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Rollup merge of #70817 - yoshuawuyts:task-ready, r=dtolnay
Add core::task::ready! macro This PR adds `ready!` as a top-level macro to `libcore` following the implementation of `futures_core::ready`, tracking issue #70922. This macro is commonly used when implementing `Future`, `AsyncRead`, `AsyncWrite` and `Stream`. And being only 5 lines, it seems like a useful and straight forward addition to std. ## Example ```rust use core::task::{Context, Poll}; use core::future::Future; use core::pin::Pin; async fn get_num() -> usize { 42 } pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> { let mut f = get_num(); let f = unsafe { Pin::new_unchecked(&mut f) }; let num = ready!(f.poll(cx)); // ... use num Poll::Ready(()) } ``` ## Naming In `async-std` we chose to nest the macro under the `task` module instead of having the macro at the top-level. This is a pattern that currently does not occur in std, mostly due to this not being possible prior to Rust 2018. This PR proposes to add the `ready` macro as `core::ready`. But another option would be to introduce it as `core::task::ready` since it's really only useful when used in conjunction with `task::{Context, Poll}`. ## Implementation questions I tried rendering the documentation locally but the macro didn't show up under `core`. I'm not sure if I quite got this right. I used the [`todo!` macro PR](https://github.com/rust-lang/rust/pull/56348/files) as a reference, and our approaches look similar. ## References - [`futures::ready`](https://docs.rs/futures/0.3.4/futures/macro.ready.html) - [`async_std::task::ready`](https://docs.rs/async-std/1.5.0/async_std/task/index.html) - [`futures_core::ready`](https://docs.rs/futures-core/0.3.4/futures_core/macro.ready.html)
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