#[unhandled] attribute for trait methods that should error at compile time

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@mathstuf

Filter<F, Repeat<T>> might be infinite depending on F's runtime behavior.

I don't actually know if this is bad though. In this case I would not mind an error forcing someone to write

if let Some(_) = it.next() {
    loop {}
}

or something equally explicit and obvious. Way more obvious than my_complicated_iter.count(). Really it's going to take a base iterator of std::iter::from_fn(something_mathy) to be a real counterexample.

@JarredAllen

Per the reference, this snippet is guaranteed to cause a compiler error if <u8 as Foo>::bar is ever called, though there aren't guarantees about compiler errors or not if it isn't called.

See Should we have another non-type never?

(wherein I'd rather we spelled it fn count(self) -> const ! {} but otherwise: agreed, this would be nice functionality)