rlua -- High level bindings between Rust and Lua

Guided Tour

This library is a high level interface between Rust and Lua. Its major goals are 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 NOT designed to be a perfect zero cost wrapper over the Lua C API, because such a wrapper cannot maintain the safety guarantees that rlua is designed to have. Every place where the Lua C API may trigger an error longjmp in any way is protected by lua_pcall, and the user of the library is protected from directly interacting with unsafe things like the Lua stack, and there is overhead associated with this safety. However, performance is a focus of the library to the extent possible while maintaining safety, so if you encounter something that egregiously worse than using the Lua C API directly, or simply something you feel could perform better, feel free to file a bug report.

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.
• Quantifying performance differences to direct use of the Lua C API.

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 16, it would be great if this was replaced with real variadic generics when this is available in Rust.

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.

Caveats to the panic / abort guarantee:

• rlua reserves the right to panic on API usage errors. Currently, the only time this will happen is when passed a handle type from a Lua instance that does not share the same main 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.
• rustc version 1.24.0 on Windows contains a bug which affects rlua error handling, turning any Lua script error into an abort. If you are using Rust 1.24.0 on windows, please upgrade to 1.24.1.

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.

rlua should also be panic safe in another way as well, which is that any Lua instances or handles should remain usable after a user triggered panic, and such panics should not break internal invariants or leak Lua stack space. This is mostly important to safely use rlua types in Drop impls, as you should not be using panics for general error handling.

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.
• 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.
• If you load the "debug" library (which requires typing "unsafe"), every safety / panic / abort guarantee goes out the window. The debug library can be used to do extremely scary things. If you use the debug library and encounter a bug, it may still very well be a bug, but try to find a reproduction that does not involve the debug library first.
• 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 cause UB.
• Lua C API errors are handled by lonjmp. ALL instances where the Lua C API would longjmp should be protected from Rust, except in internal callbacks where this is intentional. If you detect that rlua is triggering a longjmp over your Rust stack frames, this is a bug!
• If you can somehow handle a panic in a Rust callback from Lua, this is a bug.
• If you detect that, after catching a panic or during a Drop triggered from a panic, a Lua or handle method is triggering other bugs or there is a Lua stack space leak, this is a bug. rlua instances are supposed to remain fully usable in the face of user triggered panics. This guarantee does not extend to panics marked with "rlua internal error" simply because that is already indicative of a separate bug.