Calling Unsafe Functions

A function or method can be marked unsafe if it has extra preconditions you must uphold to avoid undefined behaviour:

  1. fn main() {
  2.     let emojis = "🗻∈🌏";
  4.     // Safe because the indices are in the correct order, within the bounds of
  5.     // the string slice, and lie on UTF-8 sequence boundaries.
  6.     unsafe {
  7.         println!("emoji: {}", emojis.get_unchecked(0..4));
  8.         println!("emoji: {}", emojis.get_unchecked(4..7));
  9.         println!("emoji: {}", emojis.get_unchecked(7..11));
  10.     }
  12.     println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..7) }));
  14.     // Not upholding the UTF-8 encoding requirement breaks memory safety!
  15.     // println!("emoji: {}", unsafe { emojis.get_unchecked(0..3) });
  16.     // println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..3) }));
  17. }
  19. fn count_chars(s: &str) -> usize {
  20.     s.chars().map(|_| 1).sum()
  21. }