[ER] Suboptimal error message in a case of min_const_generics inference failure

[leonardo-m]

This code:

#![feature(min_const_generics)]
fn foo<const N: usize, const K: usize>(data: [u32; N]) -> [u32; K] {
    [0; K]
}
fn main() {
    let a = foo::<_, 2>([0, 1, 2]);
}

Gives (with rustc 1.50.0-nightly 349b3b3 2020-11-29):

error[E0747]: type provided when a constant was expected
 --> ...\test.rs:6:19
  |
6 |     let a = foo::<_, 2>([0, 1, 2]);
  |                   ^

Two problems:

1. In the code I've used the underscore hoping rustc to infer the value of N, but for some reason it failed.
2. Beside the type inference failure, the error message is not the best possible. I didn't provide a type. A better error message could say "unable to infer the const argument N, please specify".

[lcnr]

Using _ for a const parameter is currently not possible, though that seems both like something we are able to support and probably should.

Until we implement this we probably want to add a help here which says that _ is not allowed for const arguments, only for types. Will open a topic on zulip about this https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/.60_.60.20as.20const.20param.20help.20msg

[varkor]

Fixing this requires essentially the same strategy as fixing #66615 (and, to a lesser extent, #77773).

[leonardo-m]

Now (rustc 1.51.0-nightly 158f8d0 2020-12-29) this code:

fn main() {
    fn init<const N: usize, const M: usize>(foos: &mut [bool; M]) {}
    const N: usize = 5;
    let mut foos = [false; 10];
    init::<N, _>(&mut foos);
}

Gives:

error[E0747]: type provided when a constant was expected
 --> ...\test.rs:5:15
  |
5 |     init::<N, _>(&mut foos);
  |               ^
  |
  = help: const arguments cannot yet be inferred with `_`
  = note: type arguments must be provided before constant arguments
  = help: reorder the arguments: consts: `<N, M>`

The last two lines (the note and the second help) seem off to me.

[varkor]

Opened #80528 for actually supporting this feature, rather than diagnostics suggesting it to be invalid.