Seamless C support

Motivation

Using C from Rust should be as easy as using C from C++: completely seamless, as though it's just another module of code. You should be able to drop Rust code into a C project and start compiling and using it in minutes.

The status quo

Today, people who want to use C and Rust together in a project have to put substantial work into infrastructure or manual bindings. Whether by creating build system infrastructure to invoke bindgen/cbindgen (and requiring the installation of those tools), or manually writing C bindings in Rust, projects cannot simply drop Rust code into a C program or C code into a Rust program. This creates a high bar for adopting or experimenting with Rust, and makes it more difficult to provide Rust bindings for a C library.

By contrast, dropping C++ code into a C project or C code into a C++ project is trivial. The same compiler understands both C and C++, and allows compiling both together or separately. The developer does not need to duplicate declarations for the two languages, and can freely call between functions in both languages.

C and C++ are still not the same language. They have different idioms and common types, and a C interface may not be the most ergonomic to use from C++. Using C++ from C involves treating the C as C++, such that it no longer works with a C compiler that has no C++ support. But nonetheless, C++ and C integrate extremely well, and C++ is currently the easiest language to integrate into an established C project.

This is the level of integration we should aspire to for Rust and C.

The next six months

To provide seamless integration between Rust and C, we need a single compiler to understand both Rust and C. Thus, the first step will be to integrate a C preprocessor and compiler frontend into the Rust compiler. For at least the initial experimentation, we could integrate components from LLVM, taking inspiration from zig cc. (In the future, we can consider other alternatives, including a native Rust implementation. We could also consider components from c2rust or similar.)

We can either generate MIR directly from C (which would be experimental and incomplete but integrate better with the compiler), or bypass MIR and generate LLVM bytecode (which would be simpler but less well integrated).

This first step would provide substantial benefits already: a C compiler that's always available on any system with Rust installed, that generates code for any supported Rust target, and that always supports cross-language optimization.

We can further improve support for calling C from Rust. We can support "importing" C header files, to permit using this support to call external libraries, and to support inline functions.

The "shiny future" we are working towards

Once C support is integrated, we can generate type information for C functions as if they were unsafe Rust functions, and then support treating the C code as a Rust module, adding the ability to import and call C functions from Rust. This would not necessarily even require header files, making it even simpler to use C from Rust. The initial support can be incomplete, supporting the subset of C that has reasonable semantics in Rust.

We will also want to add C features that are missing in Rust, to allow Rust to call any supported C code.

Once we have a C compiler integrated into Rust, we can incrementally add C extensions to support using Rust from C. For instance:

• Support importing Rust modules and calling extern "C" functions from them, without requiring a C header file.
• Support using :: for scoping names.
• Support simple Rust types (e.g. Option and Result).
• Support calling Rust methods on objects.
• Allow annotating C functions with Rust-enhanced type signatures, such as marking them as safe, using Rust references for pointer parameters, or providing simple lifetime information.

We can support mixing Rust and C in a source file, to simplify incremental porting even further.

To provide simpler integration into C build systems, we can accept a C-compiler-compatible command line (CFLAGS), and apply that to the C code we process.

We can also provide a CLI entry point that's sufficiently command-line compatible to allow using it as CC in a C project.

Design axioms

• C code should feel like just another Rust module. Integrating C code into a Rust project, or Rust code into a C project, should be trivial; it should be just as easy as integrating C with C++.

• This is not primarily about providing safe bindings. This project will primarily make it much easier to access C bindings as unsafe interfaces. There will still be value in wrapping these unsafe C interfaces with safer Rust interfaces.

• Calling C from Rust should not require writing duplicate information in Rust that's already present in a C header or source file.

• Integrating C with Rust should not require third-party tools.

• Compiling C code should not require substantially changing the information normally passed to a C compiler (e.g. compiler arguments).

Ownership and other resources

Owner: TODO

Support needed from the project

• Lang team:
  • Design meetings to discuss design changes
  • RFC reviews
• Compiler team:
  • RFC review

Outputs and milestones

Outputs

The initial output will be a pair of RFCs: one for an experimental integration of a C compiler into rustc, and the other for minimal language features to take advantage of that.

Milestones

• Compiler RFC: Integrated C compiler
• Lang RFC: Rust language support for seamless C integration

Frequently asked questions