Confusing error message with invalid format specifier

[mrmonday]

The following code:

fn main() {
    let _ = format!("{_foo}", _foo = 6u);
}

(Playpen: http://is.gd/me8pNE)

Gives the error:

<anon>:2:21: 2:29 error: invalid format string: expected `'}'`, found `'_'`
<anon>:2     let _ = format!("{_foo}", _foo = 6u);
                             ^~~~~~~~
error: aborting due to previous error

It is not too confusing in this short example, but in a longer string it is not obvious that named identifiers cannot start with an underscore.

[steveklabnik]

Updated code:

fn main() {
    let _ = format!("{_foo}", _foo = 6usize);
}

this still happens though.