Can a pointer obtained by casting `&UnsafeCell<T>` to `*mut T` be written to?

[Kestrer]

UnsafeCell::get is currently implemented by simply casting the &UnsafeCell<T> to a *mut T. A comment inside it notes that this is only able to work because libcore is a special library. Should this comment still apply? Is there benefit in forcing all writes to an UnsafeCell to go through UnsafeCell::get, or should UnsafeCell be treated as just a type-level indicator that the inner data is interior mutable? Since UnsafeCell is #[repr(transparent)] I don't think it is UB to read from a casted pointer, but I'm not sure if the same logic applies to writes.

[RalfJung]

That comment exists out of abundance of caution, since we do not yet have an official aliasing model for Rust. The comment gives the aliasing model the freedom to not just make UnsafeCell special, but to make UnsafeCell::get special. Stacked Borrows does not use that freedom, but until we have an official aliasing model and can say for sure that we do not need this freedom, I'd advise for taking the comment seriously and keeping it around, even if Miri cannot check that you are complying by it.