**Course Title:** Mastering Rust: From Basics to Systems Programming **Section Title:** Ownership, Borrowing, and Lifetimes **Topic:** Write Rust programs that demonstrate ownership and borrowing concepts. (Lab topic) In this lab, we'll put the concepts of ownership and borrowing into practice by writing Rust programs that demonstrate these concepts. We'll explore different scenarios and exercises to reinforce your understanding of ownership and borrowing rules in Rust. **Exercise 1: Ownership and Borrowing Basics** Create a new Rust project using Cargo by running the command `cargo new ownership_borrowing` in your terminal. Update the `main.rs` file with the following code: ```rust fn main() { let s = String::from("Hello, World!"); // s owns the string "Hello, World!" println!("{}", s); // prints "Hello, World!" let len = calculate_length(&s); // len borrows s println!("{}", len); // prints the length of the string "Hello, World!" } fn calculate_length(s: &String) -> usize { s.len() } ``` In this example, we define a string `s` that owns the string "Hello, World!". We then pass a reference to `s` to the `calculate_length` function, which returns the length of the string. The `&s` syntax creates a reference to `s`, allowing the `calculate_length` function to borrow `s` without taking ownership of it. **Exercise 2: Mutable References** Update the `main.rs` file with the following code: ```rust fn main() { let mut s = String::from("Hello, World!"); // s owns the string "Hello, World!" println!("{}", s); // prints "Hello, World!" let len = calculate_length(&s); // len borrows s println!("{}", len); // prints the length of the string "Hello, World!" let result = append_str(&mut s); // result borrows mutably s println!("{}", result); // prints "Append success" println!("{}", s); // prints the updated string } fn calculate_length(s: &String) -> usize { s.len() } fn append_str(s: &mut String) -> &str { s.push_str(" Appended"); "Append success" } ``` In this example, we define a mutable string `s` that owns the string "Hello, World!". We then pass a mutable reference to `s` to the `append_str` function, which appends a string to `s`. The `&mut s` syntax creates a mutable reference to `s`, allowing the `append_str` function to borrow `s` mutably. **Exercise 3: Aliasing** Update the `main.rs` file with the following code: ```rust fn main() { let mut s = String::from("Hello, World!"); // s owns the string "Hello, World!" println!("{}", s); // prints "Hello, World!" let len = calculate_length(&s); // len borrows s println!("{}", len); // prints the length of the string "Hello, World!" { let r1 = &s; // r1 borrows s println!("{}", r1); // prints "Hello, World!" // let r2 = &s; // error: cannot borrow as mutable } let r2 = &mut s; // r2 borrows mutably s println!("{}", r2); // prints "Hello, World!" } ``` In this example, we define a mutable string `s` that owns the string "Hello, World!". We then define two references `r1` and `r2` that borrow `s`. However, we can't define `r2` while `r1` is still in scope, because `r2` borrows `s` mutably and `r1` borrows `s` immutably. **Practical Takeaways** * Ownership and borrowing rules ensure memory safety in Rust programs. * Use references to borrow values without taking ownership of them. * Use mutable references to borrower values mutably. * Be careful when borrowing values to avoid aliasing issues. **External Resources** * [The Rust Book: Ownership and Borrowing](https://doc.rust-lang.org/book/ch04-01-ownership-and-borrowing.html) * [Rust Documentation: Borrowing and References](https://doc.rust-lang.org/reference/borrowing-and-references.html) **Comments and Questions** If you have any questions or need help with the exercises, feel free to leave a comment below. Next topic: **Conditional statements: if, else, match.** (From: Control Flow and Functions)

fn main() {
    let s = String::from("Hello, World!"); // s owns the string "Hello, World!"
    println!("{}", s); // prints "Hello, World!"

    let len = calculate_length(&s); // len borrows s
    println!("{}", len); // prints the length of the string "Hello, World!"
}

fn calculate_length(s: &String) -> usize {
    s.len()
}

fn main() {
    let mut s = String::from("Hello, World!"); // s owns the string "Hello, World!"
    println!("{}", s); // prints "Hello, World!"

    let len = calculate_length(&s); // len borrows s
    println!("{}", len); // prints the length of the string "Hello, World!"

    let result = append_str(&mut s); // result borrows mutably s
    println!("{}", result); // prints "Append success"
    println!("{}", s); // prints the updated string
}

fn calculate_length(s: &String) -> usize {
    s.len()
}

fn append_str(s: &mut String) -> &str {
    s.push_str(" Appended");
    "Append success"
}

fn main() {
    let mut s = String::from("Hello, World!"); // s owns the string "Hello, World!"
    println!("{}", s); // prints "Hello, World!"

    let len = calculate_length(&s); // len borrows s
    println!("{}", len); // prints the length of the string "Hello, World!"

    {
        let r1 = &s; // r1 borrows s
        println!("{}", r1); // prints "Hello, World!"

        // let r2 = &s; // error: cannot borrow as mutable
    }

    let r2 = &mut s; // r2 borrows mutably s
    println!("{}", r2); // prints "Hello, World!"
}