Function winit::platform_impl::platform::common::xkb::make_string_with

fn make_string_with<F>(scratch_buffer: &mut Vec<u8>, f: F) -> Option<SmolStr>
where
    F: FnMut(*mut c_char, usize) -> i32,

Shared logic for constructing a string with xkb_compose_state_get_utf8 and xkb_state_key_get_utf8.