Clarify and restrict when `{Arc,Rc}::get_unchecked_mut` is allowed.
(Tracking issue for `{Arc,Rc}::get_unchecked_mut`: #63292)
(I'm using `Rc` in this comment, but it applies for `Arc` all the same).
As currently documented, `Rc::get_unchecked_mut` can lead to unsoundness when multiple `Rc`/`Weak` pointers to the same allocation exist. The current documentation only requires that other `Rc`/`Weak` pointers to the same allocation "must not be dereferenced for the duration of the returned borrow". This can lead to unsoundness in (at least) two ways: variance, and `Rc<str>`/`Rc<[u8]>` aliasing. ([playground link](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d7e2d091c389f463d121630ab0a37320)).
This PR changes the documentation of `Rc::get_unchecked_mut` to restrict usage to when all `Rc<T>`/`Weak<T>` have the exact same `T` (including lifetimes). I believe this is sufficient to prevent unsoundness, while still allowing `get_unchecked_mut` to be called on an aliased `Rc` as long as the safety contract is upheld by the caller.
## Alternatives
* A less strict, but still sound alternative would be to say that the caller must only write values which are valid for all aliased `Rc`/`Weak` inner types. (This was [mentioned](https://github.com/rust-lang/rust/issues/63292#issuecomment-568284090) in the tracking issue). This may be too complicated to clearly express in the documentation.
* A more strict alternative would be to say that there must not be any aliased `Rc`/`Weak` pointers, i.e. it is required that get_mut would return `Some(_)`. (This was also mentioned in the tracking issue). There is at least one codebase that this would cause to become unsound ([here](be5a164d77/src/memtable.rs (L166)), where additional locking is used to ensure unique access to an aliased `Rc<T>`; I saw this because it was linked on the tracking issue).
`VecDeque::resize` should re-use the buffer in the passed-in element
Today it always copies it for *every* appended element, but one of those clones is avoidable.
This adds `iter::repeat_n` (https://github.com/rust-lang/rust/issues/104434) as the primitive needed to do this. If this PR is acceptable, I'll also use this in `Vec` rather than its custom `ExtendElement` type & infrastructure that is harder to share between multiple different containers:
101e1822c3/library/alloc/src/vec/mod.rs (L2479-L2492)
Attempt to reuse `Vec<T>` backing storage for `Rc/Arc<[T]>`
If a `Vec<T>` has sufficient capacity to store the inner `RcBox<[T]>`, we can just reuse the existing allocation and shift the elements up, instead of making a new allocation.
run alloc benchmarks in Miri and fix UB
Miri since recently has a "fake monotonic clock" that works even with isolation. Its measurements are not very meaningful but it means we can run these benches and check them for UB.
And that's a good thing since there was UB here: fixes https://github.com/rust-lang/rust/issues/104096.
r? ``@thomcc``
disable btree size tests on Miri
Seems fine not to run these in Miri, they can't have UB anyway. And this lets us do layout randomization in Miri.
r? ``@thomcc``
The new implementation doesn't use weak lang items and instead changes
`#[alloc_error_handler]` to an attribute macro just like
`#[global_allocator]`.
The attribute will generate the `__rg_oom` function which is called by
the compiler-generated `__rust_alloc_error_handler`. If no `__rg_oom`
function is defined in any crate then the compiler shim will call
`__rdl_oom` in the alloc crate which will simply panic.
This also fixes link errors with `-C link-dead-code` with
`default_alloc_error_handler`: `__rg_oom` was previously defined in the
alloc crate and would attempt to reference the `oom` lang item, even if
it didn't exist. This worked as long as `__rg_oom` was excluded from
linking since it was not called.
This is a prerequisite for the stabilization of
`default_alloc_error_handler` (#102318).
Remove redundant lifetime bound from `impl Borrow for Cow`
The lifetime bound `B::Owned: 'a` is redundant and doesn't make a difference,
because `Cow<'a, B>` comes with an implicit `B: 'a`, and associated types
will outlive lifetimes outlived by the `Self` type (and all the trait's
generic parameters, of which there are none in this case), so the implicit `B: 'a`
implies `B::Owned: 'a` anyway.
The explicit lifetime bound here does however [end up in documentation](https://doc.rust-lang.org/std/borrow/enum.Cow.html#impl-Borrow%3CB%3E),
and that's confusing in my opinion, so let's remove it ^^
_(Documentation right now, compare to `AsRef`, too:)_
![Screenshot_20220722_014055](https://user-images.githubusercontent.com/3986214/180332665-424d0c05-afb3-40d8-a330-a57a2c9a494b.png)
Adjust argument type for mutable with_metadata_of (#75091)
The method takes two pointer arguments: one `self` supplying the pointer value, and a second pointer supplying the metadata.
The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true.
In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. [Example](cad93775eb).
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```
This could then instead be simplified to:
```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;
let ptr = uninit.as_ptr().with_metadata_of(&**val);
```
Tracking issue: https://github.com/rust-lang/rust/issues/75091
``@dtolnay`` you're reviewed #95249, would you mind chiming in?
Remove incorrect comment in `Vec::drain`
r? ``@scottmcm``
Turns out this comment wasn't correct for 6 years, since #34951, which switched from using `slice::IterMut` into using `slice::Iter`.