omit_defaults supports factory tuple/frozenset

Previously `omit_defaults` would only detect `tuple`/`frozenset` fields
containing default values if the default was specified by value. We now
also support if these types are specified via `default_factory`.

Note that since `tuple` and `frozenset` values are immutable there's no
real reason to prefer `default_factory` for defaults of these types.

msgspec/_core.c

@@ -5675,7 +5675,15 @@ structmeta_process_default(StructMetaInfo *info, PyObject *name) {
             type = Py_TYPE(obj);
         }
         else if (f->default_factory != NODEFAULT) {
-            default_val = Factory_New(f->default_factory);
+            if (f->default_factory == (PyObject *)&PyTuple_Type) {
+                default_val = PyTuple_New(0);
+            }
+            else if (f->default_factory == (PyObject *)&PyFrozenSet_Type) {
+                default_val = PyFrozenSet_New(NULL);
+            }
+            else {
+                default_val = Factory_New(f->default_factory);
+            }
             if (default_val == NULL) return -1;
             goto done;
         }

tests/test_common.py

@@ -1744,6 +1744,22 @@ class Test(Struct, omit_defaults=True):
             res = proto.decode(proto.encode(obj))
             assert res == sol
 
+    @pytest.mark.parametrize("typ", [tuple, list, set, frozenset, dict])
+    def test_omit_defaults_collections(self, proto, typ):
+        """Check that using empty collections as default values are detected
+        regardless if they're specified by value or as a default_factory."""
+
+        class Test(Struct, omit_defaults=True):
+            a: typ = msgspec.field(default_factory=typ)
+            b: typ = msgspec.field(default=typ())
+            c: typ = typ()
+
+        ex = {"x": 1} if typ is dict else [1]
+
+        assert proto.encode(Test()) == proto.encode({})
+        for n in ["a", "b", "c"]:
+            assert proto.encode(Test(**{n: typ(ex)})) == proto.encode({n: ex})
+
     def test_omit_defaults_positional(self, proto):
         class Test(Struct, omit_defaults=True):
             a: int