Reserving "nobreak" as a keyword for Rust Edition 2021

[JelteF]

The reason I'm opening this issue is that there has been some activity on the old issues, regarding returning values with break from for and while loops:

1. Allow loops to return values other than () #961
2. Allow for and while loop to return value (without discussion of keyword names) #1767

Rereading those threads, the main issues why those were originally closed seem to be:

1. else being a confusing name for this block
2. not being able to add a new keyword that makes the meaning of the block clear
3. there was no experience with the loop version of the break <value> syntax

Number 1 is still valid (although some disagree). 3 is not valid anymore, this feature was stabilized more than 4 years ago in 2017. Number 2 seems much less of a problem these days, since we now have editions (which we didn't have when these original issues were opened).

That's why I'm proposing to reserve nobreak as a keyword in Rust Edition 2021. That way we at can add this feature in the future, using a clear keyword.

I think nobreak clearly shows the meaning of the block. It's the block that you enter when there was no break. Code would then look like this:

let found_six = for val in [1, 4, 3, 2] {
    if val == 6 {
        break true;
    }
} nobreak {
    false
}

[scottmcm]

I'm going to close this because based on #2441 (comment), lang has decided not to do speculative reservations of specific keywords any more.

The new plan is to generally reserve space with #3101, and use that reserved space as a transition plan (perhaps via something like #3098) to allow the feature to be used on the latest edition with slightly-inferior syntax until the keyword can be fully reserved in a future one, after the RFC for the whole feature has been accepted.

(Note that this is neither a statement in opposition nor in support of the feature. Just that the way forward is to RFC the whole feature, including the eventual keyword -- not to separate out the keyword first.)