Not sure that type is good advice:

    pub struct NonEmpty<T> {
        pub head: T,
        pub tail: Vec<T>,
    }

You'd have to manually implement the traits to support the ergonomics of slices and iteration and costly reallocation if you need to pass ownership as a Vec:

I'd expect:

    pub struct NonEmpty<T> {
        v: Vec<T>,
    }

The constructor would enforce the invariant and then you'd impl Deref and DerefMut for [T] to gain normal len/is_empty/indexing/iteration, passing as &[T] to other funcs and mutating values (which can't break the invariant).

To mutate length while preserving the invariant it's dealers choice e.g.

- add .into_vec() for unwrap/mutate/rewrap

- add invariant preserving mutators of your choice