Function snake_case 

pub fn snake_case(s: &str) -> String

PascalCase → snake_case (ZKeyExpr → z_key_expr). Convert a PascalCase / camelCase identifier to snake_case (a convention-free helper, re-exported for consumers composing their own CbindgenBuilder::mangle_rust_type rules). Thin alias for the core spelling, which sum-variant leaf naming shares.