Eio_unix.ResourceEio resources backed by an OS file descriptor.
type 'a t = [> `Unix_fd ] as 'a Eio.Resource.tResources that have FDs are tagged with `Unix_fd.
val fd_opt : _ Eio.Resource.t -> Fd.t optionfd_opt t returns the FD being wrapped by a generic resource, if any.
This just probes t using FD.