What are lifetimes in Rust

Lifetimes in Rust are annotations that tell the compiler how long a reference must remain valid to prevent dangling pointers. They ensure that references do not outlive the data they point to, enforcing memory safety at compile time.

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

In this example, the lifetime parameter 'a guarantees that the returned reference lives as long as the shorter of the two input references.

Lifetimes are rules that ensure a reference to data doesn't get used after the data itself is gone. Think of it like a library book: you can only borrow it while the library is open, and you must return it before the book is destroyed. This prevents your program from crashing by trying to access memory that no longer exists.