Using an interpolated variable by-ref twice in a macro causes borrowck errors

[huonw]

Using only the by_val line compiles fine:

trait Foo {
    fn by_val(self) {}
    fn by_ref(&self) {}
}
impl Foo for int {}

fn main() {
    macro_rules! t (
        ($a:expr) => { {
            // $a.by_val(); $a.by_val();
            $a.by_ref(); $a.by_ref();
        } }
    );

    t!(1i);
}

macro-use-twice.rs:15:7: 15:9 error: borrowed value does not live long enough
macro-use-twice.rs:15     t!(1i);
                             ^~
macro-use-twice.rs:8:4: 13:6 note: in expansion of t!
macro-use-twice.rs:15:4: 15:11 note: expansion site
macro-use-twice.rs:9:23: 12:9 note: borrowed pointer must be valid for the block at 9:23...
macro-use-twice.rs:9         ($a:expr) => { {
macro-use-twice.rs:10             //$a.by_val(); $a.by_val();
macro-use-twice.rs:11             $a.by_ref(); $a.by_ref();
macro-use-twice.rs:12         } }
macro-use-twice.rs:15:7: 11:37 note: ...but borrowed value is only valid for the method call at 15:7
macro-use-twice.rs:8:4: 13:6 note: in expansion of t!
macro-use-twice.rs:15:4: 15:11 note: expansion site
macro-use-twice.rs:15:7: 15:9 error: borrowed value does not live long enough
macro-use-twice.rs:15     t!(1i);
                             ^~
macro-use-twice.rs:8:4: 13:6 note: in expansion of t!
macro-use-twice.rs:15:4: 15:11 note: expansion site
macro-use-twice.rs:9:23: 12:9 note: borrowed pointer must be valid for the block at 9:23...
macro-use-twice.rs:9         ($a:expr) => { {
macro-use-twice.rs:10             //$a.by_val(); $a.by_val();
macro-use-twice.rs:11             $a.by_ref(); $a.by_ref();
macro-use-twice.rs:12         } }
macro-use-twice.rs:15:7: 11:37 note: ...but borrowed value is only valid for the method call at 15:7
macro-use-twice.rs:8:4: 13:6 note: in expansion of t!
macro-use-twice.rs:15:4: 15:11 note: expansion site
error: aborting due to 2 previous errors