safer node casting

There is a common coding pattern that goes like this:

RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
Assert(IsA(rinfo, RestrictInfo));

(Arguably, the Assert should come before the cast, but I guess it's done
this way out of convenience.)

(Not to mention the other common coding pattern of just doing the cast
and hoping for the best.)

I propose a macro castNode() that combines the assertion and the cast,
so this would become

RestrictInfo *rinfo = castNode(RestrictInfo, lfirst(lc));

This is inspired by the dynamic_cast operator in C++, but follows the
syntax of the well-known makeNode() macro.

Besides saving a bunch of code and making things safer, the function
syntax also makes some code easier to read by saving levels of
parentheses, for example:

- Assert(IsA(sstate->testexpr, BoolExprState));
- oplist = ((BoolExprState *) sstate->testexpr)->args;
+ oplist = castNode(BoolExprState, sstate->testexpr)->args;

Attached is a patch that shows how this would work. There is a lot more
that can be done, but I just stopped after a while for now.

--
Peter Eisentraut http://www.2ndQuadrant.com/
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