traits.rs

// Type you want to build
struct MyThing {}

trait MyBuilder {
  type OutputType;
  fn build(&self) -> OutputType;
}

struct MyThingBuilder {}

impl MyBuilder for MyThingBuilder {
  type OutputType = MyThing;
  fn build(&self) -> String { /* ... */ }  // Won't compile, returns the wrong type
  fn build(&self) -> MyThing { /* ... */ }  // Will compile, returns the right type
}

This pattern allows the trait to be generic without specifying a MyBuilder<T>-style thing.