unreachable!() #

The unreachable!() macro in Rust is used to indicate a section of the code that should never be reached under normal circumstances. It’s useful in places where the compiler can’t determine that some code is unreachable.

If the execution of the program ever does reach a point marked with unreachable!(), the program will immediately panic and terminate. This is because unreachable!() is essentially a shorthand for panic!() with a specific message indicating that unreachable code was reached.

Here’s an example:

fn foo(x: Option<i32>) {
    match x {
        Some(n) if n >= 0 => println!("Some(Non-negative)"),
        Some(n) if n < 0 => println!("Some(Negative)"),
        Some(_) => unreachable!(),  // This line should never be reached
        None => println!("None"),
    }
}

In this example, if foo is called with an Option<i32> that matches Some(_) but doesn’t match either of the conditions n >= 0 or n < 0, the unreachable!() macro will cause the program to panic.

It’s important to note that if the unreachable!() macro is used incorrectly and the code does reach that point, it could lead to a program crash. Therefore, it should be used with caution and only when you’re certain that the code section cannot be reached.