pass fn item pointers?

I am working on some code for testing an external binary, and it's useful if every test runs in a temporary directory as a subprocess so that . is a tempdir.

I wrote
https://github.com/cgwalters/with-procspawn-tempdir/
but realized that it'd be simpler if I had my test framework allocate the tempdir outside of the test. So I tried to send across the test function pointer fn() -> Result<()> across procspawn::spawn but of course it's not serializable.

But this library is effectively doing that "fn pointer" serialization for the function provided to spawn; could we make the MarshalledCall bits a public API like: procspawn::ThisProcessFn that was serializable with serde?

Start by reading the procspawn::spawn API and the internal MarshalledCall implementation, then compare the requested ThisProcessFn serde behavior with the external with-procspawn-tempdir use case. Determine the public API and serialization design needed to pass a test function across a subprocess; done means the documented representation works for that scenario without exposing an unstable internal detail.