[FIXED] What does returning "!" mean in Rust?

Issue

Recently I came across a function in Rust that returned ! instead of basic type, like this:

fn my_function() -> ! {
    // ...
}

What does it mean? I was unable to find piece of information about this in The Rust Book. What data does this function return with such indicator?

Solution

It means the function never returns (usually because it unconditionally panics or otherwise ends the program, or because it contains an infinite loop that prevents a return from ever happening).

The appendix describes it as:

! Always empty bottom type for diverging functions

where "diverging" means "never returns".

Answered By - ShadowRanger

Answer Checked By - Gilberto Lyons (PHPFixing Admin)