Macro core::unreachable 1.0.0[−][src]
macro_rules! unreachable {
    () => { ... };
    ($msg : expr $(,) ?) => { ... };
    ($fmt : expr, $($arg : tt) *) => { ... };
}Expand description
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 unsafe counterpart of this macro is the unreachable_unchecked function, which
will cause undefined behavior if the code is reached.
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!();
}