呼叫不安全的函式

如果函式或方法具有額外先決條件，而您必須遵循這些條件才能避免未定義的行為，那麼就可以將該函式或方法標示為 unsafe：

fn main() {
    let emojis = "🗻∈🌏";

    // Safe because the indices are in the correct order, within the bounds of
    // the string slice, and lie on UTF-8 sequence boundaries.
    unsafe {
        println!("emoji: {}", emojis.get_unchecked(0..4));
        println!("emoji: {}", emojis.get_unchecked(4..7));
        println!("emoji: {}", emojis.get_unchecked(7..11));
    }

    println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..7) }));

    // Not upholding the UTF-8 encoding requirement breaks memory safety!
    // println!("emoji: {}", unsafe { emojis.get_unchecked(0..3) });
    // println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..3) }));
}

fn count_chars(s: &str) -> usize {
    s.chars().map(|_| 1).sum()
}