example.rs

trait Trait {
    fn print(&mut self);
}

struct ImplA {
    s: String,
}

impl Trait for ImplA {
    fn print(&mut self) {
        println!("A: {:?}", self.s);
    }
}

struct ImplB {
    s: String,
}

impl Trait for ImplB {
    fn print(&mut self) {
        println!("B: {:?}", self.s);
    }
}

fn main() {
    let box_a = &mut Box::new(ImplA {s: String::from("foobar")});
    let box_b = &mut Box::new(ImplB {s: String::from("foobar")});
    let v: Vec<&mut Box<Trait>> = vec!(box_a, box_b);

    for f in v {
        f.print();
    }
}

output

zeroem@whizbang:~/projects/rust-play$ rustc vec-of-mutable-traits.rs 
vec-of-mutable-traits.rs:29:40: 29:45 error: mismatched types:
 expected `&mut Box<Trait>`,
    found `&mut Box<ImplA>`
(expected trait Trait,
    found struct `ImplA`) [E0308]
vec-of-mutable-traits.rs:29     let v: Vec<&mut Box<Trait>> = vec!(box_a, box_b);
                                                                   ^~~~~
vec-of-mutable-traits.rs:29:35: 29:53 note: in this expansion of vec! (defined in <std macros>)
vec-of-mutable-traits.rs:29:40: 29:45 help: run `rustc --explain E0308` to see a detailed explanation
vec-of-mutable-traits.rs:29:47: 29:52 error: mismatched types:
 expected `&mut Box<Trait>`,
    found `&mut Box<ImplB>`
(expected trait Trait,
    found struct `ImplB`) [E0308]
vec-of-mutable-traits.rs:29     let v: Vec<&mut Box<Trait>> = vec!(box_a, box_b);
                                                                          ^~~~~
vec-of-mutable-traits.rs:29:35: 29:53 note: in this expansion of vec! (defined in <std macros>)
vec-of-mutable-traits.rs:29:47: 29:52 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to 2 previous errors