Use the class_name for the field when building PropertyInfo

Godot uses the class_name to construct an instance of the right type in a few places,
in particular for the PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT flag.

Partial fix for godot-rust#440

godot-macros/src/class/data_models/property.rs

@@ -67,6 +67,8 @@ pub fn make_property_impl(class_name: &Ident, fields: &Fields) -> TokenStream {
             continue;
         };
 
+        let field_variant_type = util::property_variant_type(field_type);
+        let field_class_name = util::property_variant_class_name(field_type);
         let field_name = field_ident.to_string();
 
         // rustfmt wont format this if we put it in the let-else.
@@ -178,8 +180,8 @@ pub fn make_property_impl(class_name: &Ident, fields: &Fields) -> TokenStream {
             let usage = #usage_flags;
 
             let property_info = ::godot::builtin::meta::PropertyInfo {
-                variant_type: <<#field_type as ::godot::bind::property::Property>::Intermediate as ::godot::builtin::meta::VariantMetadata>::variant_type(),
-                class_name: #class_name_obj,
+                variant_type: #field_variant_type,
+                class_name: #field_class_name,
                 property_name: #field_name.into(),
                 hint,
                 hint_string,
@@ -194,7 +196,7 @@ pub fn make_property_impl(class_name: &Ident, fields: &Fields) -> TokenStream {
             unsafe {
                 ::godot::sys::interface_fn!(classdb_register_extension_class_property)(
                     ::godot::sys::get_library(),
-                    #class_name::class_name().string_sys(),
+                    #class_name_obj.string_sys(),
                     std::ptr::addr_of!(property_info_sys),
                     setter_name.string_sys(),
                     getter_name.string_sys(),

godot-macros/src/util/mod.rs

@@ -32,6 +32,16 @@ pub fn class_name_obj(class: &impl ToTokens) -> TokenStream {
     quote! { <#class as ::godot::obj::GodotClass>::class_name() }
 }
 
+pub fn property_variant_type(property_type: &impl ToTokens) -> TokenStream {
+    let property_type = property_type.to_token_stream();
+    quote! {<<#property_type as ::godot::bind::property::Property>::Intermediate as ::godot::builtin::meta::VariantMetadata>::variant_type()}
+}
+
+pub fn property_variant_class_name(property_type: &impl ToTokens) -> TokenStream {
+    let property_type = property_type.to_token_stream();
+    quote! {<<#property_type as ::godot::bind::property::Property>::Intermediate as ::godot::builtin::meta::VariantMetadata>::class_name()}
+}
+
 pub fn bail_fn<R, T>(msg: impl AsRef<str>, tokens: T) -> ParseResult<R>
 where
     T: Spanned,

itest/rust/src/object_tests/property_test.rs

@@ -6,7 +6,10 @@
 
 use godot::{
     bind::property::ExportInfo,
-    engine::{global::PropertyHint, Texture},
+    engine::{
+        global::{PropertyHint, PropertyUsageFlags},
+        Texture,
+    },
     prelude::*,
     test::itest,
 };
@@ -351,10 +354,8 @@ fn derive_export() {
         .iter_shared()
         .find(|c| c.get_or_nil("name") == "foo".to_variant())
         .unwrap();
-    assert_eq!(
-        property.get_or_nil("class_name"),
-        "DeriveExport".to_variant()
-    );
+    // `class_name` should be empty for non-Object variants.
+    assert_eq!(property.get_or_nil("class_name"), "".to_variant());
     assert_eq!(
         property.get_or_nil("type"),
         (VariantType::Int as i32).to_variant()
@@ -367,4 +368,78 @@ fn derive_export() {
         property.get_or_nil("hint_string"),
         "A:0,B:1,C:2".to_variant()
     );
+    assert_eq!(
+        property.get_or_nil("usage"),
+        PropertyUsageFlags::PROPERTY_USAGE_DEFAULT
+            .ord()
+            .to_variant()
+    );
+}
+
+#[derive(GodotClass)]
+#[class(base=Resource)]
+pub struct CustomResource {}
+
+#[godot_api]
+impl CustomResource {}
+
+#[godot_api]
+impl ResourceVirtual for CustomResource {
+    fn init(_base: godot::obj::Base<Self::Base>) -> Self {
+        Self {}
+    }
+}
+
+#[derive(GodotClass)]
+#[class(base=Node)]
+pub struct ExportResource {
+    #[export]
+    #[var(usage_flags=[PROPERTY_USAGE_DEFAULT, PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT])]
+    pub bar: Option<Gd<CustomResource>>,
+}
+
+#[godot_api]
+impl ExportResource {}
+
+#[godot_api]
+impl NodeVirtual for ExportResource {
+    fn init(_base: godot::obj::Base<Self::Base>) -> Self {
+        Self { bar: None }
+    }
+}
+
+#[itest]
+fn export_resource() {
+    let class: Gd<ExportResource> = Gd::new_default();
+
+    let property = class
+        .get_property_list()
+        .iter_shared()
+        .find(|c| c.get_or_nil("name") == "bar".to_variant())
+        .unwrap();
+    // `class_name` should be empty for non-Object variants.
+    assert_eq!(
+        property.get_or_nil("class_name"),
+        "CustomResource".to_variant()
+    );
+    assert_eq!(
+        property.get_or_nil("type"),
+        (VariantType::Object as i32).to_variant()
+    );
+    assert_eq!(
+        property.get_or_nil("hint"),
+        (PropertyHint::PROPERTY_HINT_RESOURCE_TYPE.ord()).to_variant()
+    );
+    assert_eq!(
+        property.get_or_nil("hint_string"),
+        "CustomResource".to_variant()
+    );
+    assert_eq!(
+        property.get_or_nil("usage"),
+        (PropertyUsageFlags::PROPERTY_USAGE_DEFAULT.ord()
+            | PropertyUsageFlags::PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT.ord())
+        .to_variant()
+    );
+
+    class.free();
 }