Neat that looks interesting. There's a similar Haskell idiom called session types. I have a bit of reservation about whether one can easily use Rust traits to mark out the permissible state sets that an operation can take, but that's because I don't know Rust at all. I do remember doing a hacky thing with TypeLits in Haskell to handle that. Basically you can have numbers in the type signatures and do some limited arithmetic with them at type level. I'd be interested to know if that is doable in Rust.