Extending the ability to elide commas from match arms

internals ideas (deprecated)
3

While digging through the parser to see if I could potentially implement this, I discovered that match arm RHSs are already treated during parsing similarly to statements, meaning that this wouldn’t be a breaking change at all! (On reflection, I probably should have tried compiling my own example from above that was meant to demonstrate code that worked today. :stuck_out_tongue:)

Edit: Welp, it actually totally is a breaking change, but a super(-duper) minor one. The only code that would be broken would be code like this:

match foo {
    Foo => mac! { ... }(),
    Bar => mac! { ... }[0],
    Baz => mac! { ... } & 1,
    Qux => mac! { ... } && some_predicate(),
}

I actually tried compiling this time, and it totally parses, but wouldn’t under this change. I would bet a large sum of play money that this would break literally no Rust code anywhere—in the Rust compiler itself, the sequence => <word>! { only appears three times, none of which are terminated by }(...), }[...], } & ..., or } && ....