That's not really a thing nowadays, you can't observe something like that without it being unsound for the query system and incremental - what would happen is that the necessary queries would be triggered and the type computed on-demand (this is similar to CTFE triggering type-checking, borrow-checking and only then running the MIR through miri).
It works with this tweak, which means @jschievink is right:
type X = T;
type T = impl Generator<u8>;
This is somewhat surprising to me, as I expected
impl Trait to be allowed anywhere inside the RHS of a
type alias, but apparently the feature is limited to
type Foo = impl Bar;.