Functions & Closures in Rust
From the Rust cheat sheet · Functions & Closures · verified Jul 2026
Functions & Closures
Named functions, closures, and the Fn/FnMut/FnOnce traits.
rust
// Function with parameters and return type
fn add(a: i32, b: i32) -> i32 {
a + b // No semicolon = return value
}
// Closure — anonymous function
let double = |x: i32| x * 2;
let sum = |a, b| a + b;
let result = double(5); // 10
// Closure capturing variables
let name = String::from("Alice");
let greet = || println!("Hello, {name}");💡 No semicolon on the last expression makes it the return value — this is idiomatic Rust
⚡ Closures infer types from usage — you rarely need type annotations on them
📌 Use move || to force a closure to take ownership — required for spawning threads
🟢 Fn (immutable borrow) > FnMut (mutable borrow) > FnOnce (ownership) — most closures are Fn
functionsclosuresfn-traits