Why have an Ungil trait that is equivalent to Send?

[nnmm]

As far as I can see, Ungil is implemented for exactly the same types as Send, and also cannot be implemented for non-Send user types. Why is this trait used instead of Send itself? Just to express intent more clearly?

[mejrs]

That is true on stable rust, but not when using the nightly feature. See also https://pyo3.rs/internal/doc/pyo3/marker/