In the last years type systems are being used to statically type dynamically typed languages, in Flow (JS), TypeScript (Js), MyPy (Python), Typed Racket (Scheme), etc. To make them more ergonomic their type system need to be refined and flexible, so they often contain flow typing (invented in Whiley language?). Now you鈥檙e suggesting to start introducing something like flow typing in Rust 
Once (and if) enum variant become distinct types in Rust, your idea becomes true flow typing.
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