rlua -- High level bindings between Rust and Lua

This library is a WIP high level interface between Rust and Lua. Its major goal is to expose as flexible of an API between Rust and Lua as possible, while also being completely safe.

There are other high level lua bindings systems for rust, and this crate is an exploration of a different part of the design space. The main high level interface to Lua right now is hlua which you should definitely check out and use if it suits your needs. This crate has the following differences with hlua:

* Handles to Lua values use the Lua registry, not the stack
* Handles to Lua values are all internally mutable
* Handles to Lua values use non-mutable borrows the main Lua object, so
  there can be multiple handles or long lived handles
* Targets lua 5.3

The key difference here is that rlua handles rust-side references to Lua values in a fundamentally different way than hlua, more similar to other lua bindings systems like Selene for C++. Values like LuaTable and LuaFunction that hold onto Lua values in the Rust stack, instead of pointing at values in the Lua stack, are placed into the registry with luaL_ref. In this way, it is possible to have an arbitrary number of handles to internal Lua values at any time, created and destroyed in arbitrary order. This approach IS slightly slower than the approach that hlua takes of only manipulating the lua stack, but this, combined with internal mutability, allows for a much more flexible API.

Currently exposes a somewhat complete Lua API covering values and tables and functions and userdata, but does not yet cover coroutines. This API is actually heavily inspired by the lua API that I previously wrote for Starbound, and will become feature complete with that API over time. Some capabilities that API has that are on the roadmap:

* Proper coroutine support
* Lua profiling support
* Execution limits like total instruction limits or lua <-> rust recursion
  limits
* Security limits on the lua stdlib, and general control over the loaded
  lua libraries.
* "Context" or "Sandboxing" support, this was probably a bit too heavyweight
  in Starbound's API, but there will be the ability to set the _ENV upvalue
  of a loaded chunk to a table other than _G, so that you can have different
  environments for different loaded chunks.

There are also some more general things that need to be done:

* More fleshed out Lua API, things like custom metatables and exposing the
  registry.
* MUCH better API documentation, the current API documentation is almost
  non-existent.
* Performance testing.

Additionally, there are ways I would like to change this API, once support lands in rustc. For example:

* Once ATCs land, there should be a way to wrap callbacks based on argument
  and return signature, rather than calling lua.pack / lua.unpack inside the
  callback.  Until then, it is impossible to name the type of the function
  that would do the wrapping.
* Once tuple based variadic generics land, the plan is to completely
  eliminate the lua multi macros in favor of simple tuples.

See this reddit discussion for details of the current lifetime problem with callback wrapping.

Safety

My goal is complete safety, it should not be possible to cause undefined behavior whatsoever with the API, even in edge cases. There is, however, QUITE a lot of unsafe code in this crate, and I would call the current safety level of the crate "Work In Progress". The GOAL is for the crate to handle tricky situations such as:

* Panic safety, and carrying the panic across the lua api correctly
* Lua stack size checking, and correctly handling lua being out of stack
  space
* Leaving the correct elements on the lua stack and in the correct order,
  and panicking if these invariants are not met (due to internal bugs).
* Correctly guarding the metatables of userdata so that scripts cannot, for
  example, swap the __gc methods around and cause UB.

The library currently attempts to handle each of these situations, but there are so many ways to cause unsafety with Lua that it just needs more testing.

Examples

Please look at the examples here.