no place to add evaluatable bounds to assoc const items

[psionic12]

Example:

trait Foo {
    const N: usize;
    const ARRAY: [i32;Self::N];
}

The compiler will report an error:

error: unconstrained generic constant
  --> src/main.rs:38:5
   |
38 |     const ARRAY: [i32;Self::N];
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: try adding a `where` bound using this expression: `where [(); Self::N]:`

Honestly speaking, I don't get what the suggestion means. Is it allowed to use generic_const_exprs like this? Or the only way to do this is by writing

trait Foo<const N: usize> {
    const ARRAY: [i32;N];
}

?

[fmease]

Hmm, yeah. Associated constants cannot have a where clause (contrary to types and functions) and thus the suggestion is incorrect. And adding the where clause to the trait itself would lead to a cycle error.

Labeling this as a diagnostic issue for now but I think at some point we need to allow where clauses on associated constants.

@rustbot label C-bug T-compiler A-diagnostics F-generic_const_exprs

[psionic12]

Hi @fmease, do you suggested that the code

trait Foo {
    const N: usize;
    const ARRAY: [i32;Self::N];
}

cannot be done by now by using generic_const_exprs feature?
I don't want to use trait Foo<const N: usize> since it make the code complicated to understand (in my real case), though it works.

[fmease]

cannot be done by now by using generic_const_exprs feature?

I am afraid so.

[fmease]

@psionic12 In the coming days & weeks I'm going to work on implementing generic parameters & where clauses on free & associated const items.

@rustbot claim
@rustbot label -A-diagnostics