rust-send-sync

The error means a Send bound failed; check the help line, which often names Sync on the pointee type. Fix the fields by removing sharing, moving the value, or using Sync interior mutability like atomics or Mutex instead of adding unsafe impls.

Send means a value can move to another thread; Sync means it can be shared via references across threads. The key rule is that &T is Send exactly when T is Sync, and both traits are derived field-wise by the compiler.

Arc<T> requires its payload to be both Send and Sync, because a clone hands shared access to another thread. Cell<T> is Send but never Sync, so wrap the value in a Mutex or use an atomic instead.

POSIX requires the unlocking thread to be the locking thread, so MutexGuard's Drop must run where it was created, making it !Send. It is Sync when T is Sync, so &MutexGuard can be sent to scoped threads for read-only access.

No, rustc rejects it with E0321 because cross-crate traits with default impls like Send can only be implemented for struct or enum types. Implement Sync on the owned type instead, which makes &T Send through the blanket impl.

A non-Send value like Rc is held across an .await point, so the generated future is not Send. The note names the value and the trapping await; move the value out of the await scope or write the desugared impl Future + Send signature.