`#[mlua::userdata_impl]` fails to compile when the `impl` block is in a different module than the type

[Dobee8]

Version:

mlua 0.12.0 (mlua_derive 0.12.0), features: lua51, vendored, macros

Description

#[mlua::userdata_impl] only works when it is placed in the same module (or a descendant)
as #[derive(UserData)]. In any other module it fails to compile. This makes it impossible to
split a type's Rust-side code from its Lua bindings, which is a natural layout when embedding
Lua (e.g. a game core types module + a dedicated lua/ binding module).

Minimal reproduction

// src/types.rs  (module where the type is defined)
use mlua::UserData;

#[derive(UserData)]
pub struct A {}

// src/lua.rs  (separate module)
#[mlua::userdata_impl]
impl A {
    fn hello(&self) -> mlua::Result<()> {
        Ok(())
    }
}

Expected

Compiles; A:hello is registered for Lua.

Actual

error[E0422]: cannot find struct, variant or union type __MluaUserDataRegistration_A in this scope

Root cause

• #[derive(UserData)] emits a private __MluaUserDataRegistration_A struct plus inventory::collect! and impl UserData in the type's module (mlua_derive/src/userdata/mod.rs).
• #[mlua::userdata_impl] expands to inventory::submit! { __MluaUserDataRegistration_A { ... } } referencing the bare identifier (mlua_derive/src/userdata/userdata_impl.rs), which must resolve in the module where the macro is written. From a sibling module the private type is neither in scope nor importable, so resolution fails.