rust_boxed_trait_refs.rs

use std::mem::transmute;

struct S1<'a> {
    x: &'a u32,
}

struct S2<'a, 'b> {
    x: &'a u32,
    y: &'b u32,
}

trait S {
    fn sum(&self) -> u32;
}

impl<'a> S for S1<'a> {
    fn sum(&self) -> u32 { *self.x }
}

impl<'a, 'b> S for S2<'a, 'b> {
    fn sum(&self) -> u32 { *self.x + *self.y }
}

struct Container<'a> {
    ints: Vec<Box<u32>>,
    structs: Vec<Box<S + 'a>>,
}

fn get_ref<'a, T>(x: &Box<T>) -> &'a T {
    unsafe { transmute(&**x) }
}

fn make_container<'a>() -> Container<'a> {
    let mut cont = Container { ints: Vec::new(), structs: Vec::new() };
    
    cont.ints.push(Box::new(1));
    cont.ints.push(Box::new(2));
    cont.ints.push(Box::new(3));
    
    let s1 = Box::new(S1 { x: get_ref(&cont.ints[0]) });
    let s2 = Box::new(S2 { x: get_ref(&cont.ints[1]), y: get_ref(&cont.ints[2]) });
    cont.structs.push(s1);
    cont.structs.push(s2);
    cont
}

fn main() {
    let cont = make_container();
    
    for elem in cont.structs {
        println!("{}", elem.sum());
    }
}