avm2: Implement `describeType(null)` stub (#16235)

core/src/avm2/globals/avmplus.rs

@@ -15,9 +15,12 @@ pub fn describe_type_json<'gc>(
     _this: Object<'gc>,
     args: &[Value<'gc>],
 ) -> Result<Value<'gc>, Error<'gc>> {
-    let value = args[0].coerce_to_object(activation)?;
     let flags = DescribeTypeFlags::from_bits(args.get_u32(activation, 1)?).expect("Invalid flags!");
+    if args[0] == Value::Null {
+        return describe_type_json_null(activation, flags);
+    }
 
+    let value = args[0].coerce_to_object(activation)?;
     let class_obj = value.as_class_object().or_else(|| value.instance_of());
     let object = activation
         .avm2()
@@ -80,6 +83,102 @@ bitflags::bitflags! {
     }
 }
 
+fn describe_type_json_null<'gc>(
+    activation: &mut Activation<'_, 'gc>,
+    flags: DescribeTypeFlags,
+) -> Result<Value<'gc>, Error<'gc>> {
+    if flags.contains(DescribeTypeFlags::USE_ITRAITS) {
+        return Ok(Value::Null);
+    }
+    let object = activation
+        .avm2()
+        .classes()
+        .object
+        .construct(activation, &[])?;
+
+    object.set_public_property("name", "null".into(), activation)?;
+    object.set_public_property("isDynamic", false.into(), activation)?;
+    object.set_public_property("isFinal", true.into(), activation)?;
+    object.set_public_property("isStatic", false.into(), activation)?;
+
+    let traits = activation
+        .avm2()
+        .classes()
+        .object
+        .construct(activation, &[])?;
+
+    if flags.contains(DescribeTypeFlags::INCLUDE_TRAITS) {
+        traits.set_public_property(
+            "bases",
+            if flags.contains(DescribeTypeFlags::INCLUDE_BASES) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "interfaces",
+            if flags.contains(DescribeTypeFlags::INCLUDE_INTERFACES) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "variables",
+            if flags.contains(DescribeTypeFlags::INCLUDE_VARIABLES) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "accessors",
+            if flags.contains(DescribeTypeFlags::INCLUDE_ACCESSORS) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "methods",
+            if flags.contains(DescribeTypeFlags::INCLUDE_METHODS) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "metadata",
+            if flags.contains(DescribeTypeFlags::INCLUDE_METADATA) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        traits.set_public_property(
+            "constructor",
+            if flags.contains(DescribeTypeFlags::INCLUDE_CONSTRUCTOR) {
+                ArrayObject::empty(activation)?.into()
+            } else {
+                Value::Null
+            },
+            activation,
+        )?;
+        object.set_public_property("traits", traits.into(), activation)?;
+    } else {
+        object.set_public_property("traits", Value::Null, activation)?;
+    }
+
+    Ok(object.into())
+}
+
 fn describe_internal_body<'gc>(
     activation: &mut Activation<'_, 'gc>,
     class_obj: ClassObject<'gc>,