Cross Compiling: libcore

If you get an error: can't find crate for 'core', you’re probably compiling for a different target (e.g. you’re passing the target option to cargo build). Now the compiler complains that it can’t find the core library. This document gives a quick overview how to fix this problem. For more details, see the rust-cross project.

🔗Libcore

The core library is a dependency-free library that is added implicitly when using #![no_std]. It provides basic standard library features like Option or Iterator. The core library is installed together with the rust compiler (just like the std library). But the installed libcore is specific to your architecture. If you aren’t working on x86_64 Linux and pass ‑‑target x86_64‑unknown‑linux‑gnu to cargo, it can’t find a x86_64 libcore. To fix this, you can either use rustup or xargo.

🔗rustup

Thanks to rustup, cross-compilation for official target triples is pretty easy today: Just execute rustup target add x86_64-unknown-linux-gnu.

🔗xargo

If you’re using a custom target specification, the rustup method doesn’t work. Instead, you can use xargo. Xargo is a wrapper for cargo that eases cross compilation. We can install it by executing:

cargo install xargo

If the installation fails, make sure that you have cmake and the OpenSSL headers installed. For more details, see the xargo’s dependency section.

Xargo is “a drop-in replacement for cargo”, so every cargo command also works with xargo. You can do e.g. xargo --help, xargo clean, or xargo doc. However, the build command gains additional functionality: xargo build will automatically cross compile the core library (and a few other libraries such as alloc) when compiling for custom targets.

So if your custom target file is named your-cool-target.json, you can compile your code using xargo through xargo build --target your-cool-target (note the omitted extension).