Trait `x` is not implemented for the type `x`

Question

When compiling the following code:

trait RenderTarget {}

struct RenderWindow;
impl RenderTarget for RenderWindow {}

trait Drawable {
    fn draw

        

Answer 1

I refer to Vladimir's excellent answer which explains Object's safety, however I am afraid than in the middle of the discussion the concrete problem at hand was forgotten.

As Vladimir mentions, the issue is that a method generic over types (generic over lifetimes is fine) renders the trait it belongs to unusable for run-time polymorphism; this, in Rust, is called Object Safety.

The simplest fix, therefore, is to remove the generic parameter of the method!

trait RenderTarget {}

struct RenderWindow;
impl RenderTarget for RenderWindow {}

trait Drawable {
    fn draw(&self, target: &mut RenderTarget);
}

fn main() {
    let mut win = RenderWindow;
    let mut vec: Vec> = Vec::new();

    for e in &vec {
        e.draw(&mut win);
    }
}

The main difference between:

fn draw(&self, target: &mut RT)

and

fn draw(&self, target: &mut RenderTarget)

is that the latter requires RenderTarget to be Object Safe too as it is now used in a run-time polymorphism situation (so, no static method, no generic method, no Self, ...).

Another (more technical) difference is that the former is "monorphised" at compile-time (that is RT is substituted with the real type and all relevant optimizations applied) whereas the latter is not (and so, no such optimizations occur).