detectAsyncLeaks false positive: MixedScheduler PROMISE leaks with @effect/vitest

[gabriel-ecegi]

Description

When using @effect/vitest with vitest's detectAsyncLeaks: true, every it.effect() / it.scoped() test reports false positive PROMISE leaks from Effect's MixedScheduler.

Reproduction

// vitest.config.ts
export default defineConfig({
  test: { detectAsyncLeaks: true }
});

// example.test.ts
import { it, describe, expect } from "@effect/vitest";
import { Effect } from "effect";

describe("leak demo", () => {
  it.effect("simple effect leaks PROMISE", () =>
    Effect.gen(function* () {
      expect(1).toBe(1);
    })
  );
});

Output:

⎯⎯⎯ Async Leaks 2 ⎯⎯⎯
PROMISE leaking in example.test.ts

Stack trace:

MixedScheduler.starveInternal Scheduler.js:68:17
Scheduler.js:84:47

Root Cause

MixedScheduler uses Promise.resolve(void 0).then(() => drain(depth + 1)) for fiber scheduling (Scheduler.ts#L131). Vitest tracks every PROMISE async resource via node:async_hooks and only removes them on promiseResolve. The scheduler's intermediate promises haven't resolved by the time vitest collects leaks.

What We Tried

Approach	Result
it.scoped()	Still leaks — scheduler is outside scope
Effect.scoped pipe	Same
afterAll with 200ms delay	Doesn't help — vitest measures per-test
SyncScheduler via Effect.locally	Deadlocks on any async operation (fetch)
MixedScheduler(0) (force setTimeout)	Leaks Timeout instead of PROMISE
Layer.locallyScoped(currentScheduler, SyncScheduler)	Hangs with scoped effects

Expected Behavior

@effect/vitest test helpers should properly clean up MixedScheduler resources so that detectAsyncLeaks doesn't report false positives.

Possible Solutions

1. @effect/vitest could flush the scheduler after each test before returning control to vitest
2. Use queueMicrotask instead of Promise.resolve().then() — microtasks don't create trackable async resources in node:async_hooks
3. Document the incompatibility if it can't be fixed

Environment

• effect: 3.19.19
• @effect/vitest: 0.27.0
• vitest: 4.1.0
• bun: 1.3.10
• pool: forks

Note

Vitest's leak detection does NOT fail tests (exit code 0) — leaks are warnings only. But they pollute CI output and prevent teams from using detectAsyncLeaks to catch real leaks.