Inaccurate error message for const operations in type parameters

[Nemo157]

I tried this code:

#![feature(min_const_generics)]

struct X<const S: usize>;

impl<const S: usize> X<S> {
    const LEN: usize = S + 1;
}

struct Y<const S: usize> {
  stuff: [u8; { S + 1 }],
}

This gave an error message:

error: generic parameters may not be used in const operations
  --> src/lib.rs:10:17
   |
10 |   stuff: [u8; { S + 1 }],
   |                 ^ cannot perform const operation using `S`
   |
   = help: const parameters may only be used as standalone arguments, i.e. `S`
   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions

But you can see in X::LEN that you can perform const operations using S, the restriction this hits is more subtle in that you cannot perform const operations using generic parameters when these will impact the type system (as I understand it, I may be wrong). This also comes up if a generic parameter is used in defining an associated type.

[XrXr]

Maybe "generic parameters may not be used in const operations in this context" would be better?

[Enselic]

Maybe "generic parameters may not be used in const operations in this context" would be better?

Should be fixed by #142157. I went with just adding here to the error message

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