How to write a Rust UserData method that takes a Lua file argument?

[drauschenbach]

For my UserData, I'd like to write a method that takes a Lua file argument. For example, I want to be able to call my function with arguments like this:

myUserData:with_stdout(io.stdout)

or:

local filename = os.tmpname()
local file = io.open(filename, 'w')
myUserData:with_stdout(file)

Is there any guidance in the code on how to work with Lua files on the Rust side of things?

I have this skeleton in place:

methods.add_method("with_stdout", |_lua, _this, arg: Value| {
    if let Some(arg_ud) = arg.as_userdata() {
        // What can we do now?
        
        // Can we use borrow to typecast into something that works like a file?
        if let Ok(file) = arg_ud.borrow::<__what_might_go_here?__>() {
            ...
        }
    }
    ...
}