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There are two ways that Rust implements an array of characters: str and String.

str, also known as a “string slice”, is usually used with a reference as &str.

To know why, it may help to understand why the following line of code will not compile

const MY_CONST: str = *"Hello, World!";

// the size for values of type `str` cannot be known at compilation time
// the trait `Sized` is not implemented for `str`

Even though the literal “Hello, World!” length can be known at compilation time, as a str it is a sequence of unknown size. But as a reference to a literal (&str), it has a “fat pointer” which holds the address and length of the literal in memory. Fat pointers are of a known size, and so this line will compile

const MY_CONST: &str = "Hello, World!";

A mutable binding to a string slice can have its valued replaced, but not modified in place.

Example

let mut my_str = "Hello, World!";
my_str = "Hi!";
let mut my_string = String::from("Hello");
my_string.push_str(", World!"); // there is no equivalent function to modify &str

It is less constraining to use &str instead of &String when defining a parameter for a function, as this will not compile

pub fn main() {
    let my_str: &str = "Hello, World!";
    say_hi(my_str);
}

fn say_hi(phrase: &String) {
    println!("{phrase:?}");
}

// mismatched types
// expected reference `&String`
// found reference `&str`

A &str will not be coerced into a &String. But this will compile

pub fn main() {
    let my_string = String::from("Hello, World!");
    say_hi(&my_string)
}

fn say_hi(phrase: &str) {
    println!("{phrase:?}");
}

A &String will be coerced into a &str. So to use a parameter type of &str will often be more useful.

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Originally from Exercism rust concepts