Rust

You may be better off with &[String] as a read-only view of Vec. To get &[&str] I think you need to create a new collection to hold the &str values. (String and &str have different memory representations.) But the choice depends on what you want to do - maybe providing &str values adds a convenience that is worth creating a second collection.

For the Option case I would go with Option<&[String]>. My understanding is that Option<&T> is the same size as &T for any T so an owned Option wrapper is zero-cost. If the reference pointer is null then Rust interprets that as None. Besides you usually want ownership of an Option so you can map it or whatever else you want to do.

Why not &[String]?

A vec and a string are basically the same thing (a series of bytes)

In the context of vectors I prefer my APIs to return an empty set rather than an None-option. This makes handling it much easier because you can still iterate over it, it just has nothing.

This might involve the compiler making an allocation of an empty array but most of them (gcc, ghc) will now what you are doing and optimize the null check on the empty array to a bool check.