Disentangle ForwardGenericParamBan and ConstParamTy ribs

[compiler-errors]

In #137617, the ConstParamTy rib was adjusted to act kinda like the ForwardGenericParamBan. However, this means that it no longer served its purpose banning generics from parent items. Although we still are checking for param type validity using the ConstParamTy_ trait, which means that we weren't accepting code we shouldn't, I think it's a bit strange for us not to be rejecting code like this during resolution and instead letting these malformed const generics leak into the type system:

trait Foo<T> {
  fn bar<const N: T>() {}
}

This PR does a few things:

1. Introduce a ForwardGenericParamBanReason enum, and start using the ForwardGenericParamBan rib to ban forward-declared params in const tys when generic_const_parameter_types is enabled.
2. Start using the ConstParamTy rib to ban all generics when generic_const_parameter_types is disabled.
3. Improve the diagnostics for both of the cases above, and for forward-declared params in parameter defaults too :3

r? @BoxyUwU or reassign

[BoxyUwU]

ah right 🤔 I guess at the time I thought that params would include all params in scope but that doesnt make any sense xd

[BoxyUwU]

#![feature(adt_const_params)]

trait Trait<const N: usize> {
    fn foo<const M: [u8; N]>() {}
}

Can you add a test that we forbid this? Apparently we don't have such a test already ^^'

[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[BoxyUwU]

@bors r+

this is much nicer now thx

[bors]

📌 Commit a79f97b has been approved by BoxyUwU

It is now in the queue for this repository.

[jieyouxu]

I think this failed in rollup: #138382 (comment)

2025-03-12T04:34:54.7238690Z ---- [ui] tests/ui/const-generics/generic_const_parameter_types/references-parent-generics.rs#nofeat stdout ----
2025-03-12T04:34:54.7239912Z Saved the actual stderr to "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/const-generics/generic_const_parameter_types/references-parent-generics.nofeat/references-parent-generics.nofeat.stderr"
2025-03-12T04:34:54.7240808Z diff of stderr:
2025-03-12T04:34:54.7240928Z 
2025-03-12T04:34:54.7241108Z -	error: `Self` is forbidden as the type of a const generic parameter
2025-03-12T04:34:54.7241628Z +	error[E0770]: the type of const parameters must not depend on other generic parameters
2025-03-12T04:34:54.7242657Z 2	  --> $DIR/references-parent-generics.rs:7:25
2025-03-12T04:34:54.7242960Z 3	   |
2025-03-12T04:34:54.7243405Z 4	LL |     type Assoc<const N: Self>;
2025-03-12T04:34:54.7243593Z 
2025-03-12T04:34:54.7243683Z -	   |                         ^^^^
2025-03-12T04:34:54.7243923Z -	   |
2025-03-12T04:34:54.7244201Z -	   = note: the only supported types are integers, `bool`, and `char`
2025-03-12T04:34:54.7244646Z +	   |                         ^^^^ the type must not depend on the parameter `Self`

@bors r-

[BoxyUwU]

gamer

[compiler-errors]

@bors r=BoxyUwU

[bors]

📌 Commit 42773bf has been approved by BoxyUwU

It is now in the queue for this repository.

[compiler-errors]

@bors rollup