Document features conditional on source and target versions

[adriaanm]

-Xsource:2.13

• case classes must have a non-implicit parameter list (SI-10097 Error if no non-implicit case class param scala#5585)
• empty character literal (use '\'' for single quote) (SI-10133 Deprecate unescaped single quote char literal scala#5727)
• Only methods can be marked @elidable. (SI-10068 Only permit elidable methods scala#5539)
• Higher-kinded type variable unification (Higher-kinded type variable unification. scala#6069)
• eta-expansion is changing (TODO: document when changes are in M4)
• changes to the computation of the parts of the type of an implicit value (Include the parts of a compound/refinement type in implicit scope. scala#5867, Resolve implicit instances for abstract types, according to the spec. scala#6074)

[SethTisue]

this is for 2.12? where would such documentation go?

[eed3si9n]

Related sbt/sbt#3123

If there were command line entry point that lists out list of flags, -Xsource manuals, latest book recommendations by Scala team, I'd love to surface them as an sbt task scalaHelp.

[SethTisue]

we now have a place to put this stuff: scala/docs.scala-lang#1058

[SethTisue]

nowadays, this would be about -Xsource:3.0 instead

[som-snytt]

I don't know about documenting, but I meant to add 3 to the migration guide. I'm pretty sure it's underused, because otherwise people would be complaining about it. It has, for instance, "infix on the next line." I wish 2.13 were compiled with it.

[SethTisue]

and nowadays it's -Xsource:3 and people are actually using it prepare to migrate to 3, so it's reeeeeallly time now