The latter is shown to be type-based static analysis, even though it "looks like" there's a runtime component: the Never plugs one of only two holes in Result, so it statically becomes equivalent to a newtype.

The former can't be restricted to type analysis (without something like dependent types I think?). You're right that we could apply constant folding to check in this case. My understanding is that the Rust design philosophy is to skip things like this because of the potential to produce surprising results upon refactoring.