Pretty-printer can't disambiguate alts followed by ( from function calls

[brson]

From std::lib::task:

    alt notify {
      some(c) {
        (**task_ptr).notify_enabled = 1u8;
        (**task_ptr).notify_chan = *c;
      }
      none {}
    };
    (*regs).esp = align_down((*regs).esp - 12u32) - 4u32;

Without the trailing semi, the parens are interpreted as a function call. The pretty-printer can't figure out to put the semi there (it does know how to disambiguate alts followed by unops).

[brson]

Reduced test case

fn main() {
    let regs = @{mutable eax: 0};
    alt true {
      true { }
      false { }
    };
    (*regs).eax = 1;
}

[brson]

Well, that is actually a unop, so the pretty-printer obviously doesn't know enough about how to disambiguate them. Currently, it's just checking that the next statement is a unop, and here the next statement is assignment.