[swift-evolution] Add ability to validate collection indices

[Haravikk]

> On 18 Dec 2016, at 11:56, Rien <Rien at balancingrock.nl> wrote:
> 
> I should clarify:
> 
> “works only part of the time” was meant to say that it does not cover all cases. Sometimes a warning will be given and sometimes not. When referring to time I was thinking of the programmer, not of compilation or execution time.
> 
> I think that “partially detectable” features increase the cognitive workload rather than decreasing it. If that is the case then I would rather do without.
> 
> Regards,
> Rien

Again I'd compare it more to non-escaping closures; most of the time they work just fine and you don't run into any trouble, but occasionally you want to do something that is actually non-escaping but that the compiler can't verify, which is why we have a mechanism to work around it.

Designed properly this should be fine, as it's not as if many programmers habitually scatter their indices to the four winds; in the vast majority of cases indices need to be kept somewhere close to what they refer to, otherwise they're no use at all. Thing of it like a type returning a value with the caveat "keep it where I can see it", like when I lend someone a pen; if you want to take it somewhere else you do so on the caveat that something terrible may happen to you so you best know what you're doing 😏

Anyway, my point was to just raise it as a possible solution to this that hadn't been discussed here yet; I never really made a formal proposal for it so if someone else would like to that'd be great, otherwise I'll try to at some point (alongside updating some of my other unfinished proposals).