What you're talking about is the canonical example for generic associated types, which were stabilised late 2022: https://blog.rust-lang.org/2022/11/03/Rust-1.6

What you're talking about is the canonical example for generic associated types, which were stabilised late 2022: https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#generic-associated-types-gats https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#gener...