Nested println!() statements fail

[mrmonday]

The following code

use std::fmt;

struct Foo;
impl fmt::Debug for Foo {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        println!("<Foo as Debug>::fmt()");

        write!(fmt, "")
    }
}

fn test1() {
    let foo_str = format!("{:?}", Foo);

    println!("{}", foo_str);
}

fn test2() {
    println!("{:?}", Foo);
}

fn main() {
    // This works fine
    test1();

    // This fails
    test2();
}

Playpen: http://is.gd/VBvDk4
Fails, with the following errors:

<Foo as Debug>::fmt()

thread '<main>' panicked at 'RefCell<T> already borrowed', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/cell.rs:469
playpen: application terminated with error code 101