exposing external crate's (APIs) to Lua world (as a module)

[ildar]

Greetings!
I'm discovering how to expose a Rust external crate as a Lua module.
I've already stumbled on a few things so wanted to ask community for a good advise.

1. to store some structures in Lua tables (as UserData) they should be Clone and Copy. If not, you're probably in a big trouble.
2. if you cannot store something in a Lua table (see above) you need some static storage for it, right? Maybe something with RefCell<Option<Data>>? That's a huge antipattern in Rust and I'm a little lost on how to handle it in a right way.

P.S. There are definitely patterns and antipatterns for this. Anyone wants to share?
P.P.S. I've taken a quick look at other projects and couldn't find a good example for the topic. Anyone?

[khvzak]

to store some structures in Lua tables (as UserData) they should be Clone and Copy. If not, you're probably in a big trouble.

Hm, there is no such requirement. Any data structure can be passed to Lua without implementing Clone and/or Copy.
See an example where Rectangle struct implements neither clone nor copy.

[ildar]

Partially. But I fail to access it: ``` | 33 | let test: Result<&MyUserData,LuaError> = exports.get("userdata"); | ^^^ the trait `mlua::UserData` is not implemented for `&MyUserData` ``` The `get()` getter returns the copy of the stored object. I see no way to get the ref. [full source link](https://paste.rs/b98.rs)

[khvzak]

This is because you need to use AnyUserData instead of &MyUserData to get your data from the table.
And then borrow it using borrow() or borrow_mut methods.
Eg:

let ud = exports.get::<_, AnyUserData>("userdata")?;
let mydata = ud.borrow::<MyUserData>()?;

[ildar]

Just what I need! Thanks. BTW, do you know other "expose a Rust external crate as a Lua module" examples? I found https://crates.io/crates/distant-lua but it uses mlua 0.6 and is relatively simple.

[ildar]

hmm, `borrow()` implies `'static` lifetime to UserData contents which isn't always true, I think. Need to elaborate that. Meanwhile JFYI, I'm stumbling on the error: ``` error[E0621]: explicit lifetime required in parameter type --> src/lib.rs:37:25 | 37 | let mut el = el.borrow_mut::<EventLoop>()?; | ^^^^^^^^^^ lifetime `'static` required ``` full project: https://github.com/ildar/lua-module-calloop

[khvzak]

'static lifetime is Rust requirement for Any trait: https://doc.rust-lang.org/std/any/trait.Any.html

Alternatively you can use non-static userdata types inside scope.

[ildar]

This is again more complicated then I thought.
I've made a small drawing, I wonder if I'm getting it all right.

source

[khvzak]

Yes, the diagram above is almost correct.

Clone for UserData is requirement only for FromLua implementation.

non-'static types are very tricky because they don't implement Any trait (dynamic typing). In Lua all types information is literally erased and metamethods used to match to a particular Rust type. For 'static types all userdata objects share a single metatable, for non-'static inside scope all instances have uniq metatables.

If it's possible I would recommend either go to unsafe {} territory to make things much easier or lift lifetimes to 'static.

[ildar]

Surprisingly, I managed to skip using `scope()`. After some meditation and a few experiments I declared exported function's argument as 'static: ildar/lua-module-calloop@a57ff0b#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R34 Builds and works for me.