Eliminate self-symbols from trees & types

[DarkDimius]

Following discussion with Martin, it seems that self-symbols are not needed after typer.
If this is true, it makes sense to create a Mini-Phase&InfoTransformer that would eliminate self symbols both from trees and from types.
It it's false, it would be great to know how are they used.

[liufengyun]

I've tested a simple transform locally as follows:

class ElimSelf extends MiniPhaseTransform with InfoTransformer { thisTransformer =>
  override def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context) = tp match {
    case tp: ClassInfo =>
      tp.derivedClassInfo(selfInfo = NoType)
    case _ =>
      tp
  }

  override protected def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = !(sym is Module)

  override def transformTemplate(impl: Template)(implicit ctx: Context, info: TransformerInfo): Tree = {
    cpy.Template(impl)(self = EmptyValDef)
  }
}

Elimination of self in tree works well, but elimination of selfInfo in ClassInfo have some caveats:

• Can't eliminate self type of package class, it's assumed here.
• Can't eliminate self type of module class, it results in errors in backend -- backend uses self type to access the sourceModule (log).
• TreeChecker requires self type in order to correctly retype the program.

The first two constrains us to only eliminate self of non-modules classes, however the last one makes elimination of self in ClassInfo impossible.

[DarkDimius]

@liufengyun, nice!

Can't eliminate self type of package class, it's assumed here.

I didn't mean to eliminate self-types, only self-symbols. Self-types are needed for the reasons you've indicated.

[odersky]

Classinfos definitely need a selfinfo. The hypothesis is that, after, typer,

• it can be a type, no symbol is needed.
• we don't need a self definition in the trees

Cheers

• Martin

On Sun, May 1, 2016 at 8:17 PM, liu fengyun notifications@github.com
wrote:

I've tested a simple transform locally as follows:

class ElimSelf extends MiniPhaseTransform with InfoTransformer { thisTransformer =>
override def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context) = tp match {
case tp: ClassInfo =>
tp.derivedClassInfo(selfInfo = NoType)
case _ =>
tp
}

override protected def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = !(sym is Module)

override def transformTemplate(impl: Template)(implicit ctx: Context, info: TransformerInfo): Tree = {
cpy.Template(impl)(self = EmptyValDef)
}
}

Elimination of self in tree works well, but elimination of selfInfo in
ClassInfo have some caveats:

• Can't eliminate self type of package class, it's assumed here
  https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/core/Denotations.scala#L654
  .
• Can't eliminate self type of module class, it results in errors in
  backend -- backend uses self type to access the sourceModule (log
  https://gist.github.com/liufengyun/dd41d3b69a066e6b2d53c2f587712cb8).
• TreeChecker requires self type in order to correctly retype the
  program.

The first two constrains us to only eliminate self of non-modules
classes, however the last one makes elimination of self in ClassInfo
impossible.

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#1239 (comment)

Martin Odersky
EPFL and Lightbend