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//@ run-pass #![feature(unboxed_closures)] #![feature(fn_traits)] struct Fun<F>(F); impl<F, T> FnOnce<(T,)> for Fun<F> where F: Fn(T) -> T { type Output = T; extern "rust-call" fn call_once(self, (t,): (T,)) -> T { (self.0)(t) } } fn main() { let fun = Fun(|i: isize| i * 2); println!("{}", fun(3)); }