Overview of Crubit’s C++/Rust bindings

Motivation for automating generation of FFI bindings

To call a C++ function from Rust, one can manually declare the foreign function interface (FFI) as follows:

#![allow(unused)]
fn main() {
extern "C" {
    fn cpp_multiplication_function(x: i32, y: i32) -> i64;
}
}

This works, but has some disadvantages:

• The function signature in Rust’s extern declaration needs to match the signature of the actual C++ function (e.g. we can’t say x: i16 when the C++ parameter is int32_t x - doing this would lead to Undefined Behavior). Keeping those function signatures in sync can be error prone.
• Rust does not support the C++ calling convention or any C++ features not present in C - this means that bindings for some functions may require an extra “thunk” (an extra function that exposes C ABI and forwards calls to the target function).
• It is possible to manually replicate in C++ the memory layout and ABI of #[repr(C)] and #[repr(transparent)] structs, but not the implementation-defined layout of #[repr(Rust)] structs.

Similar disadvantages exist when manually setting up FFI bindings for calling a Rust function from C++. Automating generation of FFI bindings with Crubit can help alleviate the disadvantages above.

C++ bindings for Rust APIs

Crubit can take a Rust crate and create a C++ library which exposes Rust functions and types to C++ callers.

Let’s walk through the examples/rust/function example. In the example, Bazel is instructed to provide C++ bindings for a Rust library:

load(
    "//bazel:cpp_api_from_rust.bzl",
    "cpp_api_from_rust",
)

rust_library(
    name = "example_crate",
    srcs = ["example.rs"],
)

# This declares an "example_crate_cc_api" target that provides Crubit-generated
# C++ bindings for the Rust crate behind the `":example_crate"` target.
cpp_api_from_rust(
    name = "example_crate_cc_api",
    crate = ":example_crate",
)

With the above, C++ code (e.g. cc_binary, cc_library, cc_test, etc.) can just list the bindings in its deps:

cc_binary(
    name = "main",
    srcs = ["main.cc"],

    # Declaring a dependency on C++ bindings for calling into the Rust
    # `example` library:
    deps = [ ":example_crate_cc_api" ],
)

The scaffolding above will let C++ call into Rust as follows:

#![allow(unused)]
fn main() {
// example.rs:
pub fn add_two_integers(x: i32, y: i32) -> i32 { x + y }
}

// main.cc:

// The generated bindings are in a header at the same path as the
// `example_crate` rust_library, with a `.h` suffix.
#include "examples/rust/function/example_crate.h"

int main(int argc, char* argv[]) {
  // The generated bindings are in a namespace with the same name as the
  // target crate:
  int32_t sum = example_crate::add_two_integers(2, 2);

  // ...
}

If needed, one can find and inspect the generated bindings in the “Generated Files and Directories” section for that directory in Code Search or manually building the bindings using:

$ bazel build --config=crubit-genfiles \
    //examples/rust/function:example_crate
$ cat "$(bazel info bazel-bin)/examples/rust/function/example_crate.h"

Inspecting the generated file may be useful to look at comments that Crubit leaves behind when it is unable to generate bindings for a given Rust API - e.g.:

// Error generating bindings for `reinterpret_cast` defined at path/lib.rs;l=123:
// Error formatting function name:
// `reinterpret_cast` is a C++ reserved keyword and can't be used as a C++ identifier.

TODO: Provide integration with Chromium/GN and provide GN-oriented examples.

Highlights

For a comprehensive description of the generated FFI bindings, please see the “FFI Bindings Reference”. The list below highlights a few aspects of the FFI bindings that Crubit generates:

• Crubit can generate FFI bindings for C++ and Rust functions even if they don’t follow the extern "C" calling convention.
• Crubit can replicate the memory layout of most C++ and Rust structs. This includes non-#[repr(C)] Rust structs and inheritance hierarchy of C++ classes.
• Crubit can generate bindings for APIs that use types from other libraries. For example, if a function provided by foo library returns a struct defined by a separate bar library, then foo bindings will automatically reuse bindings generated for bar.
• Crubit supports all the usual ways to pass function parameters or return values. In particular, structs can be passed by reference or by value. Values from either language can be moved across stack and heap memory on either side of the FFI boundary. This includes C++ structs with non-trivial, user-defined move constructors.
• Crubit supports generating bindings for specific instantiations of arbitrary C++ class templates. In particular, bindings for std::string_view from C++ standard library can be generated even though rs_bindings_from_cc has minimal knowledge about this specific type (except for instructing rs_bindings_from_cc to include From trait implementations from crubit/support/cc_std_impl/string_view.rs).

TODO: Help users make an informed decision when to use Crubit rather than other FFI tools. (This probably should become a separate sub-chapter of the overview. Some raw information can be currently found in the design.md doc.)