Associated constants in traits can not be used in const generics

[newpavlov]

Initially reported here.

The following code:

trait Foo {
    const N: usize;
    fn foo() -> [u8; Self::N];
}

Playground

Results in a "no associated item named N found for type Self in the current scope" compilation error on current Nightly.

Note that associated constants in impl blocks work without any issues.

cc @varkor @yodaldevoid

[dtolnay]

Potentially related to #43408.

[Dylan-DPC-zz]

The error now is:

error: constant expression depends on a generic parameter
 --> src/lib.rs:5:17
  |
5 |     fn foo() -> [u8; Self::N];
  |                 ^^^^^^^^^^^^^
  |
  = note: this may fail depending on what value the parameter takes

[varkor]

The error with min_const_generics enabled is:

error: generic parameters must not be used inside of non trivial constant values
 --> src/lib.rs:5:22
  |
5 |     fn foo() -> [u8; Self::N];
  |                      ^^^^^^^ non-trivial anonymous constants must not depend on the parameter `Self`
  |
  = note: type parameters are currently not permitted in anonymous constants

which can definitely be improved.

[varkor]

I think we ought to prioritise good diagnostics for min_const_generics from the beginning, so I'm making this issue as a blocker too.