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Add APIs for uninitialized Box, Rc, and Arc. (Plus get_mut_unchecked)
Assigning `MaybeUninit::<Foo>::uninit()` to a local variable is usually free, even when `size_of::<Foo>()` is large. However, passing it for example to `Arc::new` [causes at least one copy](https://youtu.be/F1AquroPfcI?t=4116) (from the stack to the newly allocated heap memory) even though there is no meaningful data. It is theoretically possible that a Sufficiently Advanced Compiler could optimize this copy away, but this is [reportedly unlikely to happen soon in LLVM](https://youtu.be/F1AquroPfcI?t=5431).
This PR proposes two sets of features:
* Constructors for containers (`Box`, `Rc`, `Arc`) of `MaybeUninit<T>` or `[MaybeUninit<T>]` that do not initialized the data, and unsafe conversions to the known-initialized types (without `MaybeUninit`). The constructors are guaranteed not to make unnecessary copies.
* On `Rc` and `Arc`, an unsafe `get_mut_unchecked` method that provides `&mut T` access without checking the reference count. `Arc::get_mut` involves multiple atomic operations whose cost can be non-trivial. `Rc::get_mut` is less costly, but we add `Rc::get_mut_unchecked` anyway for symmetry with `Arc`.
These can be useful independently, but they will presumably be typical when the new constructors of `Rc` and `Arc` are used.
An alternative with a safe API would be to introduce `UniqueRc` and `UniqueArc` types that have the same memory layout as `Rc` and `Arc` (and so zero-cost conversion to them) but are guaranteed to have only one reference. But introducing entire new types feels “heavier” than new constructors on existing types, and initialization of `MaybeUninit<T>` typically requires unsafe code anyway.
Summary of new APIs (all unstable in this PR):
```rust
impl<T> Box<T> { pub fn new_uninit() -> Box<MaybeUninit<T>> {…} }
impl<T> Box<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Box<T> {…} }
impl<T> Box<[T]> { pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]> {…} }
impl<T> Box<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Box<[T]> {…} }
impl<T> Rc<T> { pub fn new_uninit() -> Rc<MaybeUninit<T>> {…} }
impl<T> Rc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Rc<T> {…} }
impl<T> Rc<[T]> { pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]> {…} }
impl<T> Rc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Rc<[T]> {…} }
impl<T> Arc<T> { pub fn new_uninit() -> Arc<MaybeUninit<T>> {…} }
impl<T> Arc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Arc<T> {…} }
impl<T> Arc<[T]> { pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]> {…} }
impl<T> Arc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Arc<[T]> {…} }
impl<T: ?Sized> Rc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
impl<T: ?Sized> Arc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
```
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Co-Authored-By: Ralf Jung <post@ralfj.de>
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Hygienize use of built-in macros in the standard library
Same as https://github.com/rust-lang/rust/pull/61629, but for built-in macros.
Closes https://github.com/rust-lang/rust/issues/48781
r? @alexcrichton
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… and add a separately-unstable field to force non-exhaustive matching
(`#[non_exhaustive]` is no implemented yet on enum variants)
so that we have the option to later expose the allocator’s error value.
CC https://github.com/rust-lang/wg-allocators/issues/23
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Per https://github.com/rust-lang/rust/pull/62451#discussion_r303197278
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Bump to 1.39
r? @Centril
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Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>
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Use associated_type_bounds where applicable - closes #61738
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reduce some test sizes in Miri
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Merge recent changes into master
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bump rand in libcore/liballoc test suites
This pulls in the fix for https://github.com/rust-random/rand/issues/779, which trips Miri when running these test suites.
`SmallRng` (formerly used by libcore) is no longer built by default, it needs a feature gate. I opted to switch to `StdRng` instead. Or should I enable the feature gate?
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This mirrors the implementations of reference slices into arrays.
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Signed-off-by: Nick Cameron <nrc@ncameron.org>
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Remove additional libcore-like restrictions from liballoc, turns out the testing works ok if the tests are a part of liballoc itself.
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Turn `INCOMPLETE_FEATURES` into lint
We do this because it is annoying to see the warning when building rustc and because this is better from a "separation of concerns" POV.
The drawback to this change is that this will respect `--cap-lints`.
Also note that this is not a buffered lint so if there are fatal parser errors then the lint will not trigger.
r? @varkor
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Add doc links to liballoc crate page
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rustbuild
Remove some random unnecessary lint `allow`s
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In which we constantly improve the Vec(Deque) array PartialEq impls
Use the same approach as in https://github.com/rust-lang/rust/pull/62435 as sanctioned by https://github.com/rust-lang/rust/issues/61415#issuecomment-504155110.
r? @scottmcm
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Fix few Clippy warnings
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Right now, the order is as follows:
`pop_front()`
`push_front()`
`push_back()`
`pop_back()`
`swap_remove_back()`
`swap_remove_front()`
I believe it would be more natural, and easier to follow, if we place `pop_back()` right after the `pop_front()`, and `swap_remove_back()` after the `swap_remove_front()` like this:
`pop_front()`
`pop_back()`
`push_front()`
`push_back()`
`swap_remove_front()`
`swap_remove_back()`
The rest of the documentation (at least in this module) adheres to the same logic, where the 'front' function always precedes its 'back' equivalent.
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Add missing doc links in boxed module
r? @rust-lang/docs
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Rollup of 15 pull requests
Successful merges:
- #60066 (Stabilize the type_name intrinsic in core::any)
- #60938 (rustdoc: make #[doc(include)] relative to the containing file)
- #61884 (Stablize Euclidean Modulo (feature euclidean_division))
- #61890 (Fix some sanity checks)
- #62528 (Add joining slices of slices with a slice separator, not just a single item)
- #62707 (Add tests for overlapping explicitly dropped locals in generators)
- #62735 (Turn `#[global_allocator]` into a regular attribute macro)
- #62822 (Improve some pointer-related documentation)
- #62887 (Make the parser TokenStream more resilient after mismatched delimiter recovery)
- #62921 (Add method disambiguation help for trait implementation)
- #62930 (Add test for #51559)
- #62942 (Use match ergonomics in Condvar documentation)
- #62977 (Fix inconsistent highlight blocks.)
- #62978 (Remove `cfg(bootstrap)` code for array implementations)
- #62981 (Add note suggesting to borrow a String argument to find)
Failed merges:
- #62964 (clarify and unify some type test names)
r? @ghost
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