Allow alternation within sub-patterns

[mbrubeck]

Allowing | to be used within a sub-pattern would allow some match expressions on tuples and structs to be written much more clearly and concisely.

For example, the following match expression:

match (x, y) {
    (A|B, C|D) => 0,
    _ => 1
}

would be equivalent to this expanded version:

match (x, y) {
    (A, C) => 0,
    (A, D) => 0,
    (B, C) => 0,
    (B, D) => 0,
    _ => 1
}

[ftxqxd]

rust-lang/rfcs#99 proposed such an addition (among other things), but was postponed until post-1.0.

[alexcrichton]

As @P1start mentioned, this is currently a postponed RFC, so I'm closing in favor of that.

[mbrubeck]

rust-lang/rfcs#1882 is a more recent version of this request

[mbrubeck]

rust-lang/rfcs#2175 addresses part of this feature.

[mbrubeck]

New RFC for this feature: rust-lang/rfcs#2535.

[mbrubeck]

Update: RFC 2535 was implemented and will become stable in Rust 1.53.