Cross can be further customized by setting certain environment variables. In-depth documentation with examples can be found here.

• CROSS_CONTAINER_ENGINE: The container engine to run cross in. Defaults to docker then podman, whichever is found first (example: docker, see the FAQ).
• NIX_STORE: The directory for the Nix store (example: /nix/store).
• CROSS_CONTAINER_UID: Set the user identifier for the cross command (example: 1000).
• CROSS_CONTAINER_GID: Set the group identifier for the cross command (example: 1000).
• CROSS_CONTAINER_IN_CONTAINER: Inform cross that it is running inside a container (example: true, see the FAQ).
• CROSS_CONTAINER_OPTS: Additional arguments to provide to the container engine during $engine run (example: --env MYVAR=1 where engine=docker).
• CROSS_CONFIG: Specify the path to the cross config file (see Config File).
• CROSS_BUILD_OPTS: Space separated flags to add when building a custom image, i.e. --network=host
• CROSS_DEBUG: Print debugging information for cross.
• CROSS_COMPATIBILITY_VERSION: Use older cross behavior (example: 0.2.1).
• CROSS_CUSTOM_TOOLCHAIN: Specify that rustup is using a custom toolchain, and therefore should not try to add targets/install components. Useful with cargo-bisect-rustc.
• CROSS_REMOTE: Inform cross it is using a remote container engine, and use data volumes rather than local bind mounts. See Remote for more information using remote container engines.
• QEMU_STRACE: Get a backtrace of system calls from “foreign” (non x86_64) binaries when using cross run.
• CARGO_BUILD_TARGET: Sets the default target, similar to specifying --target.
• CROSS_BUILD_DOCKERFILE: Specify to provide a custom Docker image for all targets.
• CROSS_ROOTLESS_CONTAINER_ENGINE: Specify whether to container engine runs as root or is rootless. If set to auto or not provided, it assumes docker runs as root and all other container engines are rootless.
• CROSS_CONTAINER_USER_NAMESPACE: Custom the container user namespace. If set to none, user namespaces will be disabled. If not provided or set to auto, it will use the default namespace.
• CROSS_CUSTOM_TOOLCHAIN_COMPAT: A descriptive name for a custom toolchain so cross can convert it to a fully-qualified toolchain name.
• CROSS_CONTAINER_ENGINE_NO_BUILDKIT: The container engine does not have buildx command (or BuildKit support) when building custom images.
• CROSS_NO_WARNINGS: Set to 1 to panic on warnings from cross, before building the executables. Use 0 to disable this behaviour. The no warnings behaviour is implicitly enabled in CI pipelines.

All config file options can also be specified using environment variables. For example, setting CROSS_TARGET_AARCH64_UNKNOWN_LINUX_GNU_DOCKERFILE=foo is identical to setting target.aarch64-unknown-linux-gnu.dockerfile = foo.

Environment-Variable passthrough

By default, cross does not pass most environment variables into the build environment from the calling shell. This is chosen as a safe default as most use cases will not want the calling environment leaking into the inner execution environment. There are, however, some notable exceptions: most environment variables cross or cargo reads are passed through automatically to the build environment. The major exceptions are variables that are set by cross or conflict with our build environment, including:

• CARGO_HOME
• CARGO_TARGET_DIR
• CARGO_BUILD_TARGET_DIR
• CARGO_BUILD_RUSTC
• CARGO_BUILD_RUSTC_WRAPPER
• CARGO_BUILD_RUSTC_WORKSPACE_WRAPPER
• CARGO_BUILD_RUSTDOC
• CROSS_RUNNER
• CROSS_RUSTC_MAJOR_VERSION
• CROSS_RUSTC_MINOR_VERSION
• CROSS_RUSTC_PATCH_VERSION

Otherwise, any environment variables that start with CARGO_ or CROSS_, and a few others, will be available in the build environment. For example, RUSTFLAGS and CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS will both be automatically available in the build environment.

In the instances that you do want to pass through additional environment variables, this can be done via build.env.passthrough in your Cross.toml:

[build.env]
passthrough = [
    "RUST_BACKTRACE",
    "RUST_LOG",
    "TRAVIS",
]

To pass variables through for one target but not others, you can use this syntax instead:

[target.aarch64-unknown-linux-gnu.env]
passthrough = [
    "RUST_DEBUG",
]