Re: Catching missing Datum conversions

>
> I should probably split this into "actionable" (categories 3 and 4)
> and "noise and scaffolding" patches.
>

Breaking down the noise-and-scaffolding into some subgroups might make the
rather long patches more palatable/exceedingly-obvious:
* (Datum) 0 ---> NullDatum
* 0 ----> NullDatum
* The DatumGetPointer(allParameterTypes) null tests

Having said that, everything you did seems really straightforward, except
for

src/backend/rewrite/rewriteDefine.c
src/backend/statistics/mcv.c
src/backend/tsearch/ts_parse.c

and those seem like cases where the DatumGetXXX was a no-op before Datum
was a struct.