identity issues

gistfile1.txt

$ rustc main.rs 
main.rs:7:21: 7:29 error: unable to infer enough type information about `_`; type annotations required [E0282]
main.rs:7    let f = identity(identity);
                              ^~~~~~~~
error: aborting due to previous error

identity.rs

fn identity<T>(x: T) -> T {
   x
}

fn main() {
   let i = identity(0);
   let f = identity(identity);
}