Matthias Krüger b4513ce6f8 Rollup merge of #101310 - zachs18:rc_get_unchecked_mut_docs_soundness, r=Mark-Simulacrum ... 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).		2022-11-20 23:50:26 +01:00
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benches	update str.contains benchmarks	2022-11-14 23:03:16 +01:00
src	Rollup merge of #101310 - zachs18:rc_get_unchecked_mut_docs_soundness, r=Mark-Simulacrum	2022-11-20 23:50:26 +01:00
tests	Rollup merge of #104435 - scottmcm:iter-repeat-n, r=thomcc	2022-11-20 13:15:59 +09:00
Cargo.toml	Switch all libraries to the 2021 edition	2021-12-23 19:03:47 +08:00