transmuting &dyn Trait to &(dyn Trait + Send) is unsound in general

[lcnr]

the Send bound in &(dyn Trait + Send) is useless as &T is only Send for T: Sync. We may still rely on that Send bound in other ways however:

use std::rc::Rc;
use std::thread;

trait Trait {
    fn foo(&self)
    where
        Self: Send;
}

impl<T: Clone + 'static> Trait for T {
    fn foo(&self)
    where 
        Self: Send,
    {  
        let clone: T = self.clone();
        thread::spawn(move || clone.clone());
    }
}

fn main() {
    let rc = Rc::new(String::from("hello"));
    let x: &dyn Trait = &rc;
    loop {
        println!("strong count: {}", Rc::strong_count(&rc));
        let y: &(dyn Trait + Send) = unsafe { std::mem::transmute(x) };
        y.foo();
    }
}