Main use case:
Chromium has runtime "SequenceChecker" and "ThreadChecker" that enforce sequenced task context. Thread safety analysis can be used but neither "mutex" nor "role" seem appropriate for meaningful error messages.
As an example copied from:
int counter_ GUARDED_BY_CONTEXT(sequence_checker_);
// Member access without sequence_checker_ assertion.
Should yield: "fatal error: writing variable 'counter_' requires holding context 'sequence_checker_' exclusively"