support `-release` option

[ohze]

scalac has -release option
dotc should support this option too.

[He-Pin]

Well, so dotc will at least require Java 8 right?

[smarter]

Well, so dotc will at least require Java 8 right?

Yes.

[prolativ]

@smarter how should -release option relate to -target (which seems to be missing in scala2)?

[smarter]

-target does exist in scala 2: https://github.com/scala/scala/blob/0a4f6b47cbbbd7e3772d6842d8e049dbce839caa/src/compiler/scala/tools/nsc/settings/StandardScalaSettings.scala#L55

[prolativ]

Thanks, I was looking for it in another file. But still they look quite similar to me

[smarter]

-target determines the version number we set in the classfiles we emit, with javac, --release X implies --target X, but with scalac it looks like they're independent. I think either is fine but what javac does makes more sense here.

[prolativ]

Corresponding PR in scala2 repo: scala/scala#6362

[prolativ]

@smarter what should be the supported values for -release and -target actually? It looks like currently the accepted values in scala 2 are from 6 to 9 for -release and from 8 to 12 for -target but that's probably because someone forgot to update the values for -release while changing that for -target (at least it looks like this when I look at the history of commits) because they should be the same (unless there's something that I've fundamentally misunderstood about this issue).
But in scala 3 only jvm-1.8 and jvm-9 are allowed as -target (also note that scala 2 treats 8, jvm-8 and jvm-1.8 as equivalents while scala 3 doesn't seem to give this freedom). However internally we still seem to support older JVMs (see BCodeIdiomatic.scala)

  lazy val classfileVersion: Int = ctx.settings.target.value match {
    case "jvm-1.5"     => asm.Opcodes.V1_5
    case "jvm-1.6"     => asm.Opcodes.V1_6
    case "jvm-1.7"     => asm.Opcodes.V1_7
    case "jvm-1.8"     => asm.Opcodes.V1_8
    case "jvm-9"       => asm.Opcodes.V9
  }

[smarter]

We should support 8 up to the latest release supported by asm.Opcodes. Versions before 8 never worked with dotty even if some dead code might mention them.

[prolativ]

Should we also support alternatives naming schemes (8, jvm-8, jvm-1.8 etc.)?

[smarter]

Not worth it I think, let's just keep the version number