After coverage instrumentation and MIR transformations, we can sometimes end up
with coverage expressions that always have a value of zero. Any expression
operand that refers to an always-zero expression can be replaced with a literal
`Operand::Zero`, making the emitted coverage mapping data smaller and simpler.
This simplification step is mostly redundant with the simplifications performed
inline in `expressions_with_regions`, except that it does a slightly more
thorough job in some cases (because it checks for always-zero expressions
*after* other simplifications).
However, adding this simplification step will then let us greatly simplify that
code, without affecting the quality of the emitted coverage maps.
This code was calling `sort_unstable_by`, but failed to impose a total order on
the initial spans. That resulted in unpredictable handling of closure spans,
producing inconsistencies in the coverage maps and in user-visible coverage
reports.
This patch fixes the problem by always sorting closure spans before
otherwise-identical non-closure spans, and also switches to a stable sort in
case the ordering is still not total.
The output of these tests is too complicated to comfortably verify by hand, but
we can still use them to observe changes to the underlying mappings produced by
codegen/LLVM.
If these tests fail due to non-coverage changes (e.g. in HIR-to-MIR lowering or
MIR optimizations), it should usually be OK to just `--bless` them, as long as
the `run-coverage` test suite still works.
We compile each test file to LLVM IR assembly, and then pass that IR to a
dedicated program that can decode LLVM coverage maps and print them in a more
human-readable format. We can then check that output against known-good
snapshots.
This test suite has some advantages over the existing `run-coverage` tests:
- We can test coverage instrumentation without needing to run target binaries.
- We can observe subtle improvements/regressions in the underlying coverage
mappings that don't make a visible difference to coverage reports.
