Buggy OneShot Function Implementation

oneshot.rs

// Rust lacks closures that can only be used once so they are emulated here.
priv trait OneShot <T> {
  fn run (~self) -> T;
}

priv struct AnonymousOneShot <S, T> (S, ~fn (S) -> T);
impl <S, T> OneShot <T> for AnonymousOneShot <S, T> {
  fn run (~self) -> T {
    let ~AnonymousOneShot (x, f) = self;
    f (x)
  }
}

priv fn one_shot <S, T> (x : S, f : ~fn (S) -> T) -> ~OneShot <T> {
  ~AnonymousOneShot::<S, T> (x, f) as ~OneShot <T>
}

fn main () {
}