b670b86353
Add non-`unsafe` `.get_mut()` for `Unsafecell`
- Tracking issue: https://github.com/rust-lang/rust/issues/76943
As discussed in: https://internals.rust-lang.org/t/add-non-unsafe-get-mut-for-unsafecell/12407
- ### [Rendered documentation](https://modest-dubinsky-1f9f47.netlify.app/core/cell/struct.unsafecell)
This PR tries to move the sound `&mut UnsafeCell<T> -> &mut T` projection that all the "downstream" constructions were already relying on, up to the root abstraction, where it rightfully belongs, and officially blessing it.
- this **helps reduce the amount of `unsafe` snippets out there** (_c.f._, the second commit of this PR: https://github.com/rust-lang/rust/pull/76936/commits/09503fd1b30c83ca605546fa3f899721e41e68c6)
The fact that this getter is now expose for `UnsafeCell<T>` itself, will also help convey the idea that **`UnsafeCell` is not magical _w.r.t._ `&mut` accesses**, contrary to what some people incorrectly think.
- Even the standard library itself at some point had such a confusion, _c.f._ this comment where there is a mention of multi-threaded (and thus _shared_) access despite dealing with exclusive references over unique ownership:
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| .. | ||
| barrier | ||
| condvar | ||
| mpsc | ||
| mutex | ||
| once | ||
| rwlock | ||
| barrier.rs | ||
| condvar.rs | ||
| mod.rs | ||
| mutex.rs | ||
| once.rs | ||
| rwlock.rs | ||