Rename `double_check` to `both`

[indo-dev-0]

invariant is misleading, but double_check is too verbose. After some thought, I think pre/post/both is cleaner and semantically honest.

[ErichDonGubler]

I'm open to being persuaded that this change might be worth it, but at this point, I don't see the value of switching to something shorter or "cleaner" justifying a breaking change here. I actually think that double_check does a good job of pointing people's mental models to the purpose of a double_check block -- meanwhile, both seems overall less clear to me.

I'll leave this ticket open for a while to hear a counterargument. However, be warned that I might be a tough person to sell this particular suggestion to -- this feels like bikeshedding to me, and I'd just as soon not spend my spare time doing that.

[indolering]

I agree that this is bikeshedding, but looks are important if we want to sell this particular bikeshed to Rust core and external projects. I can't justify dropping support for double_check, at least not anytime soon.

Having a shorter keyword for invariant becomes more important when you throw in modifiers to support conditionals (like debug) or distinguishing between runtime and compile time assertions. Consider this match-like syntax:

debug|release assert|expect pre|post|both (y) {
    y - 1 == x => "error message";
}

At that point, one might as well make release, expect, and both defaults that are never used directly. Then "both" would simply serve as a pedagogical tool.

Do you idle on IRC or Matrix? I have some other design ideas I think are worth chatting about : )

[ErichDonGubler]

@indolering: More than happy to chat on Matrix; my handle is @ErichDonGubler:matrix.org.

Do you happen to be the same user as the one behind the PR OP, incidentally?

[indolering]

Yup, one is my dev machine and the other is my personal account.

[indolering]

Matrix (or the UI at least) appears to be case-sensitive, ErichDonGubler:matrix.org wasn't found but erichdongubler:matrix.org exists but is non-responsive. I'm indolering:matrix.org....