pub fn declare_module(text: &str, decl: &str) -> Option<String>Expand description
Add a module declaration (mod foo; / pub mod foo;) to a file’s module
list, placed before the first existing declaration so the list stays
readable at the top of the file.
Returns None when the declaration is already there (idempotent — the
second generator run is a no-op) or when the file declares no modules at
all and there’s no obvious place to put one. The caller reports it as a
manual step rather than guessing.