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`DynFlags.buildTag` was a field created from the set of Ways in `DynFlags.ways`. It had to be kept in sync with `DynFlags.ways` which was fragile. We want to avoid global state like this (#17957). Moreover in #14335 we also want to support loading units with different ways: target units would still use `DynFlags.ways` but plugins would use `GHC.Driver.Ways.hostFullWays`. To avoid having to deal both with build tag and with ways, we recompute the buildTag on-the-fly (should be pretty cheap) and we remove `DynFlags.buildTag` field.
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