silly cast errors related to undefined type names

[nikomatsakis]

This example:

struct Foo {
    b: Bar
}

fn main() {
    unsafe {
        let x: *const u32 = 0 as *const u32;
        let y: *const Foo = x as *const Foo;
    }
}

fn make<T>() -> T { panic!() }

gives me this error (in addition to legit errors about Bar):

error: cannot cast thin pointer `*const u32` to fat pointer `*const Foo`
 --> <anon>:8:29
8 |>         let y: *const Foo = x as *const Foo;
  |>      

[Mark-Simulacrum]

Note: Probably quite similar if not the same as #33575.