More thorough status-quo tests for `#[coverage(..)]`
In light of the stabilization push at https://github.com/rust-lang/rust/issues/84605#issuecomment-2166514660, I have written some tests to more thoroughly capture the current behaviour of the `#[coverage(..)]` attribute.
These tests aim to capture the *current* behaviour, which is not necessarily the desired behaviour. For example, some of the error message are not great, some things that perhaps ought to cause an error do not, and recursive coverage attributes have not been implemented yet.
`@rustbot` label +A-code-coverage
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.
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