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>
"Some is returned with <some value>" is an awkward construction.
The use of the passive voice is a bit odd, and doesn't seem like the
house style.
So say instead "returns X, wrapped in `Some`", for which there is some
other precedent in stdlib.
Instead of repeating "with the prefix removed", say "after the
prefix". This is a bit clearer that the original is not modified.
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
Allow generic parameters in intra-doc links
Fixes#62834.
---
The contents of the generics will be mostly ignored (except for warning
if fully-qualified syntax is used, which is currently unsupported in
intra-doc links - see issue #74563).
* Allow links like `Vec<T>`, `Result<T, E>`, and `Option<Box<T>>`
* Allow links like `Vec::<T>::new()`
* Warn on
* Unbalanced angle brackets (e.g. `Vec<T` or `Vec<T>>`)
* Missing type to apply generics to (`<T>` or `<Box<T>>`)
* Use of fully-qualified syntax (`<Vec as IntoIterator>::into_iter`)
* Invalid path separator (`Vec:<T>:new`)
* Too many angle brackets (`Vec<<T>>`)
* Empty angle brackets (`Vec<>`)
Note that this implementation *does* allow some constructs that aren't
valid in the actual Rust syntax, for example `Box::<T>new()`. That may
not be supported in rustdoc in the future; it is an implementation
detail.
Give `impl Trait` in a `const fn` its own feature gate
...previously it was gated under `#![feature(const_fn)]`.
I think we actually want to do this in all const-contexts? If so, this should be `#![feature(const_impl_trait)]` instead. I don't think there's any way to make use of `impl Trait` within a `const` initializer.
cc #77463
r? `@oli-obk`
Eliminate bounds checking in slice::Windows
This is how `<core::slice::Windows as Iterator>::next` looks right now:
```rust
fn next(&mut self) -> Option<&'a [T]> {
if self.size > self.v.len() {
None
} else {
let ret = Some(&self.v[..self.size]);
self.v = &self.v[1..];
ret
}
}
```
The line with `self.v = &self.v[1..];` relies on assumption that `self.v` is definitely not empty at this point. Else branch is taken when `self.size <= self.v.len()`, so `self.v` can be empty if `self.size` is zero. In practice, since `Windows` is never created directly but rather trough `[T]::windows` which panics when `size` is zero, `self.size` is never zero. However, the compiler doesn't know about this check, so it keeps the code which checks bounds and panics.
Using `NonZeroUsize` lets the compiler know about this invariant and reliably eliminate bounds checking without `unsafe` on `-O2`. Here is assembly of `Windows<'a, u32>::next` before and after this change ([goldbolt](https://godbolt.org/z/xrefzx)):
<details>
<summary>Before</summary>
```
example::next:
push rax
mov rcx, qword ptr [rdi + 8]
mov rdx, qword ptr [rdi + 16]
cmp rdx, rcx
jbe .LBB0_2
xor eax, eax
pop rcx
ret
.LBB0_2:
test rcx, rcx
je .LBB0_5
mov rax, qword ptr [rdi]
mov rsi, rax
add rsi, 4
add rcx, -1
mov qword ptr [rdi], rsi
mov qword ptr [rdi + 8], rcx
pop rcx
ret
.LBB0_5:
lea rdx, [rip + .L__unnamed_1]
mov edi, 1
xor esi, esi
call qword ptr [rip + core::slice::slice_index_order_fail@GOTPCREL]
ud2
.L__unnamed_2:
.ascii "./example.rs"
.L__unnamed_1:
.quad .L__unnamed_2
.asciz "\f\000\000\000\000\000\000\000\016\000\000\000\027\000\000"
```
</details>
<details>
<summary>After</summary>
```
example::next:
mov rcx, qword ptr [rdi + 8]
mov rdx, qword ptr [rdi + 16]
cmp rdx, rcx
jbe .LBB0_2
xor eax, eax
ret
.LBB0_2:
mov rax, qword ptr [rdi]
lea rsi, [rax + 4]
add rcx, -1
mov qword ptr [rdi], rsi
mov qword ptr [rdi + 8], rcx
ret
```
</details>
Note the lack of call to `core::slice::slice_index_order_fail` in second snippet.
#### Possible reasons _not_ to merge this PR:
* this changes the error message on panic in `[T]::windows`. However, AFAIK this messages are not covered by backwards compatibility policy.
Implement advance_by, advance_back_by for iter::Chain
Part of #77404.
This PR does two things:
- implement `Chain::advance[_back]_by` in terms of `advance[_back]_by` on `self.a` and `advance[_back]_by` on `self.b`
- change `Chain::nth[_back]` to use `advance[_back]_by` on `self.a` and `nth[_back]` on `self.b`
This ensures that `Chain::nth` can take advantage of an efficient `nth` implementation on the second iterator, in case it doesn't implement `advance_by`.
cc `@scottmcm` in case you want to review this
Use more intra-doc-links in `core::fmt`
This is a follow-up to #75819, which encountered some broken links due to #75176, so this PR contains the links that are blocked on #75176.
r? @jyn514
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
Implement as_ne_bytes() for integers and floats
This is related to issue #64464.
I am pretty sure that these functions are actually const-ify-able, and technically as_bits() can also be implemented for floats, but I might need some comments on both.
Use less divisions in display u128/i128
This PR is an absolute mess, and I need to test if it improves the speed of fmt::Display for u128/i128, but I think it's correct.
It hopefully is more efficient by cutting u128 into at most 2 u64s, and also chunks by 1e16 instead of just 1e4.
Also I specialized the implementations for uints to always be non-false because it bothered me that it was checked at all
Do not merge until I benchmark it and also clean up the god awful mess of spaghetti.
Based on prior work in #44583
cc: `@Dylan-DPC`
Due to work on `itoa` and suggestion in original issue:
r? `@dtolnay`
