What is NLL Non-Lexical Lifetimes

NLL (Non-Lexical Lifetimes) is Rust's modern borrow checker that tracks reference validity based on actual usage rather than syntactic scope blocks. It allows references to live only as long as they are actively used, enabling more flexible code patterns without manual lifetime annotations.

fn main() {
    let mut vec = vec![1, 2, 3];
    let first = &vec[0]; // Borrow starts here
    vec.push(4);         // Error in old checker, OK in NLL if first is unused
    println!("{:?}", first); // Borrow ends here
}

NLL is a smarter way for Rust to check if your code is safe to run. Instead of assuming a reference lasts until the end of a code block, it checks exactly where you use that reference. Think of it like a library book: you only need to keep it checked out while you are actually reading it, not for the entire time you are in the library.