Mixing Rust into an existing C shared library using Meson

Many people are interested in adding Rust to their existing projects for additional safety. For example it would be convenient to use Rust for individual high-risk things like string parsing while leaving the other bits as they are. For shared libraries you'd need to be able to do this while preserving the external plain C API and ABI. Most Rust compilation is done with Cargo, but it is not particularly suited to this task due to two things.

1. Integrating Cargo into an existing project's build system is painful, because Cargo wants to dominate the entire build process. It does not cooperate with these kind of build setups particularly well.
2. Using any Cargo dependency brings in tens or hundreds of dependency crates including five different command line parsers, test frameworks and other deps that you don't care about and don't need but which take forever to compile.

It should be noted that the latter is not strictly Cargo's fault. It is possible to use it standalone without external deps. However what seems to happen in practice is all Cargo projects experience a dependency explosion sooner or later. Thus it would seem like there should be a less invasive way to merge Rust into an existing code base. Fortunately with Meson, there is.

The sample project

To see how this can be done, we created a simple standalone C project for adding numbers. The full source code can be found in this repository. The library consists of three functions:

adder* adder_create(int number);

int adder_add(adder *a, int number);

void adder_destroy(adder*);

To add the numbers 2 and 4 together, you'd do this:

adder *two_adder = adder_create(2);

int six = adder_add(two_adder, 4);

adder_destroy(two_adder);

As adding numbers is highly dangerous, we want to implement the adder_add function in Rust and leave the other functions untouched. The implementation in all its simplicity is the following:

#[repr(C)]

pub struct Adder {

  pub number: i32

}

#[no_mangle]

pub extern fn adder_add(a: &Adder, number: i32) -> i32 {

    return a.number + number;

}

The build setup

Meson has native support for building Rust. It does not require Cargo or any other tool, it invokes rustc directly. In this particular case we need to build the Rust code as a staticlib.

rl = static_library('radder', 'adder.rs',

                    rust_crate_type: 'staticlib')

In theory all you'd need to do, then, is to link this library into the main shared library, remove the adder_add implementation from the C side and you'd be done. Unfortunately it's not that simple. Because nothing in the existing code calls this function, the linker will look at it, see that it is unused and throw it away.

The common approach in these cases is to use link_whole instead of plain linking. This does not work, because rustc adds its own metadata files inside the static library. The system linker does not know how to handle those and will exit with an error. Fortunately there is a way to make this work. You can specify additional undefined symbol names to the linker. This makes it behave as if something in the existing code had called adder_add, and grabs the implementation from the static library. This can be done with an additional kwarg to the shared_library call.

link_args: '-Wl,-u,adder_add'

With this the goal has been reached: one function implementation is done with Rust while preserving both the API and the ABI and the test suite passes as well. The resulting shared library file is about 1 kilobyte bigger than the plain C one (though if you build without optimizations enabled, it is a whopping 14 megabytes bigger).