Crubit: C++/Rust Bidirectional Interop Tool

NOTE: Crubit currently expects deep integration with the build system, and is difficult to deploy to environments dissimilar to Google's monorepo. We do not have our tooling set up to accept external contributions at this time.

Crubit is a bidirectional bindings generator for C++ and Rust, with the goal of integrating the C++ and Rust ecosystems. With Crubit, Rust can be used anywhere C++ can be, directly calling and being called by C++. Crubit does not require wrapping C++ or Rust libraries in "FFI-friendly", simplified APIs. Any C++ interface can be called or implemented by Rust code.

Current status: Crubit is aiming for an initial stable "MVP" version, comparable to bindgen and cbindgen. This will support basic datatypes like integers and pointers, (rust-movable) structs/unions/enums, and extern "C" functions.

Example

Consider the following C++ function:

extern "C" bool IsAbsPath(std::string_view path);

This function, if present in a header file which is processed by Crubit, becomes callable from Rust as if it were defined as:

pub fn IsAbsPath(path: std::string_view) -> bool {...}

There are some temporary restrictions on the API shape. For example, if path were a std::string, or a non-extern "C" function, it would not be callable from Rust directly via Crubit. (For example, std::string is not rust-movable.) These restrictions will be relaxed over time.

For actual copy-pastable examples on getting started with Crubit, see the examples/ subdirectory. examples/cpp includes code which actually calls C++ via Crubit, and a snapshot of what the generated interface looks like.

Building Crubit

$ apt install clang lld bazel
$ git clone [email protected]:google/crubit.git
$ cd crubit
$ bazel build --linkopt=-fuse-ld=/usr/bin/ld.lld //rs_bindings_from_cc:rs_bindings_from_cc_impl

Using a prebuilt LLVM tree

$ git clone https://github.com/llvm/llvm-project
$ cd llvm-project
$ CC=clang CXX=clang++ cmake -S llvm -B build -DLLVM_ENABLE_PROJECTS='clang' -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install
$ cmake --build build -j
$ # wait...
$ cmake --install build
$ cd ../crubit
$ LLVM_INSTALL_PATH=../llvm-project/install bazel build //rs_bindings_from_cc:rs_bindings_from_cc_impl

Consider retiring the "Build with Bazel provided LLVM installation" instructions

For several reasons:

The instructions as they stand, do not work. The commit hash declared in bazel/llvm.bzl is API incompatible with the crubit source code. See Instructions as written failure below.
If the commit hash is updated to something recent, the build still fails. See Failure with updated commit below.
Removing this option from the installation instructions will simplify them for users and contract the support surface.
The bazel build of LLVM doesn't seem actively maintained. The Failure with updated commit at first glance looks to be a defect in LLVM's bazel build.

Instructions as written failure

dsankel@AP-PW015KJM:~/code/crubit$ bazel build --linkopt=-fuse-ld=/usr/bin/ld.lld //rs_bindings_from_cc:rs_bindings_from_cc_impl
DEBUG: Rule 'llvm-raw' indicated that a canonical reproducible form can be obtained by modifying arguments shallow_since = "1661922120 +0800"
DEBUG: Repository llvm-raw instantiated at:
  /home/dsankel/code/crubit/WORKSPACE:127:36: in <toplevel>
  /home/dsankel/code/crubit/bazel/llvm.bzl:61:23: in llvm_loader_repository_dependencies
Repository rule new_git_repository defined at:
  /home/dsankel/.cache/bazel/_bazel_dsankel/1360c4342c51417fe5124e0a1427d9ea/external/bazel_tools/tools/build_defs/repo/git.bzl:186:37: in <toplevel>
INFO: Analyzed target //rs_bindings_from_cc:rs_bindings_from_cc_impl (121 packages loaded, 9510 targets configured).
INFO: Found 1 target...
ERROR: /home/dsankel/code/crubit/lifetime_annotations/BUILD:26:11: Compiling lifetime_annotations/type_lifetimes.cc failed: (Exit 1): clang-16 failed: error executing command /home/dsankel/code/llvm-project/install/bin/clang-16 -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -Wunused-but-set-parameter -Wno-free-nonheap-object -fcolor-diagnostics ... (remaining 88 arguments skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
lifetime_annotations/type_lifetimes.cc:742:40: error: cannot initialize a variable of type 'const clang::TemplateTypeParmDecl *' with an rvalue of type 'const TemplateTypeParmType *'
    const clang::TemplateTypeParmDecl* type_parm = targ->getReplacedParameter();
                                       ^           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lifetime_annotations/type_lifetimes.cc:779:48: error: cannot initialize a variable of type 'const clang::TemplateTypeParmDecl *' with an rvalue of type 'const TemplateTypeParmType *'
            const clang::TemplateTypeParmDecl* type_parm =
                                               ^
2 errors generated.
Target //rs_bindings_from_cc:rs_bindings_from_cc_impl failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 318.474s, Critical Path: 66.47s
INFO: 2279 processes: 1038 internal, 1241 linux-sandbox.
FAILED: Build did NOT complete successfully

Failure with updated commit

Using commit 163bb6d64e5f1220777c3ec2a8b58c0666a74d91.

dsankel@AP-PW015KJM:~/code/crubit$ bazel build --linkopt=-fuse-ld=/usr/bin/ld.lld //rs_bindings_from_cc:rs_bindings_from_cc_impl
DEBUG: Rule 'llvm-raw' indicated that a canonical reproducible form can be obtained by modifying arguments shallow_since = "1669057075 -0500"
DEBUG: Repository llvm-raw instantiated at:
  /home/dsankel/code/crubit/WORKSPACE:127:36: in <toplevel>
  /home/dsankel/code/crubit/bazel/llvm.bzl:61:23: in llvm_loader_repository_dependencies
Repository rule new_git_repository defined at:
  /home/dsankel/.cache/bazel/_bazel_dsankel/1360c4342c51417fe5124e0a1427d9ea/external/bazel_tools/tools/build_defs/repo/git.bzl:186:37: in <toplevel>
ERROR: /home/dsankel/code/crubit/rs_bindings_from_cc/BUILD:62:10: error loading package '@llvm-project//llvm': cannot load '@bazel_skylib//rules:expand_template.bzl': no such file and referenced by '//rs_bindings_from_cc:rs_bindings_from_cc_impl'
ERROR: Analysis of target '//rs_bindings_from_cc:rs_bindings_from_cc_impl' failed; build aborted: Analysis failed
INFO: Elapsed time: 98.704s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (50 packages loaded, 154 targets configured)
    currently loading: @llvm-project//llvm ... (2 packages)

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Crubit: C++/Rust Bidirectional Interop Tool

Example

Building Crubit

Using a prebuilt LLVM tree

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