mGCA: type check arguments to array expressions

[BoxyUwU]

WellFormed goals for const generics currently don't require that the constituent consts of an array valtree have the same type as the element type of the valtree. This allows the following code to pass even though Bar is not of type Foo:

#![feature(adt_const_params, min_generic_const_args)]

use std::marker::ConstParamTy;

#[derive(Eq, PartialEq, ConstParamTy)]
struct Foo;

struct Bar;

fn foo<const N: [Foo; 1]>() {}

fn main() {
    foo::<{ [Bar] }>();
}

See #150713 as an example of where this was previously implemented for tuples. Implementing this will involve extending the Tuple arm to also cover Arrays.

[meithecatte]

@rustbot claim

[biscuitrescue]

Hi @meithecatte, are you still working on this? I'm interested in working on this and would love to help out if it's no bother to you.

[meithecatte]

Hi! I ended up getting deep into the weeds. If you'd like to work on this instead, feel free.

[biscuitrescue]

Sure, thankyou. @rustbot claim