Found a workaround using lcov:

- grcov . --binary-path ./target/debug/ -s . -t lcov --branch --ignore-not-existing --ignore "*target*"  -o lcov
- lcov -r lcov "/*" 2&> out
- grep lines out

And then I use \s*lines\.*:\s*([\d\.]+%) as regexp in gitlab as suggested here: https://www.greycastle.se/how-to-show-flutter-test-coverage-in-gitlab-ci/

Any PR is welcome.
There are several examples of output so I'd say that isn't hard to do.

Would you add this as a new summary output type?

I'm fine with summary.

Duplicate of #468?

No, #468 is about generating a full report in the Cobertura format, while this ticket is about just producing a summary (nb of lines covered / total nb of lines) for GitLab summary.

This is what I currently do for Gitlab. I use xmlstarlet, which is similar to jq but for XML data.

script:
    # Coverage commands go here...
    # Extract and print percentage from Cobertura. See Gitlab issue #21549
    - >
      export COVERAGE_DEC=$(
      xmlstarlet sel -t -v "number(//coverage/@line-rate)"
      <cobertura.xml 2>/dev/null
      )
    - export COVERAGE_PCT=$(echo "100 * $COVERAGE_DEC" | bc)
    - printf "gitlab-coverage %s%%\n" "$COVERAGE_PCT"
  coverage: '/^gitlab-coverage .+%$/'

Full CI file is here.

I think --summary is a nice approach. Since I faced the same issue I can maybe share here my current Gitlab CI/CD solution using grcov in a C++ codebase.

.ctest:
    stage: test
    script:
        - ctest --test-dir ./build -j $(nproc) --no-tests=error --output-on-failure --output-junit ctest_junit.xml
        - mkdir ./build/coverage/
        - grcov ./build -s ./ --threads $(nproc) --branch --output-types html,cobertura,markdown -o ./build/coverage/ --ignore '/usr/include*' --ignore '*build*' --ignore '*lib*'
        - cat ./build/coverage/markdown.md
    coverage: /Total coverage:.*\s+(\d*\.?\d+\%)/
    artifacts:
        paths:
            - ./build/coverage/
        reports:
            junit: build/ctest_junit.xml
            coverage_report:
                coverage_format: cobertura
                path: ./build/coverage/cobertura.xml

I output as markdown and cat it to the job shell.
Then use the following regex coverage: /Total coverage:.*\s+(\d*\.?\d+\%)/ to get the values into gitlab. This was my main driver for PR #981.
I use ctest to get junit test information.
This also has cobertura information displayed correctly in gitlab web code editor, shown on MR.

@gdesmott thanks for the hint on lcov. I ended up using lcov --list <filename> for a summary, like this:

grcov_args=(
  --binary-path target/debug
  --source-dir src
  --ignore '/*'
  --branch
  # grcov needs help finding llvm-profdata, apparently
  --llvm-path "$(dirname "$(which llvm-profdata)")"
)
grcov "${grcov_args[@]}" -t html -o target/coverage/html .
grcov "${grcov_args[@]}" -t lcov -o target/coverage/lcov .
lcov --list target/coverage/lcov 2>/dev/null \
  | tee target/coverage/summary.txt

i assume --summary would also produce human-readable output. is that correct? i found my way here to file a feature request for that - it's pretty handy to have coverage info printed to the terminal without having to jump thru hoops and / or take on additional dependencies.

Yes, I think that's the idea.