Is it possible to use `impl Trait` as a function's return type in a trait definition?

Question

Is it at all possible to define functions inside of traits as having impl Trait return types? I want to create a trait that can be implemented by multiple stru

Answer 1

Fairly new to Rust, so may need checking.

You could parametrise over the return type. This has limits, but they're less restrictive than simply returning Self.

trait A where T: A {
    fn new() -> T;
}

// return a Self type
struct St1;
impl A for St1 {
    fn new() -> St1 { St1 }
}

// return a different type
struct St2;
impl A for St2 {
    fn new() -> St1 { St1 }
}

// won't compile as u32 doesn't implement A
struct St3;
impl A for St3 {
    fn new() -> u32 { 0 }
}

The limit in this case is that you can only return a type T that implements A. Here, St1 implements A, so it's OK for St2 to impl A. However, it wouldn't work with, for example,

impl A for St2 ...
impl A for St1 ...

For that you'd need to restrict the types further, with e.g.

trait A where U: A, T: A {
    fn new() -> T;
}

but I'm struggling to get my head round this last one.