CODEWITHSUNDEEP
rustreferenceclosuresmove-semanticsborrow-checker
Lucida
Lucida

Reputation: 65

Why move closure in rust can't move the ownership of a referenced variable?

Here's the example

fn call_with_f<F>(f:F) where F: FnOnce(){
    f();
}
struct Vec2{
    x:i32,
    y:i32
}
impl Vec2{
    fn new(x:i32, y:i32) -> Self{
        Self{
            x,
            y
        }
    }
    fn x(&self) -> &i32{
        &self.x
    }
}
fn main() {
    let v = Vec2::new(1,2);
    let rx = v.x();
    call_with_f(move||{
        println!("{}", v.x());
        println!("{}", rx);
    });
}

The compiler gives

error[E0505]: cannot move out of `v` because it is borrowed
  --> src\main.rs:88:14
   |
87 |     let rx = v.x();
   |              - borrow of `v` occurs here
88 |     call_with_f(move||{
   |                 ^^^^^^ move out of `v` occurs here
89 |         println!("{}", v.x());
   |                        - move occurs due to use in closure
90 |         println!("{}", rx);
   |                        -- borrow later captured here by closure

I know the complaint of compiler, but this case is necessary sometime. In a very complicated
circumstance, a bunch of work need to be done before given into the closure, a reference as an intermediate varible could simplify the preparation. v and rv have same lifetime at the beginning, they also remain the same lifetime after moved according to the semantics of the move closure. Why?

Upvotes: 0

Views: 2500

Answers (1)

Chayim Friedman
Chayim Friedman

Reputation: 71485

You cannot move a borrowed value. Period.

Moreover, such closure cannot exist (safely): it needs to store both the owned value and a reference to (part of) it, i.e. it is self-referential. See Lifetime of references in closures.

Depending on your use case, there may be various solutions, ordered from the best to the worst:

  • Create the reference inside the closure.
  • Don't move the owned value. One of the common reasons people have your question is because they need a 'static closure for threads. If this is the case, you may find scoped threads helpful.
  • Use a crate such as owning_ref (unsound!) or ourboros (no known sound holes IIRC) to create the self-referential closure. You may need to desugar the closure manually (partially, at least).
  • Use unsafe code for creating the self referential closure. WARNING: Do Not Do That Unless You Really Know What You're Doing.

Upvotes: 2

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