Do you know about unimplemented!()? I use #[test] functions for what I'd throw into a REPL and if it's not horrible you can keep them as actual tests later.
That's really nifty! Not quite was I was thinking of though, but it gave me a starting point for some research before I came across https://github.com/rust-lang/rfcs/issues/1911 which describes why unimplemented!() doesn't quite work and what I'd like instead.
