Error code E0158

An associated const, const parameter or static has been referenced in a pattern.

Erroneous code example:

#![allow(unused)]
fn main() {
enum Foo {
    One,
    Two
}

trait Bar {
    const X: Foo;
}

fn test<A: Bar>(arg: Foo) {
    match arg {
        A::X => println!("A::X"), // error: E0158: associated consts cannot be
                                  //        referenced in patterns
        Foo::Two => println!("Two")
    }
}
}

Associated consts cannot be referenced in patterns because it is impossible for the compiler to prove exhaustiveness (that some pattern will always match). Take the above example, because Rust does type checking in the generic method, not the monomorphized specific instance. So because Bar could have theoretically infinite implementations, there's no way to always be sure that A::X is Foo::One. So this code must be rejected. Even if code can be proven exhaustive by a programmer, the compiler cannot currently prove this.

The same holds true of const parameters and statics.

If you want to match against an associated const, const parameter or static consider using a guard instead:

#![allow(unused)]
fn main() {
trait Trait {
    const X: char;
}

static FOO: char = 'j';

fn test<A: Trait, const Y: char>(arg: char) {
    match arg {
        c if c == A::X => println!("A::X"),
        c if c == Y => println!("Y"),
        c if c == FOO => println!("FOO"),
        _ => ()
    }
}
}