Why is it discouraged to accept a reference to a String (&String), Vec (&Vec), or Box (&Box) as a function argument?

TL;DR: One can instead use &str, &[T] or &T to allow for more generic code.

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1. One of the main reasons to use a String or a Vec is because they allow increasing or decreasing the capacity. However, when you accept an immutable reference, you cannot use any of those interesting methods on the Vec or String.

2. Accepting a &String, &Vec or &Box also requires the argument to be allocated on the heap before you can call the function. Accepting a &str allows a string literal (saved in the program data) and accepting a &[T] or &T allows a stack-allocated array or variable. Unnecessary allocation is a performance loss. This is usually exposed right away when you try to call these methods in a test or a main method:

   awesome_greeting(&String::from("Anna"));
   

   total_price(&vec![42, 13, 1337])
   

   is_even(&Box::new(42))
   

3. Another performance consideration is that &String, &Vec and &Box introduce an unnecessary layer of indirection as you have to dereference the &String to get a String and then perform a second dereference to end up at &str.

Instead, you should accept a string slice (&str), a slice (&[T]), or just a reference (&T). A &String, &Vec<T> or &Box<T> will be automatically coerced (via deref coercion) to a &str, &[T] or &T, respectively.

fn awesome_greeting(name: &str) {
    println!("Wow, you are awesome, {}!", name);
}

fn total_price(prices: &[i32]) -> i32 {
    prices.iter().sum()
}

fn is_even(value: &i32) -> bool {
    *value % 2 == 0
}

Now you can call these methods with a broader set of types. For example, awesome_greeting can be called with a string literal ("Anna") or an allocated String. total_price can be called with a reference to an array (&[1, 2, 3]) or an allocated Vec.

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If you’d like to add or remove items from the String or Vec<T>, you can take a mutable reference (&mut String or &mut Vec<T>):

fn add_greeting_target(greeting: &mut String) {
    greeting.push_str("world!");
}

fn add_candy_prices(prices: &mut Vec<i32>) {
    prices.push(5);
    prices.push(25);
}

Specifically for slices, you can also accept a &mut [T] or &mut str. This allows you to mutate a specific value inside the slice, but you cannot change the number of items inside the slice (which means it’s very restricted for strings):

fn reset_first_price(prices: &mut [i32]) {
    prices[0] = 0;
}

fn lowercase_first_ascii_character(s: &mut str) {
    if let Some(f) = s.get_mut(0..1) {
        f.make_ascii_lowercase();
    }
}