mluau

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This repository is a fork of mlua with a greater focus on Luau, with the following changes (so far):

• More reliable coroutine and yielding support:
  • mluau allows Rust functions to yield back to Luau directly, improving support for iterators, coroutines, and task schedulers.
  • Support for Luau continuations - a Luau feature that allows a yielded Luau thread to call a Rust continuation function upon coroutine.resume, before resuming back to Luau.
• Thread stack optimizations and bug fixes:
  • Removes unnecessary copies of the main thread stack to improve resume/yield performance.
  • Uses an auxiliary thread list to prevent panicking if user code makes more than 1 million references to Rust-side code.
• Removal of async support.
  • mlua's async implementation is prone to freezes and deadlocks, and doesn't fit in as well as we'd like with Luau and the Luau ecosystem in mind.
  • Not to worry! We're looking to replace it with a dedicated Luau-focused scheduler in the future, and are working on making sure it's rock solid just like the rest of Luau.
• Improved adherence to Luau spec to minimize UB and allow for a more easily sandboxed Luau environment:
  • Removal of the __gc metamethod on userdata; although implemented by mlua, should not be supported in Luau due to memory safety and optimization considerations.
  • collectgarbage now limited to options "count" and "collect" for better sandboxing. Importantly, this disallows user code from purposely stopping the garbage collector, even when sandbox mode is disabled.
• Removal of Lua::scope, a feature we don't use that carried a slight performance penalty.
• Integration with the Lute runtime and scheduler via the luau-lute feature flag. Note that crypto and net are disabled by default due to increasing compiler times and leading to large memory usage during linking, if you want to enable crypto and net, set the luau-lute-crypto and luau-lute-net flags respectively. Prebuilt static libraries of Lute are available for Linux (GNU, x86_64 and aarch64) and Windows (x86_64) via luau-lute-prebuilt feature flag. Note that both Linux and Windows prebuilt libraries are highly experimental and may not work as expected, please report any issues you encounter.
• Support for getting metatable of non-mlua/non-Rust userdata via the unsafe AnyUserData::underlying_metatable method. This is useful for managing newproxy and (Luau only) Lute userdata.
• Thread::pop_results has been added to allow popping results directly from the thread's stack to a R which implements FromLua. This is useful when trying to interoperate with Lute runtime but should not be needed much outside this in practice.
• Thread::close has been added to allow closing Lua threads
• RawLua::stack_value correctly calls lua_checkstack to avoid a potential crash when there are no stack slots free when popping from the Lua stack (from_lua etc.)
• Namecall optimization on Luau: for methods/functions on userdata, the namecall metamethod is now defined. This allows for more efficient method calls on userdata, as it avoids the need to check for the __index metamethod on every call. This is particularly useful for performance-critical code that relies heavily on userdata methods. This optimization is enabled by default, but can be disabled using UserDataRegistry::disable_namecall_optimization() if needed.
• Due to namcall, RawUserDataRegistry is not Send.

Roadmap

• Dedicated scheduler for mluau
• Integration with C++ tooling, most importantly Lute, the Luau language's official general purpose runtime for Luau.
  • Support for Luau AST, Compiler, etc. reflection through Lute.
• Tagged userdata (performance optimization)

The below is mlua's last README which should still be accurate or mostly accurate to mluau

Note

See v0.10 release notes.

mlua is a set of bindings to the Lua programming language for Rust with a goal to provide a safe (as much as possible), high level, easy to use, practical and flexible API.

Started as an rlua fork, mlua supports Lua 5.4, 5.3, 5.2, 5.1 (including LuaJIT) and Luau and allows writing native Lua modules in Rust as well as using Lua in a standalone mode.

mlua is tested on Windows/macOS/Linux including module mode in GitHub Actions on x86_64 platforms and cross-compilation to aarch64 (other targets are also supported).

WebAssembly (WASM) is supported through wasm32-unknown-emscripten target for all Lua/Luau versions excluding JIT.

Usage

Feature flags

mlua uses feature flags to reduce the amount of dependencies and compiled code, and allow to choose only required set of features. Below is a list of the available feature flags. By default mlua does not enable any features.

• lua54: enable Lua 5.4 support
• lua53: enable Lua 5.3 support
• lua52: enable Lua 5.2 support
• lua51: enable Lua 5.1 support
• luajit: enable LuaJIT support
• luajit52: enable LuaJIT support with partial compatibility with Lua 5.2
• luau: enable Luau support (auto vendored mode)
• luau-jit: enable Luau support with JIT backend.
• luau-vector4: enable Luau support with 4-dimensional vector.
• vendored: build static Lua(JIT) libraries from sources during mlua compilation using lua-src or luajit-src
• module: enable module mode (building loadable cdylib library for Lua)

• send: make mlua::Lua: Send + Sync (adds Send requirement to mlua::Function and mlua::UserData)
• error-send: make mlua:Error: Send + Sync
• serde: add serialization and deserialization support to mlua types using serde
• macros: enable procedural macros (such as chunk!)
• anyhow: enable anyhow::Error conversion into Lua
• userdata-wrappers: opt into impl UserData for Rc<T>/Arc<T>/Rc<RefCell<T>>/Arc<Mutex<T>> where T: UserData

Serialization (serde) support

With the serde feature flag enabled, mlua allows you to serialize/deserialize any type that implements serde::Serialize and serde::Deserialize into/from mlua::Value. In addition, mlua provides the serde::Serialize trait implementation for it (including UserData support).

Example

Compiling

You have to enable one of the features: lua54, lua53, lua52, lua51, luajit(52) or luau, according to the chosen Lua version.

By default mlua uses pkg-config to find Lua includes and libraries for the chosen Lua version. In most cases it works as desired, although sometimes it may be preferable to use a custom Lua library. To achieve this, mlua supports the LUA_LIB, LUA_LIB_NAME and LUA_LINK environment variables. LUA_LINK is optional and may be dylib (a dynamic library) or static (a static library, .a archive).

An example of how to use them:

my_project $ LUA_LIB=$HOME/tmp/lua-5.2.4/src LUA_LIB_NAME=lua LUA_LINK=static cargo build

mlua also supports vendored Lua/LuaJIT using the auxiliary crates lua-src and luajit-src. Just enable the vendored feature and cargo will automatically build and link the specified Lua/LuaJIT version. This is the easiest way to get started with mlua.

Standalone mode

In standalone mode, mlua allows adding scripting support to your application with a gently configured Lua runtime to ensure safety and soundness.

Add to Cargo.toml:

[dependencies]
mlua = { version = "0.10", features = ["lua54", "vendored"] }

main.rs

use mlua::prelude::*;

fn main() -> LuaResult<()> {
    let lua = Lua::new();

    let map_table = lua.create_table()?;
    map_table.set(1, "one")?;
    map_table.set("two", 2)?;

    lua.globals().set("map_table", map_table)?;

    lua.load("for k,v in pairs(map_table) do print(k,v) end").exec()?;

    Ok(())
}

Module mode

In module mode, mlua allows creating a compiled Lua module that can be loaded from Lua code using require. In this case mlua uses an external Lua runtime which could lead to potential unsafety due to the unpredictability of the Lua environment and usage of libraries such as debug.

Example

Add to Cargo.toml:

[lib]
crate-type = ["cdylib"]

[dependencies]
mlua = { version = "0.10", features = ["lua54", "module"] }

lib.rs:

use mlua::prelude::*;

fn hello(_: &Lua, name: String) -> LuaResult<()> {
    println!("hello, {}!", name);
    Ok(())
}

#[mlua::lua_module]
fn my_module(lua: &Lua) -> LuaResult<LuaTable> {
    let exports = lua.create_table()?;
    exports.set("hello", lua.create_function(hello)?)?;
    Ok(exports)
}

And then (macOS example):

$ cargo rustc -- -C link-arg=-undefined -C link-arg=dynamic_lookup
$ ln -s ./target/debug/libmy_module.dylib ./my_module.so
$ lua5.4 -e 'require("my_module").hello("world")'
hello, world!

On macOS, you need to set additional linker arguments. One option is to compile with cargo rustc --release -- -C link-arg=-undefined -C link-arg=dynamic_lookup, the other is to create a .cargo/config.toml with the following content:

[target.x86_64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

[target.aarch64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

On Linux you can build modules normally with cargo build --release.

On Windows the target module will be linked with the lua5x.dll library (depending on your feature flags). Your main application should provide this library.

Module builds don't require Lua binaries or headers to be installed on the system.

Publishing to luarocks.org

There is a LuaRocks build backend for mlua modules: luarocks-build-rust-mlua.

Modules written in Rust and published to luarocks:

• decasify
• lua-ryaml
• tiktoken_core
• toml-edit
• typst-lua

Safety

One of mlua's goals is to provide a safe API between Rust and Lua. Every place where the Lua C API may trigger an error longjmp is protected by lua_pcall, and the user of the library is protected from directly interacting with unsafe things like the Lua stack. There is overhead associated with this safety.

Unfortunately, mlua does not provide absolute safety even without using unsafe . This library contains a huge amount of unsafe code. There are almost certainly bugs still lurking in this library! It is surprisingly, fiendishly difficult to use the Lua C API without the potential for unsafety.

Panic handling

mlua wraps panics that are generated inside Rust callbacks in a regular Lua error. Panics can then be resumed by returning or propagating the Lua error to Rust code.

For example:

let lua = Lua::new();
let f = lua.create_function(|_, ()| -> LuaResult<()> {
    panic!("test panic");
})?;
lua.globals().set("rust_func", f)?;

let _ = lua.load(r#"
    local status, err = pcall(rust_func)
    print(err) -- prints: test panic
    error(err) -- propagate panic
"#).exec();

unreachable!()

Optionally, mlua can disable Rust panic catching in Lua via pcall/xpcall and automatically resume them across the Lua API boundary. This is controlled via LuaOptions and done by wrapping the Lua pcall/xpcall functions to prevent catching errors that are wrapped Rust panics.

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

Below is a list of mlua behaviors that should be considered a bug. If you encounter them, a bug report would be very welcome:

• If you can cause UB with mlua without typing the word "unsafe", this is a bug.

• If your program panics with a message that contains the string "mlua internal error", this is a bug.

• Lua C API errors are handled by longjmp. All instances where the Lua C API would otherwise longjmp over calling stack frames should be guarded against, except in internal callbacks where this is intentional. If you detect that mlua is triggering a longjmp over your Rust stack frames, 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. mlua instances are supposed to remain fully usable in the face of user generated panics. This guarantee does not extend to panics marked with "mlua internal error" simply because that is already indicative of a separate bug.

Sandboxing

Please check the Luau Sandboxing page if you are interested in running untrusted Lua scripts in a controlled environment.

mlua provides the Lua::sandbox method for enabling sandbox mode (Luau only).

License

This project is licensed under the MIT license.