Call __post_init__ when converting struct from object

Previously a struct's `__post_init__` method wouldn't be called when
converting from a custom mapping or object with `from_attributes=True`.
This PR fixes that and expands the tests to cover this case.

msgspec/_core.c

@@ -21364,6 +21364,9 @@ convert_object_to_struct(
             should_untrack = !MS_MAYBE_TRACKED(val);
         }
     }
+
+    if (Struct_decode_post_init(struct_type, out, path) < 0) goto error;
+
     Py_LeaveRecursiveCall();
     if (is_gc && !should_untrack)
         PyObject_GC_Track(out);

tests/test_convert.py

@@ -2239,18 +2239,18 @@ class Test2(Struct, Generic[T], tag=True, array_like=array_like):
 
 
 class TestStructPostInit:
-    @pytest.mark.parametrize("array_like", [False, True])
     @pytest.mark.parametrize("union", [False, True])
-    def test_struct_post_init(self, array_like, union):
-        count = 0
+    @mapcls_from_attributes_and_array_like
+    def test_struct_post_init(self, union, mapcls, from_attributes, array_like):
+        called = False
         singleton = object()
 
         class Ex(Struct, array_like=array_like, tag=union):
             x: int
 
             def __post_init__(self):
-                nonlocal count
-                count += 1
+                nonlocal called
+                called = True
                 return singleton
 
         if union:
@@ -2262,25 +2262,23 @@ class Ex2(Struct, array_like=array_like, tag=True):
         else:
             typ = Ex
 
-        msg = Ex(1)
-        buf = to_builtins(msg)
-        res = convert(buf, type=typ)
-        assert res == msg
-        assert count == 2  # 1 for Ex(), 1 for decode
+        msg = mapcls(type="Ex", x=1) if union else mapcls(x=1)
+        res = convert(msg, type=typ, from_attributes=from_attributes)
+        assert type(res) is Ex
+        assert called
         assert sys.getrefcount(singleton) == 2  # 1 for ref, 1 for call
 
-    @pytest.mark.parametrize("array_like", [False, True])
     @pytest.mark.parametrize("union", [False, True])
     @pytest.mark.parametrize("exc_class", [ValueError, TypeError, OSError])
-    def test_struct_post_init_errors(self, array_like, union, exc_class):
-        error = False
-
+    @mapcls_from_attributes_and_array_like
+    def test_struct_post_init_errors(
+        self, union, exc_class, mapcls, from_attributes, array_like
+    ):
         class Ex(Struct, array_like=array_like, tag=union):
             x: int
 
             def __post_init__(self):
-                if error:
-                    raise exc_class("Oh no!")
+                raise exc_class("Oh no!")
 
         if union:
 
@@ -2291,16 +2289,15 @@ class Ex2(Struct, array_like=array_like, tag=True):
         else:
             typ = Ex
 
-        msg = to_builtins([Ex(1)])
-        error = True
+        msg = [mapcls(type="Ex", x=1) if union else mapcls(x=1)]
 
         if exc_class in (ValueError, TypeError):
             expected = ValidationError
         else:
             expected = exc_class
 
         with pytest.raises(expected, match="Oh no!") as rec:
-            convert(msg, type=List[typ])
+            convert(msg, type=List[typ], from_attributes=from_attributes)
 
         if expected is ValidationError:
             assert "- at `$[0]`" in str(rec.value)