rlua -- High level bindings between Rust and Lua

Guided Tour

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

rlua is designed around "registry handles" to values inside the Lua state. This means that when you get a type like rlua::Table or rlua::Function in Rust, what you actually hold is an integer key into the Lua registry. This is different from the bare Lua C API, where you create tables / functions on the Lua stack and must be aware of their stack location. This is also similar to how other Lua bindings systems like Selene for C++ work, but it means that using rlua may be slightly slower than what you could conceivably write using the C API. The reasons for this design are safety and flexibility, and to prevent the user of rlua from having to be aware of the Lua stack at all.

There are currently a few missing pieces of this API:

• Security limits on Lua code such as total instruction limits / memory limits and control over which potentially dangerous libraries (e.g. io) are available to scripts.
• Lua profiling support
• "Context" or "Sandboxing" support. There should 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.
• Benchmarks, and quantifying performance differences with what you would might write in C.

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

• Currently, variadics are handled entirely with tuples and traits implemented by macro for tuples up to size 12, it would be great if this was replaced with real variadic generics when this is available in Rust.

It is also worth it to list some non-goals for the project:

• Be a perfect zero cost wrapper over the Lua C API
• Allow the user to do absolutely everything that the Lua C API might allow

API stability

This library is very much Work In Progress, so there is a some API churn. Currently, it follows a pre-1.0 semver, so all API changes should be accompanied by 0.x version bumps.

Safety and panics

The goal of this library 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". Still, I am not currently aware of any way to cause UB, and UB is considered the most serious kind of bug, so if you find the ability to cause UB with this API at all, please file a bug report.

Another goal of this library is complete protection from panics and aborts. Currently, it should not be possible for a script to trigger a panic or abort (with some important caveats described below). Similarly to the safety goal, there ARE several internal panics and even aborts in rlua source, but they should not be possible to trigger, and if you trigger them this should be considered a bug.

There are some caveats to the panic / abort guarantee, however:

• rlua reserves the right to panic on API usage errors. Currently, the only time this will happen is when passed a registry handle type from a different main Lua state.
• Currently, there are no memory or execution limits on scripts, so untrusted scripts can always at minimum infinite loop or allocate arbitrary amounts of memory.
• The internal Lua allocator is set to use realloc from libc, but it is wrapped in such a way that OOM errors are guaranteed to abort. This is not currently such a huge deal outside of untrusted scripts, as this matches the behavior of Rust itself. Doing this allows the internals of rlua to, in certain cases, call 'm' Lua C API functions with the garbage collector disabled and know that these cannot error. Eventually, rlua will support memory limits on scripts, and those memory limits will cause regular memory errors rather than OOM aborts.

Yet another goal of the library is to, in all cases, safely handle panics generated by Rust callbacks. Panic unwinds in Rust callbacks should currently be handled correctly -- the unwind is caught and carried across the Lua API boundary as a regular Lua error in a way that prevents Lua from catching it. This is done by overriding the normal Lua 'pcall' and 'xpcall' with custom versions that cannot catch errors that are actually from Rust panics, and by handling panic errors on the receiving Rust side by resuming the panic.

In summary, here is a list of rlua behaviors that should be considered a bug. If you encounter them, a bug report would be very welcome:

• If you can cause UB at all with rlua without typing the word "unsafe", this is absolutely 100% a bug. This does not include loading the "debug" library, which requires typing "unsafe". If you load the debug library, every guarantee goes out the window.
• If your code panics / aborts with a message that contains the string "rlua internal error", this is a bug.
• The above is true even for the internal panic about running out of stack space! There are a few ways to generate normal script errors by running out of stack, but if you encounter a panic based on running out of stack, this is a bug.
• When the internal version of Lua is built using the gcc crate, and cfg!(debug_assertions) is true, Lua is built with the LUA_USE_APICHECK define set. Any abort caused by this internal Lua API checking is absolutely a bug, particularly because without LUA_USE_APICHECK it would generally be unsafe.
• Lua C API errors are handled by lonjmp. ALL instances where the Lua C API would longjmp should be protected from Rust, except in a few cases of internal callbacks where there are only Copy types on the stack. If you detect that rlua is triggering a longjmp over Rust stack frames (other than the internal ones where this is intentional), this is a bug! (NOTE: I believe it is still an open question whether technically Rust allows longjmp over Rust stack frames at all, even if there are only Copy types on the stack. Currently rlua uses this to avoid having to write a lot of messy C shims. It currently works fine, and it is difficult to imagine how it would ever NOT work, but what is and isn't UB in unsafe Rust is not precisely specified.)