Allow nested alternatives in patterns

[ben0x539]

Feature request to allow, like,

match Some(42) {
    Some(1 | 2 | 3 | 42) => 0,
    _ => 1
}

instead of

Some(1) | Some(2) | Some(3) | Some(42) => 0, 

Makes it easier to match a few inner different values deep within a common outer structure and seems like a logical extension of the existing syntax.

This might make the visual ambiguity with the binary bitwise-or | more pronounced, though.

[catamorphism]

I've wanted this sometimes, but the pattern-matching code is scary, so I'm not sure how easy it would be to implement. (I'm guessing not easy.)

[ben0x539]

(note+ping, I forgot to add I filed this because @erickt said he was interested in adding the feature)

[Aatch]

If anybody is still interested in this, an official RFC should be filed. https://github.com/rust-lang/rfcs/blob/master/active/0001-rfc-process.md