The tracking issue for this feature is #44101.

With this feature enabled, you are allowed to add a '|' to the beginning of a match arm:

#![feature(match_beginning_vert)]

enum Foo { A, B, C }

fn main() {
    let x = Foo::A;
    match x {
        | Foo::A 
        | Foo::B => println!("AB"),
        | Foo::C => println!("C"),
    }
}