Error code E0504

Note: this error code is no longer emitted by the compiler.

This error occurs when an attempt is made to move a borrowed variable into a closure.

Erroneous code example:

struct FancyNum { num: u8, } fn main() { let fancy_num = FancyNum { num: 5 }; let fancy_ref = &fancy_num; let x = move || { println!("child function: {}", fancy_num.num); // error: cannot move `fancy_num` into closure because it is borrowed }; x(); println!("main function: {}", fancy_ref.num); }

Here, fancy_num is borrowed by fancy_ref and so cannot be moved into the closure x. There is no way to move a value into a closure while it is borrowed, as that would invalidate the borrow.

If the closure can't outlive the value being moved, try using a reference rather than moving:

struct FancyNum { num: u8, } fn main() { let fancy_num = FancyNum { num: 5 }; let fancy_ref = &fancy_num; let x = move || { // fancy_ref is usable here because it doesn't move `fancy_num` println!("child function: {}", fancy_ref.num); }; x(); println!("main function: {}", fancy_num.num); }

If the value has to be borrowed and then moved, try limiting the lifetime of the borrow using a scoped block:

struct FancyNum { num: u8, } fn main() { let fancy_num = FancyNum { num: 5 }; { let fancy_ref = &fancy_num; println!("main function: {}", fancy_ref.num); // `fancy_ref` goes out of scope here } let x = move || { // `fancy_num` can be moved now (no more references exist) println!("child function: {}", fancy_num.num); }; x(); }

If the lifetime of a reference isn't enough, such as in the case of threading, consider using an Arc to create a reference-counted value:

use std::sync::Arc; use std::thread; struct FancyNum { num: u8, } fn main() { let fancy_ref1 = Arc::new(FancyNum { num: 5 }); let fancy_ref2 = fancy_ref1.clone(); let x = thread::spawn(move || { // `fancy_ref1` can be moved and has a `'static` lifetime println!("child thread: {}", fancy_ref1.num); }); x.join().expect("child thread should finish"); println!("main thread: {}", fancy_ref2.num); }