`unused_parens` doesn't fire for match arms

[clarfonthey]

Minimal example:

pub fn broken(x: Option<()>) -> i32 {
    match x {
        Some(()) => (1),
        None => (2),
    }
}

pub fn works(x: Option<()>) -> i32 {
    if x.is_none() {
        (2)
    } else {
        (1)
    }
}

I would expect that the unused_parens lint fire for the broken function, but it does not. Compare this to the works function, where it does. This can potentially show up when refactoring a function, e.g. if converting to the above from:

pub fn broken(x: Option<()>) -> i32 {
    match x {
        Some(()) => do_something_with(1),
        None => do_something_with(2),
    }
}

This could arguably be a formatting issue, as some people might like to split things on multiple lines like:

=> (
   some_long_expr
)

However, we don't do this for other cases where unused_parens is added. Take, for example:

if (
    some_long_expr
) {
   // ...
}

This still triggers the unused_parens lint.

This is a particularly weird case because users will often separate out the match arm into a block if it gets long, e.g.:

=> {
    some_long_expr
}

But not with parentheses. I think that for the case of parentheses, the precedent should go with removing them.

[wcampbell0x2a]

@rustbot claim