MIR type-checking ICE ("is not a subtype of") from lifetime parameter in scope of AnonConst.

[eddyb]

This example was found in #70452 (where it does not require #![feature(const_generics)], the only reason I have it here is to get around #43408) (playground):

#![feature(const_generics)]

const HASH_LEN: usize = 20;
struct Hash([u8; HASH_LEN]);

pub fn foo<'a>() -> &'a () {
    Hash([0; HASH_LEN]);
    &()
}

The ICE is:

error: internal compiler error:
    broken MIR in DefId(0:8 ~ playground[8116]::foo[0]) (Hash(move _2,)):
        [u8; _] is not a subtype of [u8; 20]: NoSolution

The ICE goes away when the lifetime parameter ('a) is removed, or when replaced with a type parameter, so my guess is that the MIR type-checker triggers an attempt at normalization (otherwise it would also ICE with a type parameter) but has lifetime inference variables in the Substs of the ConstKind::Unevaluated, and that prevents the normalization.

It really shouldn't, I thought we both erased lifetimes and fell back on fully polymorphic evaluation (which should also succeed here), if there are any inference variables.

[eddyb]

I'm going in circles, and the fact that I used HASH_LEN in both places isn't helping.

It's the type of the [0; HASH_LEN] passed to the Rvalue::Aggregate, that's never attempted to be normalized, not the field of Hash. So it's just another "missing normalization" bug.

[eddyb]

Looks like calls are also affected (playground):

#![feature(const_generics)]

const HASH_LEN: usize = 20;
fn init_hash(_: &mut [u8; HASH_LEN]) {}

pub fn foo<'a>() -> &'a () {
    init_hash(&mut [0; HASH_LEN]);
    &()
}

This was found in rand (see #70452 (comment)), although I wish it was before a 3-day Crater run.