Consider having a ? operator on calls (like Rust) for error propagation

[masak]

Last time I looked at Rust, they had a try! macro for this, but now they have a ? postfix operator; if "postfix" isn't stretching the terminology a bit here. (Note to self: check if it's actually implemented as a postfix ? in the parser, or whether it's the call syntax rule that has been extended. Either would work, but from slightly different directions in the grammar.)

Similar to #13, this operator affects the rest of the execution, but unlike that issue, it does it by returning early.

We don't have a Result type like Rust does; it's an open question how much such a type would require a static type checking mechanism.

[vendethiel]

I feel like this brings us back to the discussion on whether or not you should be able to use a return from a quasi; and that it should return from the function the quasi is injected into.

[masak]

Dammit! At first blush, you're right! It's here and now that my stubbornness on that issue is coming back to bite me...

I feel that, if I start turning over one stone here, I'll end up turning over them all (again) — which may or may not be worthwhile. So for the moment, let me just agree with you @vendethiel, with absolutely no further strings attached.

[masak]

I feel that, if I start turning over one stone here, I'll end up turning over them all (again) — which may or may not be worthwhile.

No, it seems I was already convinced in the past, as evidenced by #483. We just haven't Made It So yet. So all's fine, and return will return from whatever's in the mainline, not in the macro.