These are nightly-only options: so keep them in a separate file.
When it sees unstable features, stable rustfmt gives a diagnostic like:
> Warning: can't set `imports_granularity = Module`, unstable features are only available in nightly channel.
> Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.
But: _does_ otherwise format the files and exit non-zero. However, this is noisy.
We arrange that `cargo +nightly fmt-unstable` also does the right thing.
Joseph Birr-Pixton