Function arg_types_specialize_equal 

pub fn arg_types_specialize_equal(a: &[DataType], b: &[DataType]) -> bool

Equality for monomorphization and return-type cache keys.

Identical to the loose type == except that function-typed arguments compare by exact Fn id, so each distinct function passed to a higher-order function keys its own specialization. Function references are always top-level arguments (a function is passed directly, never nested inside an array or map), so only the top-level Fn case needs the stricter rule.