Is there a standard Empty Type?

I was reading the nomicon and the paragraph regarding Empty Types caught my eye:

The gist of it is that you can use an Empty Type to statically guarantee that a returned Result is never an error.

Is there a standard Empty Type like Void in the nomicon example?

I have some infallible functions that are required to return Result where such a type would be helpful.

I think you may want the soon-to-be-stable ! type?

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To add to that, the nightly documentation already has some explanations of ! (or "never" type).


I see. I didn't know you could use ! yourself. I thought it was a compiler internals thing :slight_smile:

It was/is, but it's being able to be used by regular Rust programs soon :slight_smile:

In older nightlies, the example given would compile with #![feature(never_type)], but that was changed to #![feature(exhaustive_patterns)] as ! emerges from the night.

fn main() {
    enum Void {}
    let res: Result<u32, Void> = Ok(0);
    // Err doesn't exist anymore, so Ok is actually irrefutable.
    let Ok(num) = res;

Relevant comment in the stabilization RFC as never is stabilized but exhaustive patterns are not:

So in stable rust you'll need a match arm Err(never) => never, which I suppose is the idiomatic name for "never doesn't actually have a value, and its type is ! which will coerce to any type".