We didn't return anything useful other than ErrorKind::IO anyways.
We can use io::ErrorKind::Unsupported for DummyIO.
Fixes b/271.
Change-Id: Icb231e9b38168e8b6fa473bfa405d160357b317f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8602
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
This adds a wrapper type TvixIO<T: EvalIO>, which can wrap around an
arbitrary EvalIO implementation and perform actions needed for the
Tvix CLI (marking imported paths as known, and handling __corepkgs__).
Change-Id: I5fc1ca199b9f94b21a89103b84575e0f8f58dff9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8579
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>