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); }