What exactly are the rules for `UnsafeCell::get_mut`?

[CAD97]

Miri is somewhat inconsistent here:

use std::cell::UnsafeCell;

fn main() {
    let mut cell = UnsafeCell::new(0);
    let ptr = cell.get();
    unsafe { *ptr += 1; }

    // 3 alternatives:

    *cell.get_mut() += 1; // miri doesn't complain
    // *(&mut cell).get_mut() += 1; // miri complains
    // *UnsafeCell::get_mut(&mut cell) += 1; // miri complains

    unsafe { *ptr += 1; }

    dbg!(cell.get_mut());
}

(origininally discovered by @steffahn on urlo)

[RalfJung]

This is another 2-phase borrow weirdness -- cell.get_mut() creates a two-phase borrow which has SharedReadWrite permission and thus joins the equivalence class that ptr is already in. Hence both of these pointers can be used interchangeably.

Whether that is what we want to happen I am not sure. ;)

[RalfJung]

So, closing as duplicate of #85.