Function ensure_rust 

pub fn ensure_rust(
    dir: &Path,
    members: &[&str],
) -> Result<Option<String>, String>

Makes dir a Rust environment when it is not one yet: a Cargo workspace whose members are the scripts’ crates (members, globs allowed), and rust-toolchain.toml pinning the compiler found now. The lock (Cargo.lock) is written by the first build. Returns the toolchain pinned.