test.rs

use std::boxed::Box;
use std::thread;
use std::mem;

trait Callback: Send {
    fn callback(&mut self);
}

struct CallbackHolder<CB: Callback> {
    userdata: Box<Option<CB>>
}

fn open<F, CB>(get_callback: F) -> CallbackHolder<CB>
where F: FnOnce() -> CB, CB: Callback {
    let mut userdata: Box<Option<CB>> = Box::new(None);
    *userdata = Some(get_callback());
    CallbackHolder { userdata }
}

struct MyCallback<'a> {
    data: &'a String,
}

impl<'a> Callback for MyCallback<'a> {
    fn callback(&mut self) {
        println!("{}", self.data);
    }
}

fn main() {
    let mut opt = Some(String::from("foobar"));

    let callback_holder = if let Some(ref data) = opt {
        let my_callback = MyCallback { data };
        open(|| { my_callback })
    } else {
        panic!()
    };

    opt = None;
}