Slop-zeroing thunks is problematic with nonmoving collector
To aid in sanity checking the debug RTS when run with
-N1 currently zeros the free variables of thunks when they are updated. However, this is problematic for the nonmoving collector, which may race to read free variables while a thunk is being updated. I believe the fix here is to ensure that slop-zeroing is disabled while the nonmoving collector is in use, although sadly this further restricts the sanity checking that we can safely perform.
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