How to list all functions and fields in userdata

[lammermann]

Hello guys,
I try to build a REPL with mlua and rustyline.

When it comes to tab-completion I check if the current prompt could be completed by using the method for_each from the tables. For example given the current prompt foo.b it searches the lua globals for foo and then searches all fields in foo and filters to only those beginning with b then returning bar and baz as possible completions.
This works fine for tables but I could not find something similar for userdata ( I found the method pairs in the UserDataMetatable but it does not seem to return the name of the functions of the userdata).

Is there any advice how to get this information from userdata?

Thanks in advance for any hints