Summary:
Note: this is a re-land of D44623815, which was was reverted because of a land race with D44626072
In particular this fixes `FlattenUnordered` having a nasty deadlock, but also
some other bugs:
```
# 0.3.28 - 2023-03-30
* Update to syn 2. This raises MSRV of utility crates to 1.56. (#2730, #2733)
* Fix bug in `FlattenUnordered` (#2726, #2728)
# 0.3.27 - 2023-03-11
* Add `TryFlattenUnordered` (#2577, #2590, #2606, #2607)
* Add `AbortHandle::is_aborted` (#2710)
* Add `AbortRegistration::handle` (#2712)
* Make `BiLock` strict-provenance compatible (#2716)
# 0.3.26 - 2023-01-30
* Add `Either::as_pin_mut` and `Either::as_pin_ref` (#2691)
* Add `Shared::ptr_eq` and `Shared::ptr_hash` (#2691)
* Implement `FusedStream` for `Buffered` (#2676)
* Implement `FusedStream` for all streams in `ReadyChunks` (#2693)
* Fix bug in `FuturesOrdered::push_front` (#2664)
* Remove `Fut::Output: Clone` bounds from some `Shared` methods (#2662)
* Remove `T: Debug` bounds from `Debug` implementations of `mpsc` and `oneshot` types (#2666, #2667)
# 0.3.25 - 2022-10-20
* Fix soundness issue in `join!` and `try_join!` macros (#2649)
* Implement `Clone` for `sink::Drain` (#2650)
# 0.3.24 - 2022-08-29
* Fix incorrect termination of `select_with_strategy` streams (#2635)
# 0.3.23 - 2022-08-14
* Work around MSRV increase due to a cargo bug.
```
Reviewed By: zertosh
Differential Revision: D44632588
fbshipit-source-id: bdd87cb02b3aef63a65b1f9b852579225adfedbd
Any native code (C/C++/Rust) that Mercurial (either core or extensions)
depends on should go here. Python code, or native code that depends on
Python code (e.g. #include <Python.h> or use cpython) is disallowed.
As we start to convert more of Mercurial into Rust, and write new paths
entrirely in native code, we'll want to limit our dependency on Python, which is
why this barrier exists.
See also ext/extlib/README.md, mercurial/cext/README.mb.
How do I choose between lib and extlib (and cext)?
If your code is native and doesn't depend on Python (awesome!), it goes here.
Otherwise, put it in ext/extlib (if it's only used by extensions) or
mercurial/cext (if it's used by extensions or core).
22e700ccf3