Hello,

Thanks for writing the truncation code ^^

During testing of this library, I noticed that carefully crafted inputs containing control caracters can trigger an assertion. This only happens in in debug mode => not a runtime panic for release. The used character is End of Medium.

The assertion is located here

A minimal (failing) testcase is the following

#[cfg(test)]
mod tests {
  #[test]
  fn zero_width_control_char() {
      std::assert_eq!("\u{0019}".unicode_truncate(2), ("\u{0019}", 0));
  }
}

This happens because at said point (expanded for better readability)

• let (byte_index, new_width) = (1, 0);
  

• let result = "\u{0019}".get(..1).unwrap();
  

• let result = "\u{0019}";
  

• => thus the result.width() (=1) is larger than the new_width (=0).

This case happens as (self.len(), 0) is appended to the interator.
This makes the iterator (as intended by the doccomment) eager for trailing zero width characters.
=> The asserted invariant does not hold anymore.

A possible fix would be to change the assertion to

- debug_assert_eq!(result.width(), new_width);
+ debug_assert!(result.width() >= new_width);

I can provide a PR with this fix if you'd prefer this