Closure Under de Hood in Rust

closure_with_sugar.rs

fn call_once<F: FnOnce(i32) -> i32>(f: F, x: i32) -> i32 {
    f(x)
}

fn main() {
    let outer = 10;

    let add_outer = |x| x + outer;
    let result = call_once(&add_outer, 66);
    println!("Result: {}", result);
}

closure_without_sugar.rs

// Playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=73e10710b0a077811e08a38ddab3f515

#![feature(unboxed_closures)]
#![feature(fn_traits)]

#![feature(unboxed_closures)]
#![feature(fn_traits)]

fn main() {
    let outer = 10;

    // Desugared version of closure `add_one`
    struct AddOuterEnv<'a> {
        outer: &'a i32,
    }

    impl<'a> FnOnce<(i32,)> for AddOuterEnv<'a> {
        type Output = i32;

        extern "rust-call" fn call_once(self, args: (i32,)) -> i32 {
            self.outer + args.0
        }
    }

    let result = AddOuterEnv { outer: &outer }.call_once((66,));
    println!("Result: {}", result);
}