This requires introducing a new internal type `RcUninit` (and
`ArcUninit`), which can own an `RcBox<T>` without requiring it to be
initialized, sized, or a slice. This is similar to `UniqueRc`, but
`UniqueRc` doesn't support the allocator parameter, and there is no
`UniqueArc`.
Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.
This makes their intent and expected location clearer. We see some
examples where these comments were not clearly separate from `use`
declarations, which made it hard to understand what the comment is
describing.
`UniqueRc`: support allocators and `T: ?Sized`.
Added the following (all unstable):
* Defaulted type pararameter `A: Allocator`.
* `UniqueRc::new_in()`.
* `T: ?Sized` where possible.
* `impl CoerceUnsized for UniqueRc`.
These changes are motivated by supporting the implementation of unsized `Rc::make_mut()` (PR #116113), but are also intended to be obvious generalizations of `UniqueRc` to support the things `Rc` does.
r? ``````@the8472``````
Added the following (all unstable):
* Defaulted type pararameter `A: Allocator`.
* `UniqueRc::new_in()`.
* `T: ?Sized` where possible.
* `impl CoerceUnsized for UniqueRc`.
* Drive-by doc polish: links and periods at the end of sentences.
These changes are motivated by supporting the implementation of unsized
`Rc::make_mut()` (PR #116113), but are also intended to be obvious
generalizations of `UniqueRc` to support the things `Rc` does.
Add `size_of` and `size_of_val` and `align_of` and `align_of_val` to the prelude
(Note: need to update the PR to add `align_of` and `align_of_val`, and remove the second commit with the myriad changes to appease the lint.)
Many, many projects use `size_of` to get the size of a type. However,
it's also often equally easy to hardcode a size (e.g. `8` instead of
`size_of::<u64>()`). Minimizing friction in the use of `size_of` helps
ensure that people use it and make code more self-documenting.
The name `size_of` is unambiguous: the name alone, without any prefix or
path, is self-explanatory and unmistakeable for any other functionality.
Adding it to the prelude cannot produce any name conflicts, as any local
definition will silently shadow the one from the prelude. Thus, we don't
need to wait for a new edition prelude to add it.
Fix `VecDeque::shrink_to` UB when `handle_alloc_error` unwinds.
Fixes#123369
For `VecDeque` it's relatively simple to restore the buffer into a consistent state so this PR does just that.
Note that with its current implementation, `shrink_to` may change the internal arrangement of elements in the buffer, so e.g. `[D, <uninit>, A, B, C]` will become `[<uninit>, A, B, C, D]` and `[<uninit>, <uninit>, A, B, C]` may become `[B, C, <uninit>, <uninit>, A]` if `shrink_to` unwinds. This shouldn't be an issue though as we don't make any guarantees about the stability of the internal buffer arrangement (and this case is impossible to hit on stable anyways).
This PR also includes a test with code adapted from #123369 which fails without the new `shrink_to` code. Does this suffice or do we maybe need more exhaustive tests like in #108475?
cc `@Amanieu`
`@rustbot` label +T-libs
Add opt-for-size core lib feature flag
Adds a feature flag to the core library that enables the possibility to have smaller implementations for certain algorithms.
So far, the core lib has traded performance for binary size. This is likely what most people want since they have big simd-capable machines. However, people on small machines, like embedded devices, don't enjoy the potential speedup of the bigger algorithms, but do have to pay for them. These microcontrollers often only have 16-1024kB of flash memory.
This PR is the result of some talks with project members like `@Amanieu` at RustNL.
There are some open questions of how this is eventually stabilized, but it's a similar question as with the existing `panic_immediate_abort` feature.
Speaking as someone from the embedded side, we'd rather have this unstable for a while as opposed to not having it at all. In the meantime we can try to use it and also add additional PRs to the core lib that uses the feature flag in areas where we find benefit.
Open questions from my side:
- Is this a good feature name?
- `panic_immediate_abort` is fairly verbose, so I went with something equally verbose
- It's easy to refactor later
- I've added the feature to `std` and `alloc` as well as they might benefit too. Do we agree?
- I expect these to get less usage out of the flag since most size-constraint projects don't use these libraries often.
Add `fn into_raw_with_allocator` to Rc/Arc/Weak.
Split out from #119761
Add `fn into_raw_with_allocator` for `Rc`/`rc::Weak`[^1]/`Arc`/`sync::Weak`.
* Pairs with `from_raw_in` (which already exists on all 4 types).
* Name matches `Box::into_raw_with_allocator`.
* Associated fns on `Rc`/`Arc`, methods on `Weak`s.
<details> <summary>Future PR/ACP</summary>
As a follow-on to this PR, I plan to make a PR/ACP later to move `into_raw(_parts)` from `Container<_, A: Allocator>` to only `Container<_, Global>` (where `Container` = `Vec`/`Box`/`Rc`/`rc::Weak`/`Arc`/`sync::Weak`) so that users of non-`Global` allocators have to explicitly handle the allocator when using `into_raw`-like APIs.
The current behaviors of stdlib containers are inconsistent with respect to what happens to the allocator when `into_raw` is called (which does not return the allocator)
| Type | `into_raw` currently callable with | behavior of `into_raw`|
| --- | --- | --- |
| `Box` | any allocator | allocator is [dropped](https://doc.rust-lang.org/nightly/src/alloc/boxed.rs.html#1060) |
| `Vec` | any allocator | allocator is [forgotten](https://doc.rust-lang.org/nightly/src/alloc/vec/mod.rs.html#884) |
| `Arc`/`Rc`/`Weak` | any allocator | allocator is [forgotten](https://doc.rust-lang.org/src/alloc/sync.rs.html#1487)(Arc) [(sync::Weak)](https://doc.rust-lang.org/src/alloc/sync.rs.html#2726) [(Rc)](https://doc.rust-lang.org/src/alloc/rc.rs.html#1352) [(rc::Weak)](https://doc.rust-lang.org/src/alloc/rc.rs.html#2993) |
In my opinion, neither implicitly dropping nor implicitly forgetting the allocator is ideal; dropping it could immediately invalidate the returned pointer, and forgetting it could unintentionally leak memory. My (to-be) proposed solution is to just forbid calling `into_raw(_parts)` on containers with non-`Global` allocators, and require calling `into_raw_with_allocator`(/`Vec::into_raw_parts_with_alloc`)
</details>
[^1]: Technically, `rc::Weak::into_raw_with_allocator` is not newly added, as it was modified and renamed from `rc::Weak::into_raw_and_alloc`.
Fix#124275: Implemented Default for `Arc<str>`
With added implementations.
```
GOOD Arc<CStr>
BROKEN Arc<OsStr> // removed
GOOD Rc<str>
GOOD Rc<CStr>
BROKEN Rc<OsStr> // removed
GOOD Rc<[T]>
GOOD Arc<[T]>
```
For discussion of https://github.com/rust-lang/rust/pull/124367#issuecomment-2091940137.
Key pain points currently:
> I've had a guess at the best locations/feature attrs for them but they might not be correct.
> However I'm unclear how to get the OsStr impl to compile, which file should they go in to avoid the error below? Is it possible, perhaps with some special std rust lib magic?
alloc: implement FromIterator for Box<str>
`Box<[T]>` implements `FromIterator<T>` using `Vec<T>` + `into_boxed_slice()`.
Add analogous `FromIterator` implementations for `Box<str>`
matching the current implementations for `String`.
Remove the `Global` allocator requirement for `FromIterator<Box<str>>` too.
ACP: https://github.com/rust-lang/libs-team/issues/196
LLVM currently adds a redundant check for the returned option, in addition
to the `self.ptr != self.end` check when using the default
`Iterator::fold` method that calls `vec::IntoIter::next` in a loop.
Many, many projects use `size_of` to get the size of a type. However,
it's also often equally easy to hardcode a size (e.g. `8` instead of
`size_of::<u64>()`). Minimizing friction in the use of `size_of` helps
ensure that people use it and make code more self-documenting.
The name `size_of` is unambiguous: the name alone, without any prefix or
path, is self-explanatory and unmistakeable for any other functionality.
Adding it to the prelude cannot produce any name conflicts, as any local
definition will silently shadow the one from the prelude. Thus, we don't
need to wait for a new edition prelude to add it.
Add `size_of_val`, `align_of`, and `align_of_val` as well, with similar
justification: widely useful, self-explanatory, unmistakeable for
anything else, won't produce conflicts.
