`BTreeMap::merge` optimized
This is an optimized version of https://github.com/rust-lang/rust/pull/151981. See [ACP](https://github.com/rust-lang/libs-team/issues/739#issuecomment-3873487320) for more information on `BTreeMap::merge` does.
CC @programmerjake. Let me know what you think of how I'm using `CursorMut` and `IntoIter` here and whether the unsafe code here looks good. I decided to use `ptr::read()` and `ptr::write()` to grab the value from `CursorMut` as `V` than `&mut V`, use it within the `conflict` function, and overwrite the content of conflicting key afterward.
I know this needs some polishing, especially with refactoring some redundant looking code in a nicer way, some of which could probably just be public API methods for `CursorMut`. It does pass all the tests that I currently have for `BTreeMap::merge` (inspired from `BTreeMap::append`) though, so that's good.
Add try_shrink_to and try_shrink_to_fit to Vec
Adds the functions `try_shrink_to` and `try_shrink_to_fit` to Vec to allow shrinking
in environments without global OOM handling.
The implementation differs from the tracking issue as the fallible methods return
a `TryReserveError` instead of an `AllocError` as `AllocError` is unstable and does
not contain layout information.
Tracking:
- rust-lang/rust#152350
* Add try_shrink_to and try_shrink_to_fit to Vec
Both functions are required to support shrinking a vector in
environments without global OOM handling.
* Format the try_shrink functions
* Remove excess "```" from doc
* Remove `cfg(not(no_global_oom_handling))` from rawvecinner::shrink
* Fix import cmp even if no_global_oom_handling is defined
Simplify internals of `{Rc,Arc}::default`
This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang/rust#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change.
The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.
We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it.
Conveniently, it can even be safe that way!
Pass alignments through the shim as `Alignment` (not `usize`)
We're using `Layout` on both sides, so might as well skip the transmutes back and forth to `usize`.
The mir-opt test shows that doing so allows simplifying the boxed-slice drop slightly, for example.
This commit simplifies the internal implementation of `Default` for
these two pointer types to have the same performance characteristics as
before (a side effect of changes in 131460) while avoid use of internal
private APIs of Rc/Arc. To preserve the same codegen as before some
non-generic functions needed to be tagged as `#[inline]` as well, but
otherwise the same IR is produced before/after this change.
The motivation of this commit is I was studying up on the state of
initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the
use of internal APIs and instead use public stable APIs where possible,
even in the implementation itself.
We're using `Layout` on both sides, so might as well skip the transmutes back and forth to `usize`.
The mir-opt test shows that doing so allows simplifying the boxed-slice drop slightly, for example.
Add some clarifications and fixes for fmt syntax
This tries to clarify a few things regarding fmt syntax:
- The comment on `Parser::word` seems to be wrong, as that underscore-prefixed words are just fine. This was changed in https://github.com/rust-lang/rust/pull/66847.
- I struggled to follow the description of the width argument. It referred to a "second argument", but I don't know what second argument it is referring to (which is the first?). Either way, I rewrote the paragraph to try to be a little more explicit, and to use shorter sentences.
- The description of the precision argument wasn't really clear about the distinction of an Nth argument and a named argument. I added a sentence to try to emphasize the difference.
- `IDENTIFIER_OR_KEYWORD` was changed recently in https://github.com/rust-lang/reference/pull/2049 to include bare `_`. But fmt named arguments are not allowed to be a bare `_`.
remove multiple unhelpful `reason = "..."` values from `#[unstable(...)]` invocations
The vast majority of `#[unstable()]` attributes already has no explicit reason specified. This PR removes the `reason = "..."` value for the following unhelpful or meaningless reasons:
* "recently added"
* "new API"
* "recently redesigned"
* "unstable"
An example of how the message looks with and without a reason:
```rust
fn main() {
Vec::<()>::into_parts;
Vec::<()>::const_make_global;
}
```
```
error[E0658]: use of unstable library feature `box_vec_non_null`: new API
--> src/main.rs:2:5
|
2 | Vec::<()>::into_parts;
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #130364 <https://github.com/rust-lang/rust/issues/130364> for more information
= help: add `#![feature(box_vec_non_null)]` to the crate attributes to enable
= note: this compiler was built on 2026-01-15; consider upgrading it if it is out of date
error[E0658]: use of unstable library feature `const_heap`
--> src/main.rs:3:5
|
3 | Vec::<()>::const_make_global;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #79597 <https://github.com/rust-lang/rust/issues/79597> for more information
= help: add `#![feature(const_heap)]` to the crate attributes to enable
= note: this compiler was built on 2026-01-15; consider upgrading it if it is out of date
```
Most of the remaining reasons after this are something similar to "this is an implementation detail for XYZ" or "this is not public". If this PR is approved, I'll look into those next.
The PR also removes the `fd_read` feature gate. It only consists of one attribute applied to an implementation inside a module that is already private and unstable and should not be needed.
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