Zig Makes Rust Cross-compilation Just Work

2021-05-22, Max Hollmann

This post is a spiritual successor to Loris Cro's Go cross-compilation.

The encounter

During a recent stage 2 meeting, Jakub Konka wanted to demo his progress on WASM support, but ran into problems with screen-sharing on Linux. After switching to his Mac to run it there, he found out that Wasmtime (a WASM runtime) doesn't ship aarch64-macos pre-built binaries.

But Wasmtime is an open-source Rust project, I should be able to clone the repo and build it for Jakub myself. The problem? I don't have an M1 Mac on hand. I will have to cross-compile.

The idea

Rust has built-in cross-compilation support. All you have to do is install the target's toolchain with rustup

rustup target add aarch64-apple-darwin

and build your program with --target.

cargo build --target aarch64-apple-darwin

The problems

The C dependencies

warning: cc: error: unrecognized command-line option `-arch`
error: failed to run custom build command for `wasmtime-fiber`
Caused by:
  ...
  running "cc" ...

Some Rust crates depend on C code, but the Rust toolchain does not include a C compiler. This is the same problem Loris encountered with CGO and, luckily for us, it has the same solution. Create a shell script which wraps Zig¹

#!/bin/sh
/path-to-zig/zig cc -target aarch64-macos $@

and add it to env variables as CC when invoking cargo

CC="/path-to-wrapper/zcc" cargo build --target aarch64-apple-darwin

The linker

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" ...

We're now successfully compiling Wasmtime's objects, but we're failing at the last step - linking the final binary.

The Rust toolchain includes the Rust standard library and a compiler to produce object files for the target, but it does not include a linker to link them into the final executable. Furthermore, the cargo documentation instructs us to provide it with a target linker in .cargo/config.toml. However, we can see that cargo is attempting to invoke cc like it did before with wasmtime-fiber. Can we reuse the same wrapper we used before?

Yes².

[target.aarch64-apple-darwin]
linker = "/path-to-wrapper/zcc"

The result

Success! This is a WASM "Hello, World!" example running on Wasmtime on Jakub's M1, but the Wasmtime binary was compiled inside WSL on my x86_64 PC³. Pretty wild.