Indeed, that works, and finishes in 0.00 time - here is a Go Playground to prove it (using *T instead of Optional[T]).
Here is a hacky workaround to measure the actual time using a syscall: https://go.dev/play/p/kF6Bh048siE
It shows 979ms for the original version and 0.212ms for the fast version.
If you call `time.Sleep()` the reported duration will reflect that call, but it's only simulating the delay. The duration of a CPU-heavy task is always reported as 0.00s.
Demo: https://go.dev/play/p/UZToqFseAwO
Output:
=== RUN TestSleep // calls time.Sleep(2 * time.Second)
Real duration: 554µs
--- PASS: TestSleep (2.00s)
=== RUN TestBusyLoop // for i := 0; i < 1000000000; i++ { ... }
Real duration: 390.826ms
--- PASS: TestBusyLoop (0.00s)
PASS
Concurrency is one of Go's core features, so it figures the playground would allow some form of concurrency even when the clock is fake.