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coverage: Store coverage source regions as `Span` until codegen (take 2) This is an attempt to re-land #133418: > Historically, coverage spans were converted into line/column coordinates during the MIR instrumentation pass. > This PR moves that conversion step into codegen, so that coverage spans spend most of their time stored as Span instead. > In addition to being conceptually nicer, this also reduces the size of coverage mappings in MIR, because Span is smaller than 4x u32. That PR was reverted by #133608, because in some circumstances not covered by our test suite we were emitting coverage metadata that was causing `llvm-cov` to exit with an error (#133606). --- The implementation here is *mostly* the same, but adapted for subsequent changes in the relevant code (e.g. #134163). I believe that the changes in #134163 should be sufficient to prevent the problem that required the original PR to be reverted. But I haven't been able to reproduce the original breakage in a regression test, and the `llvm-cov` error message is extremely unhelpful, so I can't completely rule out the possibility of this breaking again. r? jieyouxu (reviewer of the original PR)
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 bothcoverage-mapandcoverage-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.