Rollup merge of #68700 - withoutboats:wake-trait, r=withoutboats
Add Wake trait for safe construction of Wakers. Currently, constructing a waker requires calling the unsafe `Waker::from_raw` API. This API requires the user to manually construct a vtable for the waker themself - which is both cumbersome and very error prone. This API would provide an ergonomic, straightforward and guaranteed memory-safe way of constructing a waker. It has been our longstanding intention that the `Waker` type essentially function as an `Arc<dyn Wake>`, with a `Wake` trait as defined here. Two considerations prevented the original API from being shipped as simply an `Arc<dyn Wake>`: - We want to support futures on embedded systems, which may not have an allocator, and in optimized executors for which this API may not be best-suited. Therefore, we have always explicitly supported the maximally-flexible (but also memory-unsafe) `RawWaker` API, and `Waker` has always lived in libcore. - Because `Waker` lives in libcore and `Arc` lives in liballoc, it has not been feasible to provide a constructor for `Waker` from `Arc<dyn Wake>`. Therefore, the Wake trait was left out of the initial version of the task waker API. However, as Rust 1.41, it is possible under the more flexible orphan rules to implement `From<Arc<W>> for Waker where W: Wake` in liballoc. Therefore, we can now define this constructor even though `Waker` lives in libcore. This PR adds these APIs: - A `Wake` trait, which contains two methods - A required method `wake`, which is called by `Waker::wake` - A provided method `wake_by_ref`, which is called by `Waker::wake_by_ref` and which implementors can override if they can optimize this use case. - An implementation of `From<Arc<W>> for Waker where W: Wake + Send + Sync + 'static` - A similar implementation of `From<Arc<W>> for RawWaker`.
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src/liballoc/task.rs
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