Function call_builtin 

pub fn call_builtin<S: ValueStore>(
    store: &mut S,
    sink: &mut dyn OutputSink,
    id: BuiltinId,
    args: &[Value],
) -> RuntimeResult<Value>

Builtins run inside the runtime (no separate FFI symbols); arity/type problems are terminating errors, never Error values (design §8).