Struct PhantomNotSend 

struct PhantomNotSend {
    ghost: PhantomData<*mut ()>,
}

Technically, EnteredSpan can implement both Send and Sync safely. It doesn’t, because it has a PhantomNotSend field, specifically added in order to make it !Send.

Sending an EnteredSpan guard between threads cannot cause memory unsafety. However, it would result in incorrect behavior, so we add a PhantomNotSend to prevent it from being sent between threads. This is because it must be dropped on the same thread that it was created; otherwise, the span will never be exited on the thread where it was entered, and it will attempt to exit the span on a thread that may never have entered it. However, we still want them to be Sync so that a struct holding an Entered guard can be Sync.

Thus, this is totally safe.

Fields

ghost: PhantomData<*mut ()>

Trait Implementations

impl Debug for PhantomNotSend

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Sync for PhantomNotSend

Safety

Trivially safe, as PhantomNotSend doesn’t have any API.