mlua_derive: Make userdata_impl independent from module where UserData is defined

Before this change `#[mlua::userdata_impl]` was working only when it's placed together
with `#[derive(UserData)]`, in the same module.
Now it's flexible to allow splitting type definition and Lua binding code.

Closes #726

Author: khvzak

mlua_derive/src/userdata/mod.rs

@@ -132,25 +132,36 @@ pub fn userdata_type(item: TokenStream) -> TokenStream {
     }
 
     let registration_type_name = format_ident!("__MluaUserDataRegistration_{type_name}");
+    let registration_fn_name = format_ident!("__mlua_userdata_registration");
     let register_fields_fn_name = format_ident!("__mlua_register_{type_name}_fields");
 
     let output = quote! {
         #[doc(hidden)]
         #[allow(non_camel_case_types)]
-        struct #registration_type_name {
+        pub(crate) struct #registration_type_name {
             register: fn(&mut ::mlua::userdata::UserDataRegistry<#type_name>),
         }
 
         ::mlua::__inventory::collect!(#registration_type_name);
 
+        impl #type_name {
+            #[doc(hidden)]
+            #[allow(dead_code)]
+            pub(crate) const fn #registration_fn_name(
+                register: fn(&mut ::mlua::userdata::UserDataRegistry<Self>),
+            ) -> #registration_type_name {
+                #registration_type_name { register }
+            }
+        }
+
         #[allow(non_snake_case)]
         fn #register_fields_fn_name(registry: &mut ::mlua::userdata::UserDataRegistry<#type_name>) {
             use ::mlua::userdata::UserDataFields as _;
             #(#field_registrations)*
         }
 
         ::mlua::__inventory::submit! {
-            #registration_type_name { register: #register_fields_fn_name }
+            #type_name::#registration_fn_name(#register_fields_fn_name)
         }
 
         impl ::mlua::userdata::UserData for #type_name {

mlua_derive/src/userdata/userdata_impl.rs

@@ -323,7 +323,7 @@ pub fn userdata_impl(attr: TokenStream, item: TokenStream) -> TokenStream {
     static COUNTER: AtomicUsize = AtomicUsize::new(0);
     let unique_suffix = COUNTER.fetch_add(1, Ordering::Relaxed);
     let register_fn_name = format_ident!("__mlua_register_{type_name}_{unique_suffix}");
-    let registration_type_name = format_ident!("__MluaUserDataRegistration_{type_name}");
+    let registration_fn_name = format_ident!("__mlua_userdata_registration");
 
     let mut registration_calls = Vec::new();
     for item in &input.items {
@@ -522,7 +522,7 @@ pub fn userdata_impl(attr: TokenStream, item: TokenStream) -> TokenStream {
         }
 
         ::mlua::__inventory::submit! {
-            #registration_type_name { register: #register_fn_name }
+            #type_path::#registration_fn_name(#register_fn_name)
         }
 
         #input

tests/userdata_macro.rs

@@ -569,6 +569,36 @@ fn test_wildcard_params() {
     .unwrap();
 }
 
+mod separate_definition {
+    use mlua::UserData;
+
+    #[derive(UserData)]
+    pub(super) struct SeparateModuleUserData;
+}
+
+mod separate_implementation {
+    use super::separate_definition::SeparateModuleUserData;
+
+    #[mlua::userdata_impl]
+    impl SeparateModuleUserData {
+        #[lua(infallible)]
+        fn answer(&self) -> i32 {
+            42
+        }
+    }
+}
+
+#[test]
+fn test_userdata_impl_in_separate_module() {
+    let lua = Lua::new();
+    let userdata = lua
+        .create_userdata(separate_definition::SeparateModuleUserData)
+        .unwrap();
+    lua.globals().set("userdata", userdata).unwrap();
+
+    assert_eq!(lua.load("return userdata:answer()").eval::<i32>().unwrap(), 42);
+}
+
 #[cfg(feature = "async")]
 mod async_tests {
     use mlua::{Lua, Result, UserData};