RUST SERIES - Borrow Checker Bonus Chapter: Non-Lexical Lifetimes

#programming #rust #coding #interview

The Key Insight: Rust's borrow checker tracks the last actual use of a reference, not when it goes out of scope.

Before NLL (The Old Pain)

let chef = &mut kitchen;
chef.push("sugar");
println!("Chef's kitchen: {:?}", chef);
let inspector = &kitchen;       // ❌ ERROR in old Rust
println!("Inspector: {:?}", inspector);
// chef scope ends here

Problem: Had to create artificial scopes just to end borrows early.

After NLL (The Modern Magic)

let chef = &mut kitchen;
chef.push("sugar");
println!("Chef's kitchen: {:?}", chef);  // ← Last use ends borrow
let inspector = &kitchen;                 // ✅ Safe!
println!("Inspector: {:?}", inspector);

Solution: Borrow ends when you stop using it, not when scope ends.

Why This Matters

fn smart_processing() {
    let mut data = vec![1, 2, 3];

    // Modification phase
    let modifier = &mut data;
    modifier.push(4);
    println!("Modified: {:?}", modifier);  // Last use = borrow ends

    // Reading phase (no extra scopes needed!)
    let reader1 = &data;  // ✅ Works immediately
    let reader2 = &data;  // ✅ Multiple readers fine
    println!("Readers see: {:?} and {:?}", reader1, reader2);
}