Shared via Rust Playground

playground.rs

#![feature(unboxed_closures, fn_traits)]

fn get_fn_ptr<A, C: Fn<A>>(_: &C) -> extern "rust-call" fn(&(), A) -> C::Output {
    let fn_ptr: extern "rust-call" fn(&(), A) -> C::Output = unsafe {
        std::mem::transmute(<C as std::ops::Fn<A>>::call as usize)
    };
    fn_ptr
}

fn main() {
    let closure = |x| { println!("hi {}", x) };
    let ptr = get_fn_ptr(&closure);
    ptr(&(), ("bstrie",));
}