E0626 - Error codes index
Description: Book listing all Rust error codes

# Error code E0626

This error occurs because a borrow in a movable coroutine persists across a yield point.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] || {
        let a = &String::new(); // <-- This borrow...
        yield (); // ...is still in scope here, when the yield occurs.
        println!("{}", a);
    };
    Pin::new(&mut b).resume(());
    }

Coroutines may be either unmarked, or marked with `static`. If it is unmarked, then the coroutine is considered "movable". At present, it is not permitted to have a yield in a movable coroutine that occurs while a borrow is still in scope. To resolve this error, the coroutine may be marked `static`:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] static || { // <-- note the static keyword
        let a = &String::from("hello, world");
        yield ();
        println!("{}", a);
    };
    let mut b = std::pin::pin!(b);
    b.as_mut().resume(());
    }

If the coroutine must remain movable, for example to be used as `Unpin` without pinning it on the stack or in an allocation, we can alternatively resolve the previous example by removing the borrow and just storing the type by value:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] || {
        let a = String::from("hello, world");
        yield ();
        println!("{}", a);
    };
    Pin::new(&mut b).resume(());
    }

This is a very simple case, of course. In more complex cases, we may wish to have more than one reference to the value that was borrowed -- in those cases, something like the `Rc` or `Arc` types may be useful.

This error also frequently arises with iteration:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] || {
      let v = vec![1,2,3];
      for &x in &v { // <-- borrow of `v` is still in scope...
        yield x; // ...when this yield occurs.
      }
    };
    Pin::new(&mut b).resume(());
    }

Such cases can sometimes be resolved by iterating "by value" (or using `into_iter()`) to avoid borrowing:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] || {
      let v = vec![1,2,3];
      for x in v { // <-- Take ownership of the values instead!
        yield x; // <-- Now yield is OK.
      }
    };
    Pin::new(&mut b).resume(());
    }

If taking ownership is not an option, using indices can work too:

    #![allow(unused)]
    fn main() {
    #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
    use std::ops::Coroutine;
    use std::pin::Pin;
    let mut b = #[coroutine] || {
      let v = vec![1,2,3];
      let len = v.len(); // (*)
      for i in 0..len {
        let x = v[i]; // (*)
        yield x; // <-- Now yield is OK.
      }
    };
    Pin::new(&mut b).resume(());

    // (*) -- Unfortunately, these temporaries are currently required.
    // See <https://github.com/rust-lang/rust/issues/43122>.
    }