part1.rs

use std::marker::PhantomData;
use std::thread::{sleep, spawn, JoinHandle};
use std::time::Duration;


#[derive(Default)]
pub struct ThreadScope<'thread> {
    _marker: PhantomData<&'thread ()>,
    joins: Vec<JoinHandle<()>>,
}

impl<'thread> ThreadScope<'thread> {
    pub fn spawn<F>(&mut self, f: F)
    where
        F: FnOnce() + Send + 'thread,
    {
 
        let func: Box<dyn FnOnce() + Send + 'thread> = Box::new(f);
        // Here we 'transmute' 'thread to 'static
        let closure: Box<dyn FnOnce() + Send + 'static> = unsafe {
            std::mem::transmute(func)
        };

        let handle = spawn(closure);
        self.joins.push(handle);
    }
}

impl<'thread> Drop for ThreadScope<'thread> {
    fn drop(&mut self) {
        for handle in self.joins.drain(..) {
            handle.join().unwrap();
        }
    }
}


fn main() {
    let val = String::from("hello");
    let val_ref = val.as_str();

    let mut scope = ThreadScope::default();   

    scope.spawn(|| {
        println!("{:?}", &val_ref);
    });

    scope.spawn(|| {
        println!("{:?}", &val_ref);
    });

    drop(scope);
    drop(val);
}