Function root_binding_types 

pub fn root_binding_types(
    stmts: &[Stmt<'_>],
    interner: &Interner,
    registry: &TypeRegistry,
) -> Result<HashMap<Symbol, InferType>, LocatedTypeError>

The type of every binding the program’s ROOT declares, as the checker settled it once the whole program was read. Exact where TypeEnv is not: its variables are one flat map, so a function’s own x and the program’s x share an entry there.