rules_scala supports coverage, but it's disabled by default. You need to enable it with an extra toolchain:
bazel coverage --extra_toolchains="@io_bazel_rules_scala//scala:code_coverage_toolchain" //...
It will produce several .dat files with results for your targets.
You can also add more options to receive a combined coverage report:
bazel coverage \
--extra_toolchains="@io_bazel_rules_scala//scala:code_coverage_toolchain" \
--combined_report=lcov \
--coverage_report_generator="@bazel_tools//tools/test/CoverageOutputGenerator/java/com/google/devtools/coverageoutputgenerator:Main" \
//...
This should produce a single bazel-out/_coverage/_coverage_report.dat from all coverage files that are generated.
You can install lcov package (that supports the format Bazel uses for coverage reports) to have access to additional tools:
# Use your system package manager. E.g. on Ubuntu:
sudo apt install lcov
Having lcov package installed you can extract information from your coverage reports:
# For a summary:
lcov --summary your-coverage-report.dat
# For details:
lcov --list your-coverage-report.dat
If you prefer an HTML report, then you can use genhtml provided also by the lcov package.
An example with a bit of ceremony:
# Output html reports to a new directory.
destdir="my-coverage-reports"
mkdir -p ${destdir}
# Generate HTML report from the results.
genhtml -o ${destdir} --ignore-errors source bazel-out/_coverage/_coverage_report.dat
echo "coverage report at file://${destdir}/index.html"
Coverage support has been only tested with ScalaTest.