Bad diagnostics when using associated const on type without turbofish

[BoxyUwU]

Given the following code: here

struct Add<const N1: usize, const N2: usize>;
impl<const N1: usize, const N2: usize> Foo<N1, N2> {
    const SUM: usize = N1 + N2;
}
fn foo() -> [(); Add<10, 12>::SUM] {
    todo!()
}

The current output is:

error: expected one of `.`, `?`, `]`, or an operator, found `,`
 --> src/lib.rs:5:24
  |
5 | fn foo() -> [(); Add<10, 12>::SUM] {
  |                        ^ expected one of `.`, `?`, `]`, or an operator

error: aborting due to previous error

error: could not compile `playground`

Ideally the output should mention that the user needs to use a turbofish here:
Add::<10, 12>::SUM

cc @lcnr

[lcnr]

the help message seems to only be missing if there are 2 const arguments:

struct Foo<const N: usize, const M: usize>;
impl Foo<3, 4> {
    const SUM: () = ();
}

fn main() {
    Foo<3, 4>::SUM
}

using type parameters or just one const param causes a useful help message

struct Foo<const N: usize>;
impl Foo<3> {
    const SUM: () = ();
}

fn main() {
    Foo<3>::SUM
}

[osa1]

@rustbot claim

[osa1]

It turns out @lcnr 's repro is different than the original repro. My PR fixes @lcnr 's examples but not the original one. I'll update it to fix the original one too.

[osa1]

All fixed.