Function write_cargo_project 

pub fn write_cargo_project(
    rust_code: &str,
    dependencies: &[CrateDependency],
    output_dir: &Path,
) -> Result<(), CompileError>

Write a generated program as a runnable cargo project: src/main.rs, Cargo.toml (runtime path-deps + user ## Requires), .cargo/config.toml, and the staged runtime crates. Shared by compile_to_dir and rustc_check.