We investigate the effects on code quality of the design patterns typestate and newtype in the language Rust, by conducting three case studies from production code, evaluated through expert interviews, static code analysis and benchmarks. The findings indicate that: (1) Typestate improves code faultlessness and testability, but comes at the cost of more boilerplate code and can degrade readability. Code with extensive branching logic and many invariants likely benefits most from the pattern. (2) Newtype combined with the “Parse, don’t validate” principle improves code quality at a low cost and prevents invalid states at runtime.