Cancellation isn't possible in general. For example, if you've kicked off expensive work on another thread or passed a pointer to your future to the kernel via io_uring, you must add a layer of indirection that's quite similar to a cancelation context. You can't just accept that the expensive work you don't care about will keep happening or resume a future that has been dropped when the cqe entry bearing a pointer to it arrives. The cancellation facility provided by io_uring that guarantees that you won't receive such a thing does so by blocking your thread for a while, which is undesirable.

As implemented, async/await typically greatly harms composability. For example see here: https://nullderef.com/blog/rust-async-sync/

In the specific case of Rust, we might begin to be able to write composable libraries a couple decades from now: https://github.com/rust-lang/keyword-generics-initiative