Retention and lean()

transcript() retains every invocation until reset() or checkpoint(). Retention covers returned values — and with the runner adapters, reset() runs after your #[AfterTest] or tearDown().

For plain data that is only memory. For a value that owns an OS resource — a stream, a connection, a lock — the resource is still held while your teardown runs.

The failure this produced

A forwarding double returned real file streams. Teardown's directory removal failed with "Directory not empty" on Windows only, because POSIX unlinks open files and every Linux run stayed green.

The cookbook case has the runnable version.

Two remedies

Understudy::lean($double);                       // keep calls, not returned values
$result = Understudy::scope(fn () => ...);       // or drop the whole context early

lean()

Keeps the invocation — method, arguments, sequence — so matching, verify(), transcript() and nothingElse() work unchanged. The returned value is not retained:

	On a lean double
Invocation::returned()	raises OutcomeUnavailable, as it already does for a call that threw
The transcript line	shows returned (value not kept: lean)
Matching, verify(), nothingElse()	unchanged

It is one-way for the double's lifetime, and it also caps the per-call memory growth of a hot loop.

The one thing lean() cannot release is the stable slot behind a by-reference (&) return — callers hold a live reference into it, and the language requires the storage to outlive the call.

scope()

Drops the whole context — outcomes included — before the lifecycle teardown runs.

The rule

A double whose returned values own OS resources should be built lean, or built and used inside Understudy::scope().