What Is Non-Lexical Lifetimes (NLL) in Rust?

Non-Lexical Lifetimes (NLL) is a Rust compiler feature that determines reference validity based on actual usage points rather than the entire lexical scope of a variable. This allows references to be used after other variables in the same scope have been dropped, as long as those variables are no longer needed. The compiler implements this via the mir_borrowck query and Borrowck analysis in rustc_borrowck to track borrows precisely.

fn main() {
    let mut s = String::from("hello");
    let r1 = &s;
    let r2 = &s;
    // Without NLL, r1 would be invalid here because s is still "in scope"
    // With NLL, r1 is valid because it's not used after this point
    println!("{}", r2);
}

NLL is a smart system that checks if your code is safe by looking at exactly when you use a variable, not just where you declared it. It lets you write cleaner code by allowing references to coexist even if they seem to overlap in scope, as long as they aren't used at the same time. Think of it like a librarian who only cares if you are actually reading a book, not just if you checked it out.