Const param as PatKind::Path interpreted as a binding instead of being rejected

[Centril]

#![feature(const_generics)]

fn take_const_param<const N: usize>() {
    match 1 {
        N => {},
        _ => {},
    }
}

fn main() {
    take_const_param::<0>();
    take_const_param::<1>()
}

results in:

warning: unreachable pattern
 --> src/main.rs:6:9
  |
5 |         N => {},
  |         - matches any value
6 |         _ => {},
  |         ^ unreachable pattern
  |
  = note: `#[warn(unreachable_patterns)]` on by default

warning: unused variable: `N`
 --> src/main.rs:5:9
  |
5 |         N => {},
  |         ^ help: consider prefixing with an underscore: `_N`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: variable `N` should have a snake case name
 --> src/main.rs:5:9
  |
5 |         N => {},
  |         ^ help: convert the identifier to snake case: `n`
  |
  = note: `#[warn(non_snake_case)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.64s
     Running `target/debug/playground`

but should, similarly to associated constants, result in:

error[E0158]: const parameters cannot be referenced in patterns
 --> src/lib.rs:L:C
  |
L |     N => {}
  |     ^

[petrochenkov]

This is strange, fn fresh_binding had an assert for unexpected things in the binding position, and const parameters should be unexpected if nobody explicitly whitelisted them to behave like this.

[Centril]

The relevant code is in try_resolve_as_non_binding now (moved in #64111). The span_bug!(...) should be hit, but for some reason it seems like DefKind::ConstParam is not being created when calling resolve_ident_in_lexical_scope.

[petrochenkov]

Fixed in #70006.