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pub trait _StrExt {
    fn _is_char_boundary(&self, index: usize) -> bool;
}

impl _StrExt for str {
    #[inline]
    fn _is_char_boundary(&self, index: usize) -> bool {
        if index == self.len() {
            return true;
        }
        match self.as_bytes().get(index) {
            None => false,
            Some(&b) => b < 128 || b >= 192,
        }
    }
}