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

[sgodwincs]

Not sure if a new issue is warranted, but even with generic_const_items, this still doesn't seem to allow the following:

#![feature(generic_const_exprs)]
#![feature(generic_const_items)]

trait Foo {
    const COUNT: usize;
    const ARR: [u8; Self::COUNT]
    where
        [(); Self::COUNT]:;
}

impl<T: Foo> Foo for &T
where
    [(); T::COUNT]:,
{
    const COUNT: usize = T::COUNT;
    const ARR: [u8; Self::COUNT] = T::ARR
    where
        [(); Self::COUNT]:;
}

which gives error:

error[[E0391]](https://doc.rust-lang.org/nightly/error_codes/E0391.html): cycle detected when building an abstract representation for `<impl at src/lib.rs:11:1: 11:24>::ARR::{constant#0}`
  --> src/lib.rs:18:14
   |
18 |         [(); Self::COUNT]:;
   |              ^^^^^^^^^^^
   |
note: ...which requires building THIR for `<impl at src/lib.rs:11:1: 11:24>::ARR::{constant#0}`...
  --> src/lib.rs:18:14
   |
18 |         [(); Self::COUNT]:;
   |              ^^^^^^^^^^^
note: ...which requires type-checking `<impl at src/lib.rs:11:1: 11:24>::ARR::{constant#0}`...
  --> src/lib.rs:18:20
   |
18 |         [(); Self::COUNT]:;
   |                    ^^^^^
   = note: ...which requires evaluating trait selection obligation `&'a T: Foo`...
   = note: ...which again requires building an abstract representation for `<impl at src/lib.rs:11:1: 11:24>::ARR::{constant#0}`, completing the cycle
note: cycle used when checking that `<impl at src/lib.rs:11:1: 11:24>::ARR` is well-formed
  --> src/lib.rs:16:5
   |
16 |     const ARR: [u8; Self::COUNT] = T::ARR
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more in

See https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=ebf58d9916cfd568bb4b57eb4d107316

[fmease]

The issue you are describing is #107874 (except that the where-clause is on an impl not a trait but that is irrelevant).