fix ICE in layout computation with unnormalizable const

[lukas-code]

The first commit reverts half of 7a667d2, where I removed a case from layout_of for handling non-generic unevaluated consts in array length, that I incorrectly assumed to be unreachable. This can actually happen with the combination of feature(generic_const_exprs) and feature(trivial_bounds), because GCE makes anon consts inherit their parent's predicates and with an impossible predicate like u8: A it's possible to have an array whose length is an associated const like <u8 as A>::B that is not generic, but also can't be normalized:

#![feature(generic_const_exprs)]
#![feature(trivial_bounds)]

trait A {
    const B: usize;
}

// With GCE + trivial bounds this definition is not a compile error.
// Computing the layout of this type shouldn't ICE.
struct S([u8; <u8 as A>::B])
where
    u8: A;

---

The first commit also incidentally fixes #137308, which also managed to get an unnormalizable assoc const into an array length:

trait A {
    const B: usize;
}

impl<C: ?Sized> A for u8 { //~ ERROR: the type parameter `C` is not constrained
    const B: usize = 42;
}

// Computing the layout of this type shouldn't ICE, even with the compile error above.
struct S([u8; <u8 as A>::B]);

This happens, because we bail out from codegen_select_candidate with an error if the selected impl has unconstrained params to avoid leaking infer vars out of a query. Instance::try_resolve will then return Ok(None), which for assoc consts roughly means "this const can't be evaluated in a generic context" and is treated as such:

rust/compiler/rustc_middle/src/mir/interpret/queries.rs

Line 84 in 71e06b9

Ok(None) => Err(ErrorHandled::TooGeneric(DUMMY_SP)),

(and this can ICE if the const isn't generic: #135617).

However, here <u8 as A>::B is definitely not "too generic" and also not unresolvable due to an unsatisfiable u8: A bound, so I've included the second commit to change the result of Instance::try_resolve from Ok(None) to Err(ErrorGuaranteed) when resolving an assoc item to an impl with unconstrained generic params. This has the effect that <u8 as A>::B will now be normalized to ConstKind::Error in the example above.

This properly fixes #137308, by no longer treating <u8 as A>::B as unresolvable even though it clearly has a unique impl that it resolves to. It also has the effect of changing the layout error from Unknown ("the type may be valid but has no sensible layout") to ReferencesError ("a non-layout error is reported elsewhere") which seems more appropriate.

r? @compiler-errors

[compiler-errors]

r=me when CI is green

[lukas-code]

@bors r=compiler-errors

[bors]

📌 Commit 7fea935 has been approved by compiler-errors

It is now in the queue for this repository.