Remove lappend_cell...() family of List functions.

It seems worth getting rid of these functions because they require the
caller to retain a ListCell pointer into a List that it's modifying,
which is a dangerous practice with the new List implementation.
(The only other List-modifying function that takes a ListCell pointer
as input is list_delete_cell, which nowadays is preferentially used
via the constrained API foreach_delete_current.)

There was only one remaining caller of these functions after commit
2f5b8eb5a, and that was some fairly ugly GEQO code that can be much
more clearly expressed using a list-index variable and list_insert_nth.
Hence, rewrite that code, and remove the functions.

Discussion: https://postgr.es/m/26193.1563228600@sss.pgh.pa.us

src/backend/nodes/list.c

@@ -438,53 +438,6 @@ list_insert_nth_oid(List *list, int pos, Oid datum)
 	return list;
 }
 
-/*
- * Add a new cell to the list, in the position after 'prev_cell'. The
- * data in the cell is left undefined, and must be filled in by the
- * caller. 'list' is assumed to be non-NIL, and 'prev_cell' is assumed
- * to be non-NULL and a member of 'list'. Returns address of new cell.
- *
- * Caution: prev_cell might no longer point into the list after this!
- */
-static ListCell *
-add_new_cell_after(List *list, ListCell *prev_cell)
-{
-	/* insert_new_cell will assert that this is in-range: */
-	int			pos = prev_cell - list->elements;
-
-	return insert_new_cell(list, pos + 1);
-}
-
-/*
- * Add a new cell to the specified list (which must be non-NIL);
- * it will be placed after the list cell 'prev' (which must be
- * non-NULL and a member of 'list'). The data placed in the new cell
- * is 'datum'.
- */
-void
-lappend_cell(List *list, ListCell *prev, void *datum)
-{
-	Assert(IsPointerList(list));
-	lfirst(add_new_cell_after(list, prev)) = datum;
-	check_list_invariants(list);
-}
-
-void
-lappend_cell_int(List *list, ListCell *prev, int datum)
-{
-	Assert(IsIntegerList(list));
-	lfirst_int(add_new_cell_after(list, prev)) = datum;
-	check_list_invariants(list);
-}
-
-void
-lappend_cell_oid(List *list, ListCell *prev, Oid datum)
-{
-	Assert(IsOidList(list));
-	lfirst_oid(add_new_cell_after(list, prev)) = datum;
-	check_list_invariants(list);
-}
-
 /*
  * Prepend a new element to the list. A pointer to the modified list
  * is returned. Note that this function may or may not destructively

src/backend/optimizer/geqo/geqo_eval.c

@@ -239,6 +239,7 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, int num_gene,
 			bool force)
 {
 	ListCell   *lc;
+	int			pos;
 
 	/* Look for a clump that new_clump can join to */
 	foreach(lc, clumps)
@@ -304,21 +305,15 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, int num_gene,
 	if (clumps == NIL || new_clump->size == 1)
 		return lappend(clumps, new_clump);
 
-	/* Check if it belongs at the front */
-	lc = list_head(clumps);
-	if (new_clump->size > ((Clump *) lfirst(lc))->size)
-		return lcons(new_clump, clumps);
-
 	/* Else search for the place to insert it */
-	for (;;)
+	for (pos = 0; pos < list_length(clumps); pos++)
 	{
-		ListCell   *nxt = lnext(clumps, lc);
+		Clump	   *old_clump = (Clump *) list_nth(clumps, pos);
 
-		if (nxt == NULL || new_clump->size > ((Clump *) lfirst(nxt))->size)
-			break;				/* it belongs after 'lc', before 'nxt' */
-		lc = nxt;
+		if (new_clump->size > old_clump->size)
+			break;				/* new_clump belongs before old_clump */
 	}
-	lappend_cell(clumps, lc, new_clump);
+	clumps = list_insert_nth(clumps, pos, new_clump);
 
 	return clumps;
 }

src/include/nodes/pg_list.h

@@ -514,10 +514,6 @@ extern List *list_insert_nth(List *list, int pos, void *datum);
 extern List *list_insert_nth_int(List *list, int pos, int datum);
 extern List *list_insert_nth_oid(List *list, int pos, Oid datum);
 
-extern void lappend_cell(List *list, ListCell *prev, void *datum);
-extern void lappend_cell_int(List *list, ListCell *prev, int datum);
-extern void lappend_cell_oid(List *list, ListCell *prev, Oid datum);
-
 extern List *lcons(void *datum, List *list);
 extern List *lcons_int(int datum, List *list);
 extern List *lcons_oid(Oid datum, List *list);