Error code E0626

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

Erroneous code example:

let mut b = || {
    let a = &String::new(); // <-- This borrow...
    yield (); // ...is still in scope here, when the yield occurs.
    println!("{}", a);
};
Pin::new(&mut b).resume(());

Run

At present, it is not permitted to have a yield that occurs while a borrow is still in scope. To resolve this error, the borrow must either be “contained” to a smaller scope that does not overlap the yield or else eliminated in another way. So, for example, we might resolve the previous example by removing the borrow and just storing the integer by value:

let mut b = || {
    let a = 3;
    yield ();
    println!("{}", a);
};
Pin::new(&mut b).resume(());

Run

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:

let mut b = || {
  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(());

Run

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

let mut b = || {
  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(());

Run

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

let mut b = || {
  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>.

Run

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