get test coverage

docs/helpers.md

@@ -252,6 +252,39 @@ Whether you are developing code or writing rules, writing and running the tests
 
 The `tests` directory is similar to the `Rules` directory. If you are a translator, see the section above that describes what you should do.
 
+Rust provides testing support with the command `cargo test`.
+
+### Test Coverage
+
+This section explains test coverage with `llvm-cov` and `grcov` on _MacOS_.
+Another option is using built-in coverage-support of your IDE, for example _RustRover_ by _JetBrains_.
+
+Using other operating systems should also work with [grcov](https://github.com/mozilla/grcov),
+but may require some tweaks regarding LLVM, paths, etc.
+
+```bash
+# One-time setup
+rustup component add llvm-tools-preview
+cargo install grcov
+
+export LLVM_PROFILE_FILE="target/coverage/%p-%m.profraw"
+RUSTFLAGS="-Cinstrument-coverage" cargo test
+# Example with a single test:
+# RUSTFLAGS="-Cinstrument-coverage" cargo test Languages::zh::tw::units::without_prefix_powers_of_2
+
+# Generate report
+grcov . \
+  --source-dir . \
+  --binary-path ./target/debug/deps \
+  -t html \
+  --branch \
+  --ignore-not-existing \
+  --ignore "target/*" \
+  -o target/coverage/html
+
+open target/coverage/html/index.html
+```
+
 
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