Allow instantiating trait object binder in ptr-to-ptr casts

[lukas-code]

For unsizing coercions between trait objects with the same principal, we already allow instantiating the for binder. For example, coercing Box<dyn for<'a> Trait<'a> to Box<dyn Trait<'static>> is allowed.

Since ptr-to-ptr casts will insert an unsizing coercion before the cast if possible, this has the consequence that the following compiles already:

// This compiles today.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut dyn Trait<'b> {
    // lowered as (roughly)
    // tmp: *mut dyn Trait<'?0> = Unsize(x)     // requires dyn for<'a> Trait<'a> <: dyn Trait<'?0>
    // ret: *mut dyn Trait<'b> = PtrToPtr(tmp)  // requires dyn Trait<'?0> == dyn Trait<'b>
    x as _
}

However, if no unsizing coercion is inserted then this currently fails to compile as one type is more general than the other. This PR will allow this code to compile, too, by changing ptr-to-ptr casts of pointers with vtable metadata to use sutyping instead of type equality.

// This will compile after this PR.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut Wrapper<dyn Trait<'b>> {
    // lowered as (roughly)
    // no Unsize here!
    // ret: *mut Wrapper<dyn Trait<'b>> = PtrToPtr(x)  // requires dyn for<'a> Trait<'a> == dyn Trait<'b>
    x as _
}

Note that it is already possible to work around the current restrictions and make the code compile before this PR by splitting the cast in two, so this shouldn't allow a new class of programs to compile:

// Workaround that compiles today.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut Wrapper<dyn Trait<'b>> {
    x as *mut dyn Trait<'_> as _
}

r? @compiler-errors
cc @WaffleLapkin

[compiler-errors]

Yeah makes sense. Somewhat related to #130866.

@bors r+

[bors]

📌 Commit 53f45c4 has been approved by compiler-errors

It is now in the queue for this repository.