Error code E0161

A value was moved whose size was not known at compile time.

Erroneous code example:

trait Bar {
    fn f(self);
}

impl Bar for i32 {
    fn f(self) {}
}

fn main() {
    let b: Box<dyn Bar> = Box::new(0i32);
    b.f();
    // error: cannot move a value of type dyn Bar: the size of dyn Bar cannot
    //        be statically determined
}

In Rust, you can only move a value when its size is known at compile time.

To work around this restriction, consider “hiding” the value behind a reference: either &x or &mut x. Since a reference has a fixed size, this lets you move it around as usual. Example:

trait Bar {
    fn f(&self);
}

impl Bar for i32 {
    fn f(&self) {}
}

fn main() {
    let b: Box<dyn Bar> = Box::new(0i32);
    b.f();
    // ok!
}