Change 3.1 to future

[odersky]

It's now

-source future
-source future-migration

import language.future
import language.`future-migration`

instead of

-source 3.1
-source 3.1-migration

import language.`3.1`
import language.`3.1-migration`

[som-snytt]

FTR, scala 2 moved away from -Xfuture to more specific -Xsource.

scala/scala#7328

I'm reminded there was an experiment with import language.future.X.

I had got used to -strict then -source 3.1. I agree with -source future to mean "our best understanding of where we're going, irrespective of when features land (or ship) in fact."

Scala 2 had suffered from the conflation of future and experimental, where "experimental" is "the future that never arrives", or, possibly, "the status quo."

[odersky]

@som-snytt We will keep 3.x for versions that we know (i.e. all past versions and the next one once planning for it is complete), and future for all things that are planned to be shipped yet don't have an assigned version yet.