How to create dynamic userdata types?

[dubrowgn]

I have struct definitions coming from a DSL interpreted at runtime, and want to create lua userdata types with a custom __index implementation for field accessors. However, I don't see any userdata APIs that don't take a compile-time rust type, so how can I create userdata types at runtime?

Conceptually, I would imagine this is something like lua_newuserdata followed by lua_setmetatable. It would be nice to be able to reference mlua types like Table, but I don't see a way to access the index field for these.

Am I forced to re-implement the high-level mlua API's on top of ffi to implement dynamic lua types at runtime? Am I missing something?

[khvzak]

that don't take a compile-time rust type

Compile-time rust type is required to provide typing guarantee and correct Drop on GC collection.

It would be nice to be able to reference mlua types like Table, but I don't see a way to access the index field for these.

You can access index using Table::metatable

Am I forced to re-implement the high-level mlua API's on top of ffi to implement dynamic lua types at runtime? Am I missing something?
I have struct definitions coming from a DSL interpreted at runtime,

How your struct definitions are represented in Rust? They should have some type?

[dubrowgn]

Thanks for the quick response.

You can access index using Table::metatable

I meant the struct field index on ValueRef, e.g. tbl.0.index. I don't see how to reference a Table via the low-level API (e.g. calling ffi::lua_setmetatable on my own userdata) without access to this field. Table::metatable just returns another Table.

How your struct definitions are represented in Rust?

They're not. They are defined at runtime in a C library (flecs, although it shouldn't matter). I get back reflection info, and need to build some kind of glue so these types can be used in lua. So far, I can build a metatable with an appropriate __index implementation, but I cannot instantiate one from rust because I don't have a rust backing type. Lua lets you do this, but mlua doesn't appear to expose this kind of thing at all through the high level API.

[khvzak]

I meant the struct field index on ValueRef, e.g. tbl.0.index. I don't see how to reference a Table via the low-level API (e.g. calling ffi::lua_setmetatable on my own userdata) without access to this field.

You can do this using Lua::exec_raw.
You can pass any mlua IntoLua type and it will be pushed into the stack.