Function walk_src_max_width 

pub fn walk_src_max_width(dir: impl AsRef<Path>, strip_prefix: &str) -> usize

Walk dir recursively, find every .rs file, and return max(path_len_after_stripping(strip_prefix)) + 4.

Returns [FALLBACK_WIDTH] (40) if dir does not exist or contains no .rs files — safe to call unconditionally from a build.rs.