Retrieve data and vtable from Waker

[comex]

Currently, you can use RawWaker::new to create a RawWaker from a data pointer and vtable pointer, and you can use Waker::from_raw to create a Waker from a RawWaker.

But there are no APIs to perform either conversion in the opposite direction. Why not?

Retrieving the data and vtable pointer would be particularly useful for a Future that expects to be run on a particular executor: it could downcast the data pointer to perform custom operations on it, after verifying that the vtable pointer was an expected value.

[comex]

(...On a related note, although it's probably too late to change without breaking the API, why does Context store a reference to a Waker instead of storing it by value? It's not like you can rely on particular data being stored after the Waker itself, since it's safe to clone one. In lieu of that, it's only two words, cheap to copy when creating a Context – and the Context itself is passed through futures by reference already.)