[mypyc] Simplify code generated for SetAttr and non-pointer type (#12916

)

Previously we sometimes generated an if statement with an empty body.

mypyc/codegen/emitfunc.py

@@ -373,12 +373,13 @@ def visit_set_attr(self, op: SetAttr) -> None:
         else:
             # ...and struct access for normal attributes.
             attr_expr = self.get_attr_expr(obj, op, decl_cl)
-            if not op.is_init:
+            if not op.is_init and attr_rtype.is_refcounted:
+                # This is not an initalization (where we know that the attribute was
+                # previously undefined), so decref the old value.
                 always_defined = cl.is_always_defined(op.attr)
                 if not always_defined:
                     self.emitter.emit_undefined_attr_check(attr_rtype, attr_expr, '!=')
-                if attr_rtype.is_refcounted:
-                    self.emitter.emit_dec_ref(attr_expr, attr_rtype)
+                self.emitter.emit_dec_ref(attr_expr, attr_rtype)
                 if not always_defined:
                     self.emitter.emit_line('}')
             # This steals the reference to src, so we don't need to increment the arg

mypyc/test/test_emitfunc.py

@@ -326,6 +326,13 @@ def test_set_attr(self) -> None:
                cpy_r_r0 = 1;
             """)
 
+    def test_set_attr_non_refcounted(self) -> None:
+        self.assert_emit(
+            SetAttr(self.r, 'x', self.b, 1),
+            """((mod___AObject *)cpy_r_r)->_x = cpy_r_b;
+               cpy_r_r0 = 1;
+            """)
+
     def test_dict_get_item(self) -> None:
         self.assert_emit(CallC(dict_get_item_op.c_function_name, [self.d, self.o2],
                                dict_get_item_op.return_type, dict_get_item_op.steals,