Autocxx

This project is a tool for calling C++ from Rust in a heavily automated, but safe, fashion.

The intention is that it has all the fluent safety from cxx whilst generating interfaces automatically from existing C++ headers using a variant of bindgen. Think of autocxx as glue which plugs bindgen into cxx.

Intended eventual interface

It's intended that eventually this exposes a single procedural macro, something like this:

class Bob {
public:
    Bob(std::string name);
    ...
    void do_a_thing();
}

use autocxx::include_cxx;

include_cxx!(
    Header("base/bob.h"),
    Allow("Bob"),
)

let a = ffi::base::Bob::make_unique("hello".into());
a.do_a_thing();

The existing cxx facilities are used to allow safe ownership of C++ types from Rust; specifically things like std::unique_ptr and std::string - so the Rust code should not typically require use of unsafe code, unlike with normal bindgen bindings.

The macro and code generator will both need to know the include path to be passed to bindgen. At the moment, this is passed in via an environment variable, AUTOCXX_INC. See the demo/build.rs file for details.

How it works

It is effectively a two-stage procedural macro, which:

• First, runs bindgen to generate some bindings suitable for cxx::bridge
• Second, runs cxx::bridge to convert them to Rust code.

The same code can be passed through tools that generate .cc and .h bindings too:

• First, runs bindgen to generate some bindings suitable for cxx::bridge (in exactly the same way as above)
• Second, runs the codegen code from cxx to generate .cc and .h files

Current state of affairs

There is an example of this macro working within the demo directory. At the moment, it will work with only the very simplest functions (ints and voids only)!

The project also contains test code which does this end-to-end, for all sorts of C++ types and constructs which we eventually would like to support. They nearly all fail :)

Build environment

This crate is not yet on crates.io and currently depends on a hacked-up version of bindgen and a hacked-up version of cxx.

To try it out,

• Fetch the code using git.
• cargo update
• cargo test --all -- --test-threads=1.

This will fetch a specific fork of bindgen (see the Cargo.toml for the repo and branch) and use that as the dependency. The same applies to cxx.

At present, many of the tests fail, and thus the test run fails overall. Individual tests can be run, and some pass.

Because this uses bindgen, and bindgen may depend on the state of your system C++ headers, it is somewhat sensitive. The following known build problems exist:

• It requires Rust nightly due to #[proc_macro_span]
• Tests fail if they run in parallel, hence why you should use cargo test --all -- --test-threads=1. Looks perhaps like bindgen has global state.
• On Linux including any system header: bindgen generates pub type __uint32_t = ::std::os::raw::c_uint; which cxx can't cope with. This is just a matter of munging bindgen more. This currently stops the demo building on my Linux box, and prevents all but one test passing.
• On Linux using cargo 1.47 nightly: trybuild is unable to pull in dependencies from git repositories because it's in offline mode. Running cargo update first seems to solve this.
• (Fixed, I think: On Linux: tests fail: cc can't link because it can't find __gxx_personality_v0; resolved by adding another -lstdc++ argument at the end of the rustc linker line.)

Directory structure

• demo - a demo example
• engine - all the core code. Currently a Rust library, but we wouldn't want to support these APIs for external users, so maybe it needs to be a directory of code symlinked into all the other sub-crates. All the following three sub-crates are thin wrappers for part of this engine. This also contains the test code.
• macro - the procedural macro include_cxx as described above.
• gen/build - a library to be used from build.rs scripts to generate .cc and .h files from an include_cxx section.
• gen/cmd - a command-line tool which does the same. Except this isn't written yet.
• src - currently, nothing.

Next steps

Plans:

• Upstream the cxx change if possible.
• Fix a few of the annoying TODOs (the oddest one being in demo/build.rs)
• Then, start working on the bindgen fork to add support for more C++ types and see how far we can get through the test suite.

License and usage notes

This is not an officially supported Google product.