Stacked Borrows: redo retag-to-raw

[RalfJung]

@matthewjasper realized that the current rules for retag-to-raw are not working well when the redundant reborrows around raw ptr casts are gone:

let mut x = 12;
let x = &mut x;

&*x; // Stack is frozen

let raw = &raw mut *x; // No longer creates a temporary `&mut i32`
unsafe { *raw = 42; } // Error because no Raw item got pushed

However, this will likely mean that &raw const x and &raw mut x are not the same operation, because the following code must not be UB:

let mut x = 12;
let x = &mut x;

let shr = &*x; // Stack is frozen

let raw = &raw const *x; // If this unfreezes...
let _val = *shr; // ... then this is UB

Also see this discussion on Zulip.

[RalfJung]

Also see this long post of mine on an old topic about coercing mutable references to *const T, which I only now realized is closely related.

[RalfJung]

I also just realized that currently, x as *mut T where x: &mut T effectively counts as a write access, because of the implicit reborrow. Things would likely break if that were to change. And it actually makes some sense, we want to push a new tag after all and we should only do that after popping old stuff away.

[RalfJung]

I think this is basically solved on the Stacked Borrows side, but we need to figure out what we really want (x: &mut T) as *const T to mean. That is tracked at rust-lang/rust#56604.