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#![feature(unsized_locals)]
#![feature(unboxed_closures)]
#![feature(tuple_trait)]
pub trait FnOnce<Args: std::marker::Tuple> {
type Output;
extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
}
struct A;
impl FnOnce<()> for A {
type Output = String;
extern "rust-call" fn call_once(self, (): ()) -> Self::Output {
format!("hello")
}
}
struct B(i32);
impl FnOnce<()> for B {
type Output = String;
extern "rust-call" fn call_once(self, (): ()) -> Self::Output {
format!("{}", self.0)
}
}
struct C(String);
impl FnOnce<()> for C {
type Output = String;
extern "rust-call" fn call_once(self, (): ()) -> Self::Output {
self.0
}
}
struct D(Box<String>);
impl FnOnce<()> for D {
type Output = String;
extern "rust-call" fn call_once(self, (): ()) -> Self::Output {
*self.0
}
}
fn main() {
let x = *(Box::new(A) as Box<dyn FnOnce<(), Output = String>>);
assert_eq!(x.call_once(()), format!("hello"));
let x = *(Box::new(B(42)) as Box<dyn FnOnce<(), Output = String>>);
assert_eq!(x.call_once(()), format!("42"));
let x = *(Box::new(C(format!("jumping fox"))) as Box<dyn FnOnce<(), Output = String>>);
assert_eq!(x.call_once(()), format!("jumping fox"));
let x = *(Box::new(D(Box::new(format!("lazy dog")))) as Box<dyn FnOnce<(), Output = String>>);
assert_eq!(x.call_once(()), format!("lazy dog"));
}
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