UTF8 safe truncate for Rust strings

utf8_truncate.rs

fn utf8_truncate(input : &mut String, maxsize: usize) {
  let mut utf8_maxsize = input.len();
  if utf8_maxsize >= maxsize {
    { let mut char_iter = input.char_indices();
    while utf8_maxsize >= maxsize {
      utf8_maxsize = match char_iter.next_back() {
        Some((index, _)) => index,
        _ => 0
      };
    } } // Extra {} wrap to limit the immutable borrow of char_indices()
    input.truncate(utf8_maxsize);
  }
}

Thanks to aatch from IRC, for the minimal UTF8 smiley mutlilation example demonstrating why this is needed:

playbot: let mut s = String::from("☺");  s.truncate(2);
playbot: thread '<main>' panicked at 'assertion failed: 
             self.is_char_boundary(new_len)', ../src/libcollections/string.rs:530

While .truncate() and .len() work on raw bytes, UTF8-encoded characters often use several bytes, so blind truncating would break the encoding.

P.S. The irony that I couldn't use the original example's smiley char because GitHub refuses high-end Unicode:

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