generator: support listelement-ref

Support for listelement-ref needs the following three changes
(in the order as they appear in the patch):

* making the current list-element accessible with the variable
  xcb_listelement which is a pointer to the list-element

* supporting lists of simple-type for sumof with a nested expression

* using the variable for resolving a listelement-ref expression

Changes for V2 of this patch:
- adapt to removal of patch "libxcb 2/6" from patchset "ListInputDevices".

Changes for V3 of this patch:
- adapt to V2 of patch "libxcb 5/6" from patchset "ListInputDevices"

Changes for V4 of this patch:
- adapt to revision 2 of the patchset "ListInputDevices"

Message-ID: <545743A0.50907@DemoRecorder.com>
Patch-Thread-Subject: [Xcb] support popcount of a list and associated xml changes
Patch-Set: PopcountList
Patch-Number: libxcb 4/4
Patch-Version: V4
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>

src/c_client.py

@@ -1646,6 +1646,14 @@ def _c_accessor_get_expr(expr, field_mapping):
         _c_pre.code("for (%s = 0; %s < %s; %s++) {", loopvar, loopvar, lengthvar, loopvar)
         _c_pre.indent()
 
+        # define and set xcb_listelement, so that it can be used by
+        # listelement-ref expressions.
+        if expr.contains_listelement_ref:
+            _c_pre.code(
+                "const %s *xcb_listelement = %s;",
+                field.c_field_type, listvar)
+
+        # summation
         if expr.rhs is None:
             _c_pre.code("%s += *%s;", sumvar, listvar)
         else:
@@ -1655,6 +1663,7 @@ def _c_accessor_get_expr(expr, field_mapping):
             # field mapping for the subexpression needs to include
             # the fields of the list-member type
             scoped_field_mapping = field_mapping.copy()
+            if not field.type.member.is_simple:
                 scoped_field_mapping.update(
                     _c_helper_field_mapping(
                         field.type.member,
@@ -1675,6 +1684,8 @@ def _c_accessor_get_expr(expr, field_mapping):
         _c_pre.code("/* sumof end. Result is in %s */", sumvar)
         _c_pre.end()
         return sumvar
+    elif expr.op == 'listelement-ref':
+        return '(*xcb_listelement)'
     elif expr.op != None:
         return ('(' + _c_accessor_get_expr(expr.lhs, field_mapping) +
                 ' ' + expr.op + ' ' +