Fix ICE while computing type layout

[samueltardieu]

If a type is incomplete, for example if generic parameters are not available yet, although they are not escaping, its layout may not be computable. Calling TyCtxt::layout_of() would create a delayed bug in the compiler.

changelog: [zero_sized_map_values]: fix ICE

Fixes #14822

r? @Jarcho

[Jarcho]

Looks like the delay_bug had just been moved around a bit, not actually removed.

The test case can be simplified to:

trait Trait { type T; }
struct S<T: Trait>(T::T);
type Map<T> = HashMap<u32, *const S<T>>;

The delay_bug happens when evaluating the pointer metadata of S<T> which depends on whether T::T is Sized. Since the type alias doesn't have a trait bound of T: Trait evaluating T::T: Sized ultimately fails with NoSolution.

Can you add a comment roughly explaining what actually causes the delay_bug?

[samueltardieu]

Can you add a comment roughly explaining what actually causes the delay_bug?

I have reused your explanation as-is, and the simplified test case.

[Jarcho]

Thank you.