Hint Functions

Compiler hints และ optimization

unreachable_unchecked

fn main() {
    // unreachable_unchecked - UNSAFE optimization hint

    fn process(n: u32) -> u32 {
        match n {
            0 => 0,
            1 => 1,
            _ => n * 2,
        }
    }

    // Safe version
    fn fast_divide(a: u32, b: u32) -> u32 {
        if b == 0 {
            panic!("division by zero");
        }
        a / b
    }

    // Unsafe version - only when 100% certain
    unsafe fn fast_divide_unchecked(a: u32, b: u32) -> u32 {
        if b == 0 {
            std::hint::unreachable_unchecked();
        }
        a / b
    }

    println!("Safe: {}", fast_divide(10, 2));

    unsafe {
        println!("Unsafe: {}", fast_divide_unchecked(10, 2));
    }

    // When to use:
    // - Performance critical code
    // - When you can PROVE the branch is never taken
    // - After profiling shows it helps

    // DANGER: UB if reached!
    println!("Done safely!");
}