An example I can give: Rust doesn't support return type inference. You can use impl Trait for that. You can also write -> _ and the compiler will tell you what type it would have inferred for you to write down there. Global return type inference won't ever be allowed in Rust. But maybe we could allow it inside of a single module? A single crate? I would expect the DX to be crap, but we could technically do it without losing too much on correctness.