Add min specialisation for RangeFrom

[varkor]

Calling min on RangeFrom currently causes an infinite loop. Although other methods such as max also result in an infinite loop, it is strictly incorrect in the case of min. Adding a specialisation fixes this.

Separated from #47180 because this technically changes behaviour; it’s not just an optimisation, so it’s a little different.

r? @alexcrichton

[alexcrichton]

@bors: r+

[bors]

📌 Commit 554fbc2 has been approved by alexcrichton

[ollie27]

If this is valid implementation of min then should Repeat also implement min, max etc. like this?

[varkor]

@ollie27 — I was waiting for this change to be approved before correcting the behavior of other infinite iterators, but you're right: I have a PR for Repeat and Cyclehere now.

[scottmcm]

This is an insta-stable behavior change, not just an optimization, so I really think it needs an FCP.

It results in min() and min_by_key(|x| x) doing different things, which feels really weird to me.

[sfackler]

I am also not totally on board with this change.

[alexcrichton]

@bors: r-

er sorry I didn't realize this was a behavior change!

@varkor mind detaling why you think this behavior change is ok? e.g. enabling this on this iterator?

[varkor]

@alexcrichton: Sorry, I should have made it clearer! It makes sense to have this discussion in one place, so maybe let's move it to #47370 where it's already going on.

[alexcrichton]

Sure yeah sounds good to me!

[varkor]

Better to close this one for now as per the discussions in #47370. Sorry for the confusion!