coverage: Avoid a possible query stability hazard in CoverageCounters
#119401
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#119252 revealed a possible query stability hazard in
CoverageCounters
: we iterate over the entries of anFxHashMap
in a way that allows the iteration order to potentially affect the relative creation order of MIR blocks.I'm not sure whether there's an actual stability problem or not in practice, but it's certainly a hazard, and I don't see any reason not to switch over to
FxIndexMap
to avoid potential issues.This can either be merged on its own, or incorporated into #119252.
cc @Enselic
r? @cjgillot