()` produces different MIR then one found in bound checks. This optimization pass replaces terminators that are calls to `core::slice::len` with just a MIR operand and Goto terminator.
It uses some heuristics to remove the outer borrow that is made to call `core::slice::len`, but I assume it can be eliminated, just didn't find how.
Would like to express my gratitude to `@oli-obk` who helped me a lot on Zullip
This method on the Iterator trait is doc(hidden), and about half of
implementations were doc(hidden). This adds the attribute to the
remaining implementations.
Integrate binary search codes of binary_search_by and partition_point
For now partition_point has own binary search code piece.
It is because binary_search_by had called the comparer more times and the author (=me) wanted to avoid it.
However, now binary_search_by uses the comparer minimum times. (#74024)
So it's time to integrate them.
The appearance of the codes are a bit different but both use completely same logic.
Implement indexing slices with pairs of core::ops::Bound<usize>
Closes#49976.
I am not sure about code duplication between `check_range` and `into_maybe_range`. Should be former implemented in terms of the latter? Also this PR doesn't address code duplication between `impl SliceIndex for Range*`.
Instruct LLVM that binary_search returns a valid index
This allows removing bound checks when the return value of `binary_search` is used to index into the slice it was call on. I also added a codegen test for this, not sure if it's the right thing to do (I didn't find anything on the dev guide), but it felt so.
Add function core::iter::zip
This makes it a little easier to `zip` iterators:
```rust
for (x, y) in zip(xs, ys) {}
// vs.
for (x, y) in xs.into_iter().zip(ys) {}
```
You can `zip(&mut xs, &ys)` for the conventional `iter_mut()` and
`iter()`, respectively. This can also support arbitrary nesting, where
it's easier to see the item layout than with arbitrary `zip` chains:
```rust
for ((x, y), z) in zip(zip(xs, ys), zs) {}
for (x, (y, z)) in zip(xs, zip(ys, zs)) {}
// vs.
for ((x, y), z) in xs.into_iter().zip(ys).zip(xz) {}
for (x, (y, z)) in xs.into_iter().zip((ys.into_iter().zip(xz)) {}
```
It may also format more nicely, especially when the first iterator is a
longer chain of methods -- for example:
```rust
iter::zip(
trait_ref.substs.types().skip(1),
impl_trait_ref.substs.types().skip(1),
)
// vs.
trait_ref
.substs
.types()
.skip(1)
.zip(impl_trait_ref.substs.types().skip(1))
```
This replaces the tuple-pair `IntoIterator` in #78204.
There is prior art for the utility of this in [`itertools::zip`].
[`itertools::zip`]: https://docs.rs/itertools/0.10.0/itertools/fn.zip.html
Generalize and inline slice::fill specializations
This makes the memset specialization applicable to more types. And since the code now lives in a generic method it is also eligible for cross-crate inlining which should fix#83235
Add `as_str` method for split whitespace str iterators
This PR adds `as_str` methods to `SplitWhitespace` and `SplitAsciiWhitespace`
str iterators. The methods return the remainder, similar to `as_str` methods on
`Chars` and other split iterators. This PR is a continuation of https://github.com/rust-lang/rust/pull/75265, which added `as_str` for all other str split iterators.
The feature gate for new methods is `#![feature(str_split_whitespace_as_str)]`.
`SplitWhitespace` and `SplitAsciiWhitespace` use iterators under the hood, so to implement `as_str` it's required to either
1. Make fields of some iterators `pub(crate)`
2. Add getter methods (like `into_inner`, `inner`, `inner_mut`...) to some (all) iterators
3. Completely rewrite `SplitWhitespace` and `SplitAsciiWhitespace`
This PR uses the 1. approach since it's easier to implement and requires fewer changes (and no changes to the public API). If you think that's not the right way, please, tell me.
r? `@m-ou-se`
Improve slice.binary_search_by()'s best-case performance to O(1)
This PR aimed to improve the [slice.binary_search_by()](https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search_by)'s best-case performance to O(1).
# Noticed
I don't know why the docs of `binary_search_by` said `"If there are multiple matches, then any one of the matches could be returned."`, but the implementation isn't the same thing. Actually, it returns the **last one** if multiple matches found.
Then we got two options:
## If returns the last one is the correct or desired result
Then I can rectify the docs and revert my changes.
## If the docs are correct or desired result
Then my changes can be merged after fully reviewed.
However, if my PR gets merged, another issue raised: this could be a **breaking change** since if multiple matches found, the returning order no longer the last one instead of it could be any one.
For example:
```rust
let mut s = vec![0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
let num = 1;
let idx = s.binary_search(&num);
s.insert(idx, 2);
// Old implementations
assert_eq!(s, [0, 1, 1, 1, 1, 2, 2, 3, 5, 8, 13, 21, 34, 42, 55]);
// New implementations
assert_eq!(s, [0, 1, 1, 1, 2, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);
```
# Benchmarking
**Old implementations**
```sh
$ ./x.py bench --stage 1 library/libcore
test slice::binary_search_l1 ... bench: 59 ns/iter (+/- 4)
test slice::binary_search_l1_with_dups ... bench: 59 ns/iter (+/- 3)
test slice::binary_search_l2 ... bench: 76 ns/iter (+/- 5)
test slice::binary_search_l2_with_dups ... bench: 77 ns/iter (+/- 17)
test slice::binary_search_l3 ... bench: 183 ns/iter (+/- 23)
test slice::binary_search_l3_with_dups ... bench: 185 ns/iter (+/- 19)
```
**New implementations (1)**
Implemented by this PR.
```rust
if cmp == Equal {
return Ok(mid);
} else if cmp == Less {
base = mid
}
```
```sh
$ ./x.py bench --stage 1 library/libcore
test slice::binary_search_l1 ... bench: 58 ns/iter (+/- 2)
test slice::binary_search_l1_with_dups ... bench: 37 ns/iter (+/- 4)
test slice::binary_search_l2 ... bench: 76 ns/iter (+/- 3)
test slice::binary_search_l2_with_dups ... bench: 57 ns/iter (+/- 6)
test slice::binary_search_l3 ... bench: 200 ns/iter (+/- 30)
test slice::binary_search_l3_with_dups ... bench: 157 ns/iter (+/- 6)
$ ./x.py bench --stage 1 library/libcore
test slice::binary_search_l1 ... bench: 59 ns/iter (+/- 8)
test slice::binary_search_l1_with_dups ... bench: 37 ns/iter (+/- 2)
test slice::binary_search_l2 ... bench: 77 ns/iter (+/- 2)
test slice::binary_search_l2_with_dups ... bench: 57 ns/iter (+/- 2)
test slice::binary_search_l3 ... bench: 198 ns/iter (+/- 21)
test slice::binary_search_l3_with_dups ... bench: 158 ns/iter (+/- 11)
```
**New implementations (2)**
Suggested by `@nbdd0121` in [comment](https://github.com/rust-lang/rust/pull/74024#issuecomment-665430239).
```rust
base = if cmp == Greater { base } else { mid };
if cmp == Equal { break }
```
```sh
$ ./x.py bench --stage 1 library/libcore
test slice::binary_search_l1 ... bench: 59 ns/iter (+/- 7)
test slice::binary_search_l1_with_dups ... bench: 37 ns/iter (+/- 5)
test slice::binary_search_l2 ... bench: 75 ns/iter (+/- 3)
test slice::binary_search_l2_with_dups ... bench: 56 ns/iter (+/- 3)
test slice::binary_search_l3 ... bench: 195 ns/iter (+/- 15)
test slice::binary_search_l3_with_dups ... bench: 151 ns/iter (+/- 7)
$ ./x.py bench --stage 1 library/libcore
test slice::binary_search_l1 ... bench: 57 ns/iter (+/- 2)
test slice::binary_search_l1_with_dups ... bench: 38 ns/iter (+/- 2)
test slice::binary_search_l2 ... bench: 77 ns/iter (+/- 11)
test slice::binary_search_l2_with_dups ... bench: 57 ns/iter (+/- 4)
test slice::binary_search_l3 ... bench: 194 ns/iter (+/- 15)
test slice::binary_search_l3_with_dups ... bench: 151 ns/iter (+/- 18)
```
I run some benchmarking testings against on two implementations. The new implementation has a lot of improvement in duplicates cases, while in `binary_search_l3` case, it's a little bit slower than the old one.
Turn may_have_side_effect into an associated constant
The `may_have_side_effect` is an implementation detail of `TrustedRandomAccess`
trait. It describes if obtaining an iterator element may have side effects. It
is currently implemented as an associated function.
Turn `may_have_side_effect` into an associated constant. This makes the
value immediately available to the optimizer.
Convert primitives in the standard library to intra-doc links
Blocked on https://github.com/rust-lang/rust/pull/80181. I forgot that this needs to wait for the beta bump so the standard library can be documented with `doc --stage 0`.
Notably I didn't convert `core::slice` because it's like 50 links and I got scared 😨
Specialize slice::fill with Copy type and u8/i8/bool
I don't expect rustperf could measure any perf improvements with this changes
since `slice::fill` is newly added.
Godbolt link for this change: <https://rust.godbolt.org/z/r3fzee>.
r? `@matthewjasper` since this patch added new specialization.
This commit adds `as_str` methods to `SplitWhitespace` and `SplitAsciiWhitespace`
str iterators. The methods return the remainder, similar to `as_str` methods on
`Chars` and other split iterators.
This commit also makes fields of some iterators `pub(crate)`.
Improve design of `assert_len`
It was discussed in the [tracking issue](https://github.com/rust-lang/rust/issues/76393#issuecomment-761765448) that `assert_len`'s name and usage are confusing. This PR improves them based on a suggestion by ``@scottmcm`` in that issue.
I also improved the documentation to make it clearer when you might want to use this method.
Old example:
```rust
let range = range.assert_len(slice.len());
```
New example:
```rust
let range = range.ensure_subset_of(..slice.len());
```
Fixes#81157
The `may_have_side_effect` is an implementation detail of `TrustedRandomAccess`
trait. It describes if obtaining an iterator element may have side effects. It
is currently implemented as an associated function.
Turn `may_have_side_effect` into an associated constant. This makes the
value immediately available to the optimizer.
Stabilize `core::slice::fill_with`
_Tracking issue: https://github.com/rust-lang/rust/issues/79221_
This stabilizes the `slice_fill_with` feature for Rust 1.51, following the stabilization of `slice_fill` in 1.50. This was requested by libs team members in https://github.com/rust-lang/rust/pull/79213.
This PR also adds the "memset" alias for `slice::fill_with`, mirroring the alias set on the `slice::fill` sibling API. This will ensure someone looking for "memset" will find both variants.
r? `@Amanieu`
Rework diagnostics for wrong number of generic args (fixes#66228 and #71924)
This PR reworks the `wrong number of {} arguments` message, so that it provides more details and contextual hints.
Stabilize split_inclusive
### Contents of this MR
This stabilises:
* `slice::split_inclusive`
* `slice::split_inclusive_mut`
* `str::split_inclusive`
Closes#72360.
### A possible concern
The proliferation of `split_*` methods is not particularly pretty. The existence of `split_inclusive` seems to invite the addition of `rsplit_inclusive`, `splitn_inclusive`, etc. We could instead have a more general API, along these kinds of lines maybe:
```
pub fn split_generic('a,P,H>(&'a self, pat: P, how: H) -> ...
where P: Pattern
where H: SplitHow;
pub fn split_generic_mut('a,P,H>(&'a mut self, pat: P, how: H) -> ...
where P: Pattern
where H: SplitHow;
trait SplitHow {
fn reverse(&self) -> bool;
fn inclusive -> bool;
fn limit(&self) -> Option<usize>;
}
pub struct SplitFwd;
...
pub struct SplitRevInclN(pub usize);
```
But maybe that is worse.
### Let us defer that? ###
This seems like a can of worms. I think we can defer opening it now; if and when we have something more general, these two methods can become convenience aliases. But I thought I would mention it so the lang API team can consider it and have an opinion.
Stabilize slice::strip_prefix and slice::strip_suffix
These two methods are useful. The corresponding methods on `str` are already stable.
I believe that stablising these now would not get in the way of, in the future, extending these to take a richer pattern API a la `str`'s patterns.
Tracking PR: #73413. I also have an outstanding PR to improve the docs for these two functions and the corresponding ones on `str`: #75078
I have tried to follow the [instructions in the dev guide](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr). The part to do with `compiler/rustc_feature` did not seem applicable. I assume that's because these are just library features, so there is no corresponding machinery in rustc.
The return of the GroupBy and GroupByMut iterators on slice
According to https://github.com/rust-lang/rfcs/pull/2477#issuecomment-742034372, I am opening this PR again, this time I implemented it in safe Rust only, it is therefore much easier to read and is completely safe.
This PR proposes to add two new methods to the slice, the `group_by` and `group_by_mut`. These two methods provide a way to iterate over non-overlapping sub-slices of a base slice that are separated by the predicate given by the user (e.g. `Partial::eq`, `|a, b| a.abs() < b.abs()`).
```rust
let slice = &[1, 1, 1, 3, 3, 2, 2, 2];
let mut iter = slice.group_by(|a, b| a == b);
assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
assert_eq!(iter.next(), Some(&[3, 3][..]));
assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
assert_eq!(iter.next(), None);
```
[An RFC](https://github.com/rust-lang/rfcs/pull/2477) was open 2 years ago but wasn't necessary.
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`.
Add array_windows fn
This mimicks the functionality added by array_chunks, and implements a const-generic form of
`windows`. It makes egregious use of `unsafe`, but by necessity because the array must be
re-interpreted as a slice of arrays, and unlike array_chunks this cannot be done by casting the
original array once, since each time the index is advanced it needs to move one element, not
`N`.
I'm planning on adding more tests, but this should be good enough as a premise for the functionality.
Notably: should there be more functions overwritten for the iterator implementation/in general?
~~I've marked the issue as #74985 as there is no corresponding exact issue for `array_windows`, but it's based of off `array_chunks`.~~
Edit: See Issue #75027 created by @lcnr for tracking issue
~~Do not merge until I add more tests, please.~~
r? @lcnr
Updated issue to #75027
Update to rm oob access
And hopefully fix docs as well
Fixed naming conflict in test
Fix test which used 1-indexing
Nth starts from 0, woops
Fix a bunch of off by 1 errors
See https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=757b311987e3fae1ca47122969acda5a
Add even more off by 1 errors
And also write `next` and `next_back` in terms of `nth` and `nth_back`.
Run fmt
Fix forgetting to change fn name in test
add nth_back test & document unsafe
Remove as_ref().unwrap()
Documented occurrences of unsafe, noting what invariants are maintained
Add `slice::array_chunks_mut`
This follows `array_chunks` from #74373 with a mutable version, `array_chunks_mut`. The implementation is identical apart from mutability. The new tests are adaptations of the `chunks_exact_mut` tests, plus an inference test like the one for `array_chunks`.
I reused the unstable feature `array_chunks` and tracking issue #74985, but I can separate that if desired.
r? `@withoutboats`
cc `@lcnr`
Indent a note to make folding work nicer
Sublime Text folds code based on indentation. It maybe an unnecessary change, but does it look nicer after that ?
rustdoc: do not use plain summary for trait impls
Fixes#38386.
Fixes#48332.
Fixes#49430.
Fixes#62741.
Fixes#73474.
Unfortunately this is not quite ready to go because the newly-working links trigger a bunch of linkcheck failures. The failures are tough to fix because the links are resolved relative to the implementor, which could be anywhere in the module hierarchy.
(In the current docs, these links end up rendering as uninterpreted markdown syntax, so I don't think these failures are any worse than the status quo. It might be acceptable to just add them to the linkchecker whitelist.)
Ideally this could be fixed with intra-doc links ~~but it isn't working for me: I am currently investigating if it's possible to solve it this way.~~ Opened #73829.
EDIT: This is now ready!
Get rid of bounds check in slice::chunks_exact() and related function…
…s during construction
LLVM can't figure out in
let rem = self.len() % chunk_size;
let len = self.len() - rem;
let (fst, snd) = self.split_at(len);
and
let rem = self.len() % chunk_size;
let (fst, snd) = self.split_at(rem);
that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.
Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.
LLVM can't figure out in
let rem = self.len() % chunk_size;
let len = self.len() - rem;
let (fst, snd) = self.split_at(len);
and
let rem = self.len() % chunk_size;
let (fst, snd) = self.split_at(rem);
that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.
Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.
These are unsafe variants of the non-unchecked functions and don't do
any bounds checking.
For the time being these are not public and only a preparation for the
following commit. Making it public and stabilization can follow later
and be discussed in https://github.com/rust-lang/rust/issues/76014 .
`alloc::slice` uses `core::slice` functions, documentation are copied
from there and the links as well without resolution. `crate::ptr...`
cannot be resolved in `alloc::slice`, but `ptr` itself is imported in
both `alloc::slice` and `core::slice`, so we used that instead.
