Missing unconstrained generic constant error if adt_const_params feature is included

[b-naber]

Based on the example in #90455, which after the changes in #90529 would output an unconstrained generic constant error. The following code, however, does not output such an error, although I believe it should.

Given the following code:

#![feature(generic_const_exprs, adt_const_params)]

const fn num_limbs(_: &str) -> usize {
    5   
}

fn foo<const N: &'static str>(_arr: [u64; num_limbs(N)]) where [u64; num_limbs(N)]: {}

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

The current output is:

error[E0284]: type annotations needed: cannot satisfy `the constant `foo::<{_: &'static str}>::{constant#0}` can be evaluated`
  --> test2.rs:17:3
   |
17 |   foo(arr);
   |   ^^^ cannot satisfy `the constant `foo::<{_: &'static str}>::{constant#0}` can be evaluated`
   |
note: required by a bound in `foo`
  --> test2.rs:13:70
   |
13 | fn foo<const N: &'static str>(_arr: [u64; num_limbs(N)]) where [u64; num_limbs(N)]: {}
   |                                                                      ^^^^^^^^^^^^ required by this bound in `foo`

error: aborting due to previous error; 2 warnings emitted

This only happens if the adt_const_params is included. This also compiles if you don't include the call of foo in main.

[BoxyUwU]

dont think this is related to adt_const_params as this has the same error:

#![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);
}

[lcnr]

the reason this happens is because from some_expr(N) == 5 we don't infer the value for N.

The error message should be improved by using emit_inference_failure_err in this branch

rust/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

Lines 2160 to 2173 in 7036449

	_ => {
	if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
	return;
	}
	let mut err = struct_span_err!(
	self.tcx.sess,
	span,
	E0284,
	"type annotations needed: cannot satisfy `{}`",
	predicate,
	);
	err.span_label(span, &format!("cannot satisfy `{}`", predicate));
	err
	}

for that the predicates should be searched for a relevant inference variable.

[lcnr]

going to mentor this issue in case someone wants to fix this

[atsuzaki]

@rustbot claim