Better error message for generic_const_exprs inference failure

[atsuzaki]

Fixes #90531

This code:

#![feature(generic_const_exprs)]

fn foo<const N: usize>(_arr: [u64; N + 1]) where [u64; N + 1]: {}

fn main() {
  let arr = [5; 5];       
  foo(arr);
}

Will now emit the following error:

warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> test.rs:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information

error[E0284]: type annotations needed
 --> test.rs:8:7
  |
8 |       foo(arr);
  |       ^^^ cannot infer the value of the const parameter `N` declared on the function `foo`
  |
note: required by a bound in `foo`
 --> test.rs:3:56
  |
3 | fn foo<const N: usize>(_arr: [u64; N + 1]) where [u64; N + 1]: {}
  |                                                        ^^^^^ required by this bound in `foo`
help: consider specifying the generic argument
  |
8 |       foo::<N>(arr);
  |          +++++

error: aborting due to previous error; 1 warning emitted

cc: @lcnr thanks a lot again for the help on this

[rust-highfive]

r? @cjgillot

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[lcnr]

Thanks ❤️

r? @lcnr

@bors r+ rollup

[bors]

📌 Commit 96d34dc has been approved by lcnr

It is now in the queue for this repository.