Rust's ownership model is one of its most unique and powerful features, ensuring memory safety without needing a garbage collector. Ownership in Rust is governed by a set of rules that the compiler checks at compile time. Understanding these rules is crucial for writing efficient and safe Rust code.

In Rust, each value has a variable that's called its owner. There can only be one owner at a time, and when the owner goes out of scope, the value is dropped. Here are the basic rules of ownership:

1. Each value in Rust has a variable that's called its owner.
2. There can only be one owner at a time.
3. When the owner goes out of scope, the value will be dropped (no longer valid).

{
    let s = String::from("hello"); // s is the owner of the String
} // s goes out of scope and "hello" is dropped

Rust allows you to create references to a value, which lets you access it without taking ownership. This is called borrowing. Borrowing can be immutable or mutable.

You can create multiple immutable references to a value, but you cannot have a mutable reference while immutable references exist. This allows you to read from the value without changing it.

fn main() {
    let s1 = String::from("hello");
 
    let len = calculate_length(&s1); // borrow s1 as immutable
    println!("The length of '{}' is {}.", s1, len);
}
 
fn calculate_length(s: &String) -> usize { // s is an immutable reference to a String
    s.len()
}

In the example above, &s1 creates an immutable reference to s1. This means that calculate_length borrows s1 but does not take ownership of it. The & symbol is used to denote a reference in Rust. This allows the function to access the value without taking ownership, which means s1 can still be used after the function call.

Similarly, in the function signature fn calculate_length(s: &String) -> usize, &String indicates that the parameter s is an immutable reference to a String. This allows the function to read from the String without modifying it or taking ownership.

In this challenge, you will create a function calculate_length that takes an immutable reference to a String, calculates its length, and returns the length.

The task is designed to help you understand the concepts of ownership and immutable borrowing in Rust.

• The calculate_length function should take an immutable reference to the input String and return its length.

let s1 = String::from("hello");
let len = calculate_length(&s1);
assert_eq!(len, 5);

• The String type in Rust has a method len() which returns its length.