Function collect_rs_files_from_module_tree 

fn collect_rs_files_from_module_tree(
    src_dir: &Path,
    out: &mut Vec<PathBuf>,
) -> Result<(), String>

Collect Rust source files by walking the module tree from lib.rs, following mod child; declarations and respecting #[cfg(feature = "...")] gates via CARGO_FEATURE_* environment variables.