Add element to immutable vector rust

Question

I am trying to create a user input validation function in rust utilising functional programming and recursion. How can I return an immutable vector with one element concatenated onto the end?

fn get_user_input(output_vec: Vec<String>) -> Vec<String> {
    // Some code that has two variables: repeat(bool) and new_element(String)
    if !repeat {
        return output_vec.add_to_end(new_element); // What function could "add_to_end" be?
    }
    get_messages_from_user(output_vec.add_to_end(new_element)) // What function could "add_to_end" be?
}

There are functions for everything else:
push adds a mutable vector to a mutable vector
append adds an element to the end of a mutable vector
concat adds an immutable vector to an immutable vector
??? adds an element to the end of a immutable vector

The only solution I have been able to get working is using:

[write_data, vec![new_element]].concat()

but this seems inefficient as I'm making a new vector for just one element (so the size is known at compile time).