py/modbuiltins: Make built-in dir support the __dir__ special method.

If MICROPY_PY_ALL_SPECIAL_METHODS is enabled then dir() will now delegate
to the special method __dir__ if the object it is listing has this method.

py/makeqstrdata.py

@@ -114,6 +114,9 @@ def parse_input_headers(infiles):
                 if ident == "":
                     # Sort empty qstr above all still
                     order = -200000
+                elif ident == "__dir__":
+                    # Put __dir__ after empty qstr for builtin dir() to work
+                    order = -190000
                 elif ident.startswith("__"):
                     order -= 100000
                 qstrs[ident] = (order, ident, qstr)

py/modbuiltins.c

@@ -190,6 +190,13 @@ STATIC mp_obj_t mp_builtin_dir(size_t n_args, const mp_obj_t *args) {
             mp_obj_t dest[2];
             mp_load_method_protected(args[0], i, dest, false);
             if (dest[0] != MP_OBJ_NULL) {
+                #if MICROPY_PY_ALL_SPECIAL_METHODS
+                // Support for __dir__: see if we can dispatch to this special method
+                // This relies on MP_QSTR__dir__ being first after MP_QSTR_
+                if (i == MP_QSTR___dir__ && dest[1] != MP_OBJ_NULL) {
+                    return mp_call_method_n_kw(0, 0, dest);
+                }
+                #endif
                 mp_obj_list_append(dir, MP_OBJ_NEW_QSTR(i));
             }
         }

tests/basics/special_methods2.py

@@ -94,6 +94,9 @@ class Cud():
         print("__isub__ called")
         return self
 
+    def __dir__(self):
+        return ['a', 'b', 'c']
+
 cud1 = Cud()
 cud2 = Cud()
 
@@ -113,6 +116,12 @@ cud2 // cud1
 cud1 += cud2
 cud1 -= cud2
 
+# test that dir() delegates to __dir__ special method
+print(dir(cud1))
+
+# test that dir() does not delegate to __dir__ for the type
+print('a' in dir(Cud))
+
 # TODO: the following operations are not supported on every ports
 #
 # ne is not supported, !(eq) is called instead