Macro std::unreachable

macro_rules! unreachable {
    (  ) => { ... };
    (
$ msg : expr ) => { ... };
    (
$ msg : expr , ) => { ... };
    (
$ fmt : expr , $ ( $ arg : tt ) * ) => { ... };
}

Indicates unreachable code.

This is useful any time that the compiler can't determine that some code is unreachable. For example:

• Match arms with guard conditions.
• Loops that dynamically terminate.
• Iterators that dynamically terminate.

If the determination that the code is unreachable proves incorrect, the program immediately terminates with a panic!. The function unreachable_unchecked, which belongs to the std::hint module, informs the compiler to optimize the code out of the release version entirely.

Panics

This will always panic!

Examples

Match arms:

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!(), // compile error if commented out
        None              => println!("None")
    }
}

Iterators:

fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
    for i in 0.. {
        if 3*i < i { panic!("u32 overflow"); }
        if x < 3*i { return i-1; }
    }
    unreachable!();
}