Pre-RFC: Break with value in for/while loops

[withoutboats]

@dtolnay

Could you point out some of the downsides of this approach, having for and while evaluate to Option<T> if they contain break T; and evaluate to () it they contain break; or no break? From the perspective of reading and writing Rust code this seems perfectly natural to me, and it is not a breaking change because break T; is not currently allowed there. And in some ways it is consistent with break inside loop in that loop evaluates to T if it contains break T; and evaluates to () if it contains break; .

It's mainly just that its inconsistent and needs to be explained to everyone. It's not the same as loops evaluating to T or (), because break; is regarded as equivalent to break ();, just like return is. In order for it to be the same, normal loops would have to evaluate to Option<()>.