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

Alternative	Use When	Don't Use When
`Arc` + move	Shared read across threads	Single owner
Scoped threads	Borrow stack data	Detached thread
Channel send	Hand off ownership	Shared state needed

FAQs

move all captures?

Yes - keyword applies to inferred capture set.

tokio spawn?

Same move + Send + 'static.

clone before move?

Keep copy in parent thread.

FnOnce after move?

Consuming captures often FnOnce.

copy types?

Copied into closure environment.

ref in move?

Still moves owned refs (e.g. String).

scoped threads?

May borrow without move - scoped API.

closure size?

Large captured struct increases closure size.

wasm?

thread spawn limited - move still for callbacks.

lint?

redundant_clone around move patterns.

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+.

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

Alternative	Use When	Don't Use When
`Arc` + move	Shared read across threads	Single owner
Scoped threads	Borrow stack data	Detached thread
Channel send	Hand off ownership	Shared state needed

FAQs

move all captures?

Yes - keyword applies to inferred capture set.

tokio spawn?

Same move + Send + 'static.

clone before move?

Keep copy in parent thread.

FnOnce after move?

Consuming captures often FnOnce.

copy types?

Copied into closure environment.

ref in move?

Still moves owned refs (e.g. String).

scoped threads?

May borrow without move - scoped API.

closure size?

Large captured struct increases closure size.

wasm?

thread spawn limited - move still for callbacks.

lint?

redundant_clone around move patterns.

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+.

move Closures

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move Closures