move Closures

move closures capture variables by value into the closure environment, required for thread::spawn and long-lived callbacks.

Recipe

use std::thread;
 
fn main() {
    let data = vec![1, 2, 3];
    thread::spawn(move || println!("{:?}", data));
}

When to reach for this: Threads, tokio::spawn, timers, and when closure may outlive stack borrows.

Working Example

fn spawn_sum(nums: Vec<i32>) -> std::thread::JoinHandle<i32> {
    std::thread::spawn(move || nums.iter().sum())
}
 
fn main() {
    let h = spawn_sum(vec![1, 2, 3]);
    println!("{}", h.join().unwrap());
}

What this demonstrates:

• nums moved into closure
• Main thread cannot use nums after spawn
• JoinHandle returns result from thread

Deep Dive

move applies to all captured variables, not selective. Use clone before move if both thread and main need data.

Gotchas

• move when borrow would work - Extra clones. Fix: Omit move for short-lived closures.
• move copies Copy types - Still "move" semantics but bits copied.
• Arc needed for shared ownership - Multiple threads need Arc clone before move.
• RefCell across threads - Not Send. Fix: Mutex.
• async move block - Captures owned for future.

Alternatives

FAQs

Related

• Fn, FnMut & FnOnce
• 'static & Bounds
• Scoped Threads
• Shared State in Async

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+.