Fn, FnMut & FnOnce

Closures implement Fn (shared borrow), FnMut (mut borrow), or FnOnce (consume captures) based on usage.

Recipe

fn call_twice(f: impl Fn() -> i32) -> i32 {
    f() + f()
}
 
fn main() {
    let n = 10;
    println!("{}", call_twice(|| n));
}

When to reach for this: Bounding closure parameters in generic APIs (thread::spawn needs FnOnce).

Working Example

fn with_mut(f: impl FnMut(&mut Vec<i32>)) {
    let mut v = vec![1];
    f(&mut v);
}
 
fn main() {
    with_mut(|v| v.push(2));
}

What this demonstrates:

• FnMut for mutating captured state or mut ref args
• FnOnce required if closure consumes captured String
• Compiler picks minimal trait impl

Deep Dive

Fn is subtrait of FnMut is subtrait of FnOnce. Prefer impl Fn when possible for flexibility.

Gotchas

• Spawn requires FnOnce + Send + 'static - Captures must be owned. Fix: move closure.
• Storing FnMut trait object - Box<dyn FnMut()>. Fix: Box or generic.
• Calling FnOnce twice - Compile error. Fix: Call once or clone data before.
• Conflicting captures - Move and borrow same variable. Fix: Restructure captures.
• Method refs in closure - Auto trait impl may be FnMut.

Alternatives

FAQs

Related

• Closures Basics
• move Closures
• Returning Closures
• Higher-Order Functions

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.