MGCA: forbid generic parameters from being in used in the types of type_const items

[BoxyUwU]

Support for const generics with types that reference generic parameters is gated behind the generic_const_parameter_types feature gate:

#![feature(adt_const_params, unsized_const_params)]

// This errors as the type in `const N: T` uses generic parameters
fn generic_const_param_ty<T: core::marker::ConstParamTy_, const N: T>() {}

We should enforce the same restriction on const items marked as type_const :

#![feature(adt_const_params, unsized_const_params, min_generic_const_args, generic_const_items)]

#[type_const]
const FOO<T: core::marker::ConstParamTy_>: [T; 0] = [];

This should error due to the type [T; 0] naming generic parameters. Until #150612 is implemented this will also error due to [] not being wrapped in a const block

cc @theemathas we'd talked about you working on this so I've assigned you but if you don't want to work on this anymore just unassign yourself :)

[theemathas]

I already forgot about that issue 😅

Although, last time I checked, it seemed too hard for me so I guess I'mma unassign myself

[enthropy7]

may i try to fight with this issue? i wanna get into codebase and try to solve it :>

[BoxyUwU]

go for it! I would recommend looking into how this is already handled for struct Foo<T, const N: T> and then apply that to the types of const items marked type_const. If you're not already on the rust zulip I'd recommend joining so you can ask questions in the project-const-generics channel.

[enthropy7]

Thank you :333 joined Zulip, reading docs.. @rustbot claim