rustc/tests/ui/consts/const_in_pattern/issue-78057.rs

#![deny(unreachable_patterns)]

#[derive(PartialEq)]
struct Opaque(i32);

impl Eq for Opaque {}

const FOO: Opaque = Opaque(42);

fn main() {
    match FOO {
        FOO => {},
        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
        _ => {}
    }
}