Swift: allow skipping fields in cppgen

Some fields of base classes pose some problems with diamond hierarchies,
and we don't use them any way as we are emitting them using directly
trap entries instead of structured C++ classes.

This introduces a `cpp_skip` pragma to skip generation of those fields
in structured generated C++ classes, and applies it to `is_unknown` and
`location`.

swift/codegen/generators/cppgen.py

@@ -13,6 +13,7 @@ Each class in the schema gets a corresponding `struct` in `TrapClasses.h`, where
 
 import functools
 import pathlib
+import typing
 from typing import Dict
 
 import inflection
@@ -34,22 +35,25 @@ def _get_type(t: str) -> str:
     return t
 
 
-def _get_field(cls: schema.Class, p: schema.Property) -> cpp.Field:
-    trap_name = None
-    if not p.is_single:
-        trap_name = inflection.camelize(f"{cls.name}_{p.name}")
-        if not p.is_predicate:
-            trap_name = inflection.pluralize(trap_name)
-    args = dict(
-        field_name=p.name + ("_" if p.name in cpp.cpp_keywords else ""),
-        type=_get_type(p.type),
-        is_optional=p.is_optional,
-        is_repeated=p.is_repeated,
-        is_predicate=p.is_predicate,
-        trap_name=trap_name,
-    )
-    args.update(cpp.get_field_override(p.name))
-    return cpp.Field(**args)
+def _get_fields(cls: schema.Class) -> typing.Iterable[cpp.Field]:
+    for p in cls.properties:
+        if "cpp_skip" in p.pragmas:
+            continue
+        trap_name = None
+        if not p.is_single:
+            trap_name = inflection.camelize(f"{cls.name}_{p.name}")
+            if not p.is_predicate:
+                trap_name = inflection.pluralize(trap_name)
+        args = dict(
+            field_name=p.name + ("_" if p.name in cpp.cpp_keywords else ""),
+            type=_get_type(p.type),
+            is_optional=p.is_optional,
+            is_repeated=p.is_repeated,
+            is_predicate=p.is_predicate,
+            trap_name=trap_name,
+        )
+        args.update(cpp.get_field_override(p.name))
+        yield cpp.Field(**args)
 
 
 class Processor:
@@ -65,7 +69,7 @@ class Processor:
         return cpp.Class(
             name=name,
             bases=[self._get_class(b) for b in cls.bases],
-            fields=[_get_field(cls, p) for p in cls.properties],
+            fields=list(_get_fields(cls)),
             final=not cls.derived,
             trap_name=trap_name,
         )

swift/codegen/schema.yml

@@ -13,14 +13,18 @@ _directories:
   stmt: Stmt$
 
 Element:
-  is_unknown: predicate
+  is_unknown:
+    type: predicate
+    _pragma: cpp_skip  # this is emitted using trap entries directly
   _pragma: qltest_skip
 
 File:
   name: string
 
 Locatable:
-  location: Location?
+  location:
+    type: Location?
+    _pragma: cpp_skip  # this is emitted using trap entries directly
   _pragma: qltest_skip
 
 Location:

swift/codegen/test/test_cppgen.py

@@ -165,5 +165,18 @@ def test_classes_with_dirs(generate_grouped):
     }
 
 
+def test_cpp_skip_pragma(generate):
+    assert generate([
+        schema.Class(name="A", properties=[
+            schema.SingleProperty("x", "foo"),
+            schema.SingleProperty("y", "bar", pragmas=["x", "cpp_skip", "y"]),
+        ])
+    ]) == [
+        cpp.Class(name="A", final=True, trap_name="As", fields=[
+            cpp.Field("x", "foo"),
+        ]),
+    ]
+
+
 if __name__ == '__main__':
     sys.exit(pytest.main([__file__] + sys.argv[1:]))