Comment by Sharlin

Even the newtype-based "parse, don't validate" is tremendously useful in practice, though. The big thing is that if you have a bare string, you don't know "where it's been". It doesn't carry with it information whether it's already been validated. Even if a newtype can't provide you full correctness by construction, it's vastly easier to be convinced of the validity of an encapsulated value compared to a naked one.

For full-on parse-don't-validate, you essentially need a dependent type system. As a more light-weight partial solution, Rust has been prototyping pattern types, which are types constrained by patterns. For instance a range-restricted integer type could be simply spelled `i8 is 0..100`, or a nonempty slice as `[T] is [_, ..]`. Such a feature would certainly make correctness-by-construction easier in many cases.

The non-empty list implemented as a (T, Vec<T>) is, btw, a nice example of the clash between practicality and theoretical purity. It can't offer you a slice (consecutive view) of its elements without storing the first element twice (which requires immutability and that T: Clone, unlike normal Vec<T>), which makes it fairly useless as a vector. It's okay if you consider it just an abstract list with a more restricted interface.