How to Use Nested Functions in Rust

You define a nested function by writing a fn block inside another function's body, making it visible only to the outer function. This keeps helper logic encapsulated and prevents name collisions in the global scope.

fn outer() {
    fn inner() {
        println!("Hello from inside!");
    }
    inner();
}

In this example, inner is defined within outer and can only be called from within outer. Attempting to call inner from main or any other function will result in a compile-time error because it is out of scope.

A nested function is a helper function defined inside another function, so it only exists while that outer function runs. It is useful for keeping small, one-time-use logic close to where it is needed without cluttering the rest of your code. Think of it like a private note written on a sticky note inside a specific folder; only people looking inside that folder can see it.