Cross-compiling Toolchain environment for Raspberry Pi 3 B+ in Rust

Run docker build -t raspi3bp-rust ., then docker run -it --rm raspi3bp-rust. The compiled binary will be found at /root/hello/target/../debug/hello in the container. Copy it to an raspi3b+ and run it. It should say "Hello, world!".

This is based on the instructions found in the rust-cross project.