core::unreachable!

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

A utility macro for indicating 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.

Panics

This will always panic.

Examples

Match arms:

fn main() { #[allow(dead_code)] 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") } } }

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 main() { #[allow(dead_code)] 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!(); } }

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!();
}