Variables used in `when` still considered unused

[nikomatsakis]

The following code gets an error about an unused variable b:

use std;

fn foo(a: int, b: int) {
    alt a {
      _ when a == b { log_err "hi"; }
      _ { log_err "ho"; }
    }
}

fn main() {
    foo(1,1);
    foo(1,2);
}