https://bugzilla.redhat.com/show_bug.cgi?id=2150509
--- Comment #4 from Fabio Valentini decathorpe@gmail.com --- Yes, you can include the upstream license text as a separate source and include it in the package manually until upstream publishes a new release, i.e. something like
Source1: https://github.com/kamadak/mutate_once-rs/raw/0dc8ba7/LICENSE
%prep (...) cp -pav %{SOURCE1} .
%files %license %{crate_instdir}/LICENSE (...)