Why doesn't Rust support trait object upcasting?

Question

Given this code:

trait Base {
    fn a(&self);
    fn b(&self);
    fn c(&self);
    fn d(&self);
}

trait Derived : Base {
    fn e(&sel         


        

Answer 1

I ran into the same wall when I started with Rust. Now, when I think about traits, I have a different image in mind than when I think about classes.

trait X: Y {} means when you implement trait X for struct S you also need to implement trait Y for S.

Of course this means that a &X knows it also is a &Y, and therefore offers the appropriate functions. It would require some runtime-effort (more pointer dereferences) if you needed to traverse pointers to Y's vtable first.

Then again, the current design + additional pointers to other vtables probably wouldn't hurt much, and would allow easy casting to be implemented. So maybe we need both? This is something to be discussed on internals.rust-lang.org