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+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 coverage-run` (runs a test binary and checks its coverage report)
+- `./x.py test coverage` (runs both `coverage-map` and `coverage-run`)
+
+## 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
+`coverage-run` 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.