Pass features along during expansion
Use the set of passed features to detect uses of feature-gated macros without the corresponding feature enabled.
Fix#22234.
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Also, the framework this add (passing along a reference to the features in the expansion context) is a necessary precursor for landing a properly feature-gated desugaring-based overloaded-`box` and placement-`in` (#22181).
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This is fixing a bug, but since there might be code out there that is unknowingly taking advantage of that bug, I feel obligated to mark this as a:
[breaking-change]
020e4e4ad9