If you are in a situation where rubocop is hanging on a file but you don't know which one,
afaik there is no way to have rubocop pring each filename as it runs it. None of the formatters
do this and there is no verbose mode. Even --debug does not do this.
Here is a hack to (very slowly) invoke rubocop once for each candidate file:
bundle exec rubocop --list-target-files > files.txt
cat files.txt | xargs -L1 -n1 bundle exec rubocop # this works on macos/bsd, use --verbose instead of -t on linux