feat: Implement static executors

Resolves #111. Creates a `StaticExecutor` type under a feature flag and allows 
constructing it from an `Executor` via `Executor::leak`. Unlike the executor 
it came from, it's a wrapper around a `State` and omits all changes to 
`active`.

Note, unlike the API proposed in #111, this PR also includes a unsafe 
`StaticExecutor::spawn_scoped` for spawning non-'static tasks, where the 
caller is responsible for ensuring that the task doesn't outlive the borrowed 
state. This would be required for Bevy to migrate to this type, where we're 
currently using lifetime transmutation on `Executor` to enable 
`Thread::scope`-like APIs for working with borrowed state. `StaticExecutor` 
does not have an external lifetime parameter so this approach is infeasible 
without such an API.

The performance gains while using the type are substantial:

```
single_thread/executor::spawn_one
                        time:   [1.6157 µs 1.6238 µs 1.6362 µs]
Found 6 outliers among 100 measurements (6.00%)
  3 (3.00%) high mild
  3 (3.00%) high severe
single_thread/executor::spawn_batch
                        time:   [28.169 µs 29.650 µs 32.196 µs]
Found 19 outliers among 100 measurements (19.00%)
  10 (10.00%) low severe
  3 (3.00%) low mild
  3 (3.00%) high mild
  3 (3.00%) high severe
single_thread/executor::spawn_many_local
                        time:   [6.1952 ms 6.2230 ms 6.2578 ms]
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) high mild
  3 (3.00%) high severe
single_thread/executor::spawn_recursively
                        time:   [50.202 ms 50.479 ms 50.774 ms]
Found 6 outliers among 100 measurements (6.00%)
  5 (5.00%) high mild
  1 (1.00%) high severe
single_thread/executor::yield_now
                        time:   [5.8795 ms 5.8883 ms 5.8977 ms]
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild

multi_thread/executor::spawn_one
                        time:   [1.2565 µs 1.2979 µs 1.3470 µs]
Found 8 outliers among 100 measurements (8.00%)
  7 (7.00%) high mild
  1 (1.00%) high severe
multi_thread/executor::spawn_batch
                        time:   [38.009 µs 43.693 µs 52.882 µs]
Found 22 outliers among 100 measurements (22.00%)
  21 (21.00%) high mild
  1 (1.00%) high severe
Benchmarking multi_thread/executor::spawn_many_local: Warming up for 3.0000 s
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase target time to 386.6s, or reduce sample count to 10.
multi_thread/executor::spawn_many_local
                        time:   [27.492 ms 27.652 ms 27.814 ms]
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) low mild
  3 (3.00%) high mild
Benchmarking multi_thread/executor::spawn_recursively: Warming up for 3.0000 s
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase target time to 16.6s, or reduce sample count to 30.
multi_thread/executor::spawn_recursively
                        time:   [165.82 ms 166.04 ms 166.26 ms]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild
multi_thread/executor::yield_now
                        time:   [22.469 ms 22.649 ms 22.798 ms]
Found 8 outliers among 100 measurements (8.00%)
  5 (5.00%) low severe
  3 (3.00%) low mild

single_thread/leaked_executor::spawn_one
                        time:   [1.4717 µs 1.4778 µs 1.4832 µs]
Found 9 outliers among 100 measurements (9.00%)
  3 (3.00%) low severe
  2 (2.00%) low mild
  3 (3.00%) high mild
  1 (1.00%) high severe
single_thread/leaked_executor::spawn_many_local
                        time:   [4.2622 ms 4.3065 ms 4.3489 ms]
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) low mild
single_thread/leaked_executor::spawn_recursively
                        time:   [26.566 ms 26.899 ms 27.228 ms]
single_thread/leaked_executor::yield_now
                        time:   [5.7200 ms 5.7270 ms 5.7342 ms]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

multi_thread/leaked_executor::spawn_one
                        time:   [1.3755 µs 1.4321 µs 1.4892 µs]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild
multi_thread/leaked_executor::spawn_many_local
                        time:   [4.1838 ms 4.2394 ms 4.2989 ms]
Found 7 outliers among 100 measurements (7.00%)
  7 (7.00%) high mild
multi_thread/leaked_executor::spawn_recursively
                        time:   [43.074 ms 43.159 ms 43.241 ms]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) low mild
multi_thread/leaked_executor::yield_now
                        time:   [23.210 ms 23.257 ms 23.302 ms]
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) low mild
```

.github/workflows/ci.yml

@@ -45,6 +45,7 @@ jobs:
         if: startsWith(matrix.rust, 'nightly')
         run: cargo check -Z features=dev_dep
       - run: cargo test
+      - run: cargo test --all-features
       - run: cargo check --all --all-features --target wasm32-unknown-unknown
       - run: cargo hack build --all --all-features --target wasm32-unknown-unknown --no-dev-deps
 
@@ -82,6 +83,10 @@ jobs:
         env:
           MIRIFLAGS: -Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation
           RUSTFLAGS: ${{ env.RUSTFLAGS }} -Z randomize-layout
+      - run: cargo miri test --all-features
+        env:
+          MIRIFLAGS: -Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation -Zmiri-ignore-leaks
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} -Z randomize-layout
 
   security_audit:
     permissions:

Cargo.toml

@@ -6,17 +6,21 @@ name = "async-executor"
 version = "1.11.0"
 authors = ["Stjepan Glavina <[email protected]>", "John Nunley <[email protected]>"]
 edition = "2021"
-rust-version = "1.60"
+rust-version = "1.63"
 description = "Async executor"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/async-executor"
 keywords = ["asynchronous", "executor", "single", "multi", "spawn"]
 categories = ["asynchronous", "concurrency"]
 exclude = ["/.*"]
 
+[features]
+# Adds support for executors optimized for use in static variables.
+static = []
+
 [dependencies]
 async-task = "4.4.0"
-concurrent-queue = "2.0.0"
+concurrent-queue = "2.5.0"
 fastrand = "2.0.0"
 futures-lite = { version = "2.0.0", default-features = false }
 slab = "0.4.4"
@@ -37,3 +41,4 @@ once_cell = "1.16.0"
 [[bench]]
 name = "executor"
 harness = false
+required-features = ["static"]