If `unsafe` were viral it would be useless. Syntactically ill-typed code can still be proven semantically well-typed, and `unsafe` is intended as a marker that

If `unsafe` were viral it would be useless. Syntactically ill-typed code can still be proven semantically well-typed, and `unsafe` is intended as a marker that the proof of semantic soundness lies outside of Rust's type system. If there is no proof (at least an informal one), you shouldn't be using unsafe, generally speaking.