Stabilize `core::slice::fill`
Tracking issue https://github.com/rust-lang/rust/issues/70758
Stabilizes the `core::slice::fill` API in Rust 1.50, adding a `memset` doc alias so people coming from C/C++ looking for this operation can find it in the docs. This API hasn't seen any changes since we changed the signature in https://github.com/rust-lang/rust/pull/71165/, and it seems like the right time to propose stabilization. Thanks!
r? `@m-ou-se`
Deprecate atomic compare_and_swap method
Finish implementing [RFC 1443](https://github.com/rust-lang/rfcs/blob/master/text/1443-extended-compare-and-swap.md) (https://github.com/rust-lang/rfcs/pull/1443).
It was decided to deprecate `compare_and_swap` [back in Rust 1.12 already](https://github.com/rust-lang/rust/issues/31767#issuecomment-215903038). I can't find any info about that decision being reverted. My understanding is just that it has been forgotten. If there has been a decision on keeping `compare_and_swap` then it's hard to find, and even if this PR does not go through it can act as a place where people can find out about the decision being reverted.
Atomic operations are hard to understand, very hard. And it does not help that there are multiple similar methods to do compare and swap with. They are so similar that for a reader it might be hard to understand the difference. This PR aims to make that simpler by finally deprecating `compare_and_swap` which is essentially just a more limited version of `compare_exchange`. The documentation is also updated (according to the RFC text) to explain the differences a bit better.
Even if we decide to not deprecate `compare_and_swap`. I still think the documentation for the atomic operations should be improved to better describe their differences and similarities. And the documentation can be written nicer than the PR currently proposes, but I wanted to start somewhere. Most of it is just copied from the RFC.
The documentation for `compare_exchange` and `compare_exchange_weak` indeed describe how they work! The problem is that they are more complex and harder to understand than `compare_and_swap`. So for someone who does not fully grasp this they might fall back to using `compare_and_swap`. Making the documentation outline the similarities and differences might build a bridge for people so they can cross over to the more powerful and sometimes more efficient operations.
The conversions I do to avoid the `std` internal deprecation errors are very straight forward `compare_and_swap -> compare_exchange` changes where the orderings are just using the mapping in the new documentation. Only in one place did I use `compare_exchange_weak`. This can probably be improved further. But the goal here was not for those operations to be perfect. Just to not get worse and to allow the deprecation to happen.
Added [T; N]::zip()
This is my first PR to rust so I hope I have done everything right, or at least close :)
---
This is PR adds the array method `[T; N]::zip()` which, in my mind, is a natural extension to #75212.
My implementation of `zip()` is mostly just a modified copy-paste of `map()`. Should I keep the comments? Also am I right in assuming there should be no way for the `for`-loop to panic, thus no need for the dropguard seen in the `map()`-function?
The doc comment is in a similar way a slightly modified copy paste of [`Iterator::zip()`](https://doc.rust-lang.org/beta/std/iter/trait.Iterator.html#method.zip)
`@jplatte` mentioned in [#75490](https://github.com/rust-lang/rust/pull/75490#issuecomment-677790758) `zip_with()`,
> zip and zip_with seem like they would be useful :)
is this something I should add (assuming there is interest for this PR at all :))
Fix memory leak in test "mem::uninit_write_slice_cloned_no_drop"
This fixes#80116. I replaced the `Rc` based method I was using with a type that panics when dropped.
Move {f32,f64}::clamp to core.
`clamp` was recently stabilized (tracking issue: https://github.com/rust-lang/rust/issues/44095). But although `Ord::clamp` was added in `core` (because `Ord` is in `core`), the versions for the `f32` and `f64` primitives were added in `std` (together with `floor`, `sin`, etc.), not in `core` (together with `min`, `max`, `from_bits`, etc.).
This change moves them to `core`, such that `clamp` on floats is available in `no_std` programs as well.
Integer types have a `count_ones` method that end up calling
`intrinsics::ctpop`.
On some architectures, that intrinsic is translated as a corresponding
CPU instruction know as "popcount" or "popcnt".
This PR makes it so that searching for those names in rustdoc shows those methods.
CC https://blog.rust-lang.org/2020/11/19/Rust-1.48.html#adding-search-aliases
MaybeUninit::copy/clone_from_slice
This PR adds 2 new methods to MaybeUninit under the feature of `maybe_uninit_write_slice`: `copy_from_slice` and `clone_from_slice`.
These are useful for initializing uninitialized buffers (such as the one returned by `Vec::spare_capacity_mut` for example) with initialized data.
The methods behave similarly to the methods on slices, but the destination is uninitialized and they return the destination slice as an initialized slice.
std::iter: document iteration over `&T` and `&mut T`
A colleague of mine is new to Rust, and mentioned that it was “slightly
confusing” to figure out what `&mut` does in iterating over `&mut foo`:
```rust
for value in &mut self.my_vec {
// ...
}
```
My colleague had read the `std::iter` docs and not found the answer
there. There is a brief section at the top about “the three forms of
iteration”, which mentions `iter_mut`, but it doesn’t cover the purpose
of `&mut coll` for a collection `coll`. This patch adds an explanatory
section to the docs. I opted to create a new section so that it can
appear after the note that `impl<I: Iterator> IntoIterator for I`, and
it’s nice for the existing “three forms of iteration” to appear near the
top.
Test Plan:
Ran `./x.py doc library/core`, and the result looked good, including
links. Manually copy-pasted the two doctests into the playground and ran
them.
wchargin-branch: doc-iter-by-reference
Add some core::cmp::Ordering helpers
...to allow easier equal-to-or-greater-than and less-than-or-equal-to
comparisons.
Prior to Rust 1.42 a greater-than-or-equal-to comparison might be written
either as a match block, or a traditional conditional check like this:
```rust
if cmp == Ordering::Equal || cmp == Ordering::Greater {
// Do something
}
```
Which requires two instances of `cmp`. Don't forget that while `cmp` here
is very short, it could be something much longer in real use cases.
From Rust 1.42 a nicer alternative is possible:
```rust
if matches!(cmp, Ordering::Equal | Ordering::Greater) {
// Do something
}
```
The commit adds another alternative which may be even better in some cases:
```rust
if cmp.is_equal_or_greater() {
// Do something
}
```
The earlier examples could be cleaner than they are if the variants of
`Ordering` are imported such that `Equal`, `Greater` and `Less` can be
referred to directly, but not everyone will want to do that.
The new solution can shorten lines, help avoid logic mistakes, and avoids
having to import `Ordering` / `Ordering::*`.
...to allow easier greater-than-or-equal-to and less-than-or-equal-to
comparisons, and variant checking without needing to import the enum,
similar to `Option::is_none()` / `Option::is_some()`, in situations where
you are dealing with an `Ordering` value. (Simple `PartialOrd` / `Ord`
based evaluation may not be suitable for all situations).
Prior to Rust 1.42 a greater-than-or-equal-to comparison might be written
either as a match block, or a traditional conditional check like this:
```rust
if cmp == Ordering::Equal || cmp == Ordering::Greater {
// Do something
}
```
Which requires two instances of `cmp`. Don't forget that while `cmp` here
is very short, it could be something much longer in real use cases.
From Rust 1.42 a nicer alternative is possible:
```rust
if matches!(cmp, Ordering::Equal | Ordering::Greater) {
// Do something
}
```
The commit adds another alternative which may be even better in some cases:
```rust
if cmp.is_ge() {
// Do something
}
```
The earlier examples could be cleaner than they are if the variants of
`Ordering` are imported such that `Equal`, `Greater` and `Less` can be
referred to directly, but not everyone will want to do that.
The new solution can shorten lines, help avoid logic mistakes, and avoids
having to import `Ordering` / `Ordering::*`.
Constier maybe uninit
I was playing around trying to make `[T; N]::zip()` in #79451 be `const fn`. One of the things I bumped into was `MaybeUninit::assume_init`. Is there any reason for the intrinsic `assert_inhabited<T>()` and therefore `MaybeUninit::assume_init` not being `const`?
---
I have as best as I could tried to follow the instruction in [library/core/src/intrinsics.rs](https://github.com/rust-lang/rust/blob/master/library/core/src/intrinsics.rs#L11). I have no idea what I am doing but it seems to compile after some slight changes after the copy paste. Is this anywhere near how this should be done?
Also any ideas for name of the feature gate? I guess `const_maybe_assume_init` is quite misleading since I have added some more methods. Should I add test? If so what should be tested?
