cargo-xbuild 0.6.4

Automatically cross-compiles the sysroot crates core, compiler_builtins, and alloc.


Cargo-xbuild is a wrapper for cargo build, which cross compiles the sysroot crates core, compiler_builtins, and alloc for custom targets. It is a simplified fork of xargo, which is in maintainance mode.

Alternative: The build-std feature of cargo

Cargo now has its own feature for cross compiling the sysroot: build-std. You can use it by passing -Z build-std=core,alloc to cargo build. Alternatively, you can specify the following in a .cargo/config.toml file:

build-std = ["core", "compiler_builtins", "alloc"]

The above requires at least Rust nightly 2020–07–15. With the above config in place, the normal cargo build command will now automatically cross-compile the specified sysroot crates.

The compiler may emit references to memset, memcpy, etc which are usually provided by the platform's libc but luckily compiler_builtins has a mem feature that will provide implementations of those functions. To enable that feature we can use the unstable cargo flag -Z build-std-features=compiler-builtins-mem or specify the following in a config.toml:

build-std = ["core", "compiler_builtins", "alloc"]
+build-std-features = ["compiler-builtins-mem"]

Note that using the compiler-builtins-mem requires at least Rust nightly 2020-09-30. For older versions you need to add a dependency on the rlibc crate to provide implementations of memset, memcpy, etc, which the compiler expects. Note that you need to add an extern crate rlibc statement in order for this to work (even in the 2018 edition of Rust). This is required to get cargo to link the otherwise unused crate.

Compared to cargo-xbuild, there are many advantages of using cargo's own feature:

  • the normal cargo {check, build, run, test} commands can be used
  • no external tool must be installed
  • less bugs and breakage because it is always up to date with rustc/cargo
  • faster compilation since the compiler can build the sysroot concurrently to the project crates
  • it might be stablized one day

So it is strongly recommended to try the build-std feature of cargo instead of using this crate.


  • The rust-src component, which you can install with rustup component add rust-src.

  • Rust and Cargo.

Installation of cargo-xbuild

In case you decide to use cargo-xbuild instead of cargo's build-std feature for some reason, you can install this crate through:

$ cargo install cargo-xbuild

Note: The latest version of cargo-xbuild supports all nightlies after 2020-07-30. If you are on an older nightly, you need to install version 0.5.35: cargo install cargo-xbuild --version 0.5.35.


Just use cargo xbuild instead of cargo build when cross-compiling for a custom target.

cargo xbuild --target your-target-name.json

Instead of the "can't find crate for core" error you would get with a plain cargo build, this crate cross-compiles the core, compiler_builtins, and alloc crates and then invokes cargo build with a modified sysroot. The sysroot is compiled in the target directory of your crate.

All additional arguments (e.g. --release or --verbose) are forwarded to cargo build.


To configure cargo-xbuild create a package.metadata.cargo-xbuild table in your Cargo.toml. The following options are available:

memcpy = true
sysroot_path = "target/sysroot"
panic_immediate_abort = false
  • The memcpy flag defines whether the mem feature of the compiler_builtins crate should be activated. Turning this flag off allows to specify own versions of the memcpy, memset etc. functions.
  • The sysroot_path flag specifies the directory where the sysroot should be placed.
  • The panic_immediate_abort flag specifies whether the panic_immediate_abort feature the of core crate should be defined.

Environment Variables

In addition to the above configuration keys, cargo-xbuild can be also configured through the following environment variables:

  • The XBUILD_SYSROOT_PATH variable can be used to specify where cargo-xbuild should place the generated sysroot. This variables takes precendence over the package.metadata.cargo-xbuild.sysroot_path configuration key.
  • When the XBUILD_KEEP_TEMP variable is set, the temporary directory used for compiling the sysroot is not deleted. This is useful for debugging. For convenience, cargo-xbuild also prints the directory name when the environment variable is set.

Dev channel

If you want to use a local Rust source instead of rust-src rustup component, you can set the XARGO_RUST_SRC environment variable.

# The source of the `core` crate must be in `$XARGO_RUST_SRC/core`
$ export XARGO_RUST_SRC=/path/to/rust/src

$ cargo xbuild --target msp430-none-elf.json

Using on Android

It's possible to run cargo-xbuild on your Android phone:

Install Termux and Nightly Rustc

  • Install termux
  • Install fish shell and set as default (optional): pkg install fish; chsh -s fish; fish
  • Install some basic tools: pkg install wget tar
  • Add the community repository by its-pointless: wget; bash
  • Install rust nightly: pkg install rustc cargo rustc-nightly
  • Prepend the nightly rustc path to your $PATH in order to use nightly (fish syntax): set -U fish_user_paths $PREFIX/opt/rust-nightly/bin/ $fish_user_paths
  • rustc --version should now return a nightly version

(Optional) Install Git and Clone your Repository

  • Install git: pkg install git
  • Clone a repository of your choice: git clone

Install Xbuild

  • Install cargo-xbuild: cargo install cargo-xbuild
  • Add the cargo bin directory to your $PATH (fish syntax): set -U fish_user_paths ~/.cargo/bin/ $fish_user_paths
  • Now cargo xbuild should be available.

It does not work yet because it needs access to the rust source code. By default it tries to use rustup for this, but we have no rustup support so we need a different way.

Providing the Rust Source Code

The Rust source code corresponding to our installed nightly is available in the its-pointless repository:

  • Download it: wget
  • Extract it: tar xf rust-src-nightly.tar.xz
  • Set the XARGO_RUST_SRC environment variable to tell cargo-xbuild the source path (fish syntax): set -Ux XARGO_RUST_SRC ~/rust-src-nightly/rust-src/lib/rustlib/src/rust/src

Now cargo-xbuild should no longer complain about a missing rust-src component. However it will throw an I/O error after building the sysroot. The problem is that the downloaded Rust source code has a different structure than the source provided by rustup. We can fix this by adding a symbolic link:

ln -s ~/../usr/opt/rust-nightly/bin ~/../usr/opt/rust-nightly/lib/rustlib/aarch64-linux-android/bin

Now cargo xbuild --target your-target.json should work!


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