Algebraic datatypes, which naturally begets Option and Either, predate OCaml.
I am fully aware of it, I was just making the point that it wasn't invented by Rust as the title suggests, and first versions of Rust were written in OCaml, hence why I chose that ML flavour for the example.
If you want to go pedatic we can start discussing the type theory papers they appear on.
When I say "inspired by Rust Result type", I mean the function names.
I could implement map/fmap/bind/lift/unlift/return/...