Interoperability with C++

The Rust community offers multiple options for C++/Rust interop, with new tools being developed all the time. At the moment, Chromium uses a tool called CXX.

You describe your whole language boundary in an interface definition language (which looks a lot like Rust) and then CXX tools generate declarations for functions and types in both Rust and C++.

Overview diagram of cxx, showing that the same interface definition is used to create both C++ and Rust side code which then communicate via a lowest common denominator C API

See the CXX tutorial for a full example of using this.

Talk through the diagram. Explain that behind the scenes, this is doing just the same as you previously did. Point out that automating the process has the following benefits:

  • The tool guarantees that the C++ and Rust sides match (e.g. you get compile errors if the #[cxx::bridge] doesn't match the actual C++ or Rust definitions, but with out-of-sync manual bindings you'd get Undefined Behavior)
  • The tool automates generation of FFI thunks (small, C-ABI-compatible, free functions) for non-C features (e.g. enabling FFI calls into Rust or C++ methods; manual bindings would require authoring such top-level, free functions manually)
  • The tool and the library can handle a set of core types - for example:
    • &[T] can be passed across the FFI boundary, even though it doesn't guarantee any particular ABI or memory layout. With manual bindings std::span<T> / &[T] have to be manually destructured and rebuilt out of a pointer and length - this is error-prone given that each language represents empty slices slightly differently)
    • Smart pointers like std::unique_ptr<T>, std::shared_ptr<T>, and/or Box are natively supported. With manual bindings, one would have to pass C-ABI-compatible raw pointers, which would increase lifetime and memory-safety risks.
    • rust::String and CxxString types understand and maintain differences in string representation across the languages (e.g. rust::String::lossy can build a Rust string from non-UTF8 input and rust::String::c_str can NUL-terminate a string).