Relax UserDataBorrowRef restrictions to allow recursive calls

Author: khvzak

src/userdata/cell.rs

@@ -291,11 +291,7 @@ pub(crate) struct UserDataBorrowRef<'a, T>(&'a UserDataVariant<T>);
 impl<T> Drop for UserDataBorrowRef<'_, T> {
     #[inline]
     fn drop(&mut self) {
-        if !cfg!(feature = "send") || is_sync::<T>() {
-            unsafe { self.0.raw_lock().unlock_shared() };
-        } else {
-            unsafe { self.0.raw_lock().unlock_exclusive() };
-        }
+        unsafe { self.0.raw_lock().unlock_shared() };
     }
 }
 
@@ -314,11 +310,11 @@ impl<'a, T> TryFrom<&'a UserDataVariant<T>> for UserDataBorrowRef<'a, T> {
 
     #[inline(always)]
     fn try_from(variant: &'a UserDataVariant<T>) -> Result<Self> {
-        if !cfg!(feature = "send") || is_sync::<T>() {
-            if !variant.raw_lock().try_lock_shared() {
-                return Err(Error::UserDataBorrowError);
-            }
-        } else if !variant.raw_lock().try_lock_exclusive() {
+        // We don't need to check for `T: Sync` because when this method is used (internally),
+        // Lua mutex is already locked.
+        // If non-`Sync` userdata is already borrowed by another thread (via `UserDataRef`), it will be
+        // exclusively locked.
+        if !variant.raw_lock().try_lock_shared() {
             return Err(Error::UserDataBorrowError);
         }
         Ok(UserDataBorrowRef(variant))

tests/send.rs

@@ -20,12 +20,11 @@ fn test_userdata_multithread_access_send_only() -> Result<()> {
         fn add_methods<M: UserDataMethods<Self>>(methods: &mut M) {
             methods.add_method("method", |lua, this, ()| {
                 let ud = lua.globals().get::<AnyUserData>("ud")?;
-                assert!((ud.call_method::<()>("method2", ()).err().unwrap().to_string())
-                    .contains("error borrowing userdata"));
+                assert_eq!(ud.call_method::<String>("method2", ())?, "method2");
                 Ok(this.0.clone())
             });
 
-            methods.add_method("method2", |_, _, ()| Ok(()));
+            methods.add_method("method2", |_, _, ()| Ok("method2"));
         }
     }