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Manish Goregaokar
d82588b45a
Add core::future::{poll_fn, PollFn}
This is a sibling PR to #70834, adding `future::poll_fn`. This is a small helper function that helps bridge the gap between "poll state machines" and "async/await". It was first introduced in [futures@0.1.7](https://docs.rs/futures/0.1.7/futures/future/fn.poll_fn.html) in December of 2016, and has been tried and tested as part of the ecosystem for the past 3.5 years.
## Implementation
Much of the same reasoning from #70834 applies: by returning a concrete struct rather than an `async fn` we get to mark the future as `Unpin`. It also becomes named which allows storing it in structs without boxing. This implementation has been modified from the implementation in `futures-rs`.
## References
- [`futures::future::poll_fn`](https://docs.rs/futures/0.3.5/futures/future/fn.poll_fn.html)
- [`async_std::future::poll_fn`](https://docs.rs/async-std/1.5.0/async_std/future/fn.poll_fn.html)