invalid_reference_casting fires for correct usage of UnsafeCell

[RalfJung]

Code

use std::cell::UnsafeCell;

unsafe fn get_mut_unchecked<T>(ptr: &UnsafeCell<T>) -> &mut T {
    let t = ptr as *const UnsafeCell<T> as *mut T;
    unsafe { &mut *t }
}

Current output

--> src/lib.rs:5:14
  |
4 |     let t = ptr as *const UnsafeCell<T> as *mut T;
  |             ------------------------------------- casting happened here
5 |     unsafe { &mut *t }
  |              ^^^^^^^
  |
  = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
  = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
  = note: `#[deny(invalid_reference_casting)]` on by default

Desired output

(none)

Rationale and extra context

#159730 relaxed the rules for UnsafeCell. Under the new rules, the given code is equivalent to

unsafe fn get_mut_unchecked<T>(ptr: &UnsafeCell<T>) -> &mut T {
    let t = ptr.get();
    unsafe { &mut *t }
}

This does not raise a diagnostic either, so there's no longer a reason to raise a diagnostic for the original code.

Cc @Urgau

Other cases

Rust Version

nightly

Anything else?

No response