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runtime: fix Stacked Borrows violation in
LocalOwnedTasks
(#5099)
## Motivation It turns out that `LocalSet` has...never actually passed Miri. This is because all of the tests for `LocalSet` are defined as integration tests in `tokio/tests/task_local_set.rs`, rather than lib tests in `tokio/src`, and we never run Miri for integration tests. PR #5095 added a new test reproducing an unrelated bug in `LocalSet`, which had to be implemented as a lib test, as it must make internal assertions about the state of the `LocalSet` that cannot be tested with just the public API. This test failed under Miri, and it turns out that the reason for this is unrelated to the change in #5095 --- `LocalSet` uses the `LocalOwnedTasks` type, which turns out to contain a stacked borrows violation due to converting an `&Header` into an `NonNull<Header>` when removing a task from the `LocalOwnedTasks` list. ## Solution Fortunately, this was actually quite easy to fix. The non-`Local` version of `OwnedTasks` already uses `Task::header_ptr()` to get a `NonNull<Header>` in its version of `remove`, which avoids this issue. Therefore, the fix was as simple as updating `LocalOwnedTasks` to do the same. I've also added a very simple lib test in `src/task/local_set.rs` that just creates a `LocalSet` and spawns a single task on it under a current-thread runtime. This test fails under Miri without the fix in this PR: https://github.com/tokio-rs/tokio/actions/runs/3237072694/jobs/5303682530 We should probably keep the test so that we're always testing at least the most trivial `LocalSet` usage under Miri...
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