rust/tests/coverage/README.md

The tests in this directory are shared by two different test modes, and can be run in multiple different ways:

• ./x.py test coverage-map (compiles to LLVM IR and checks coverage mappings)
• ./x.py test run-coverage (runs a test binary and checks its coverage report)
• ./x.py test coverage (runs both coverage-map and run-coverage)

Maintenance note

These tests can be sensitive to small changes in MIR spans or MIR control flow, especially in HIR-to-MIR lowering or MIR optimizations.

If you haven't touched the coverage code directly, and the tests still pass in run-coverage mode, then it should usually be OK to just re-bless the mappings as necessary with ./x.py test coverage-map --bless, without worrying too much about the exact changes.