Keep export clause until first transform, use for incremental compilation

[bishabosha]

not sure what else should cause enough indirection for this to fail

now:

• Keep export clause trees until FirstTransform
• Add Export trees to TASTy format and QuoteContext reflection API
• Use Export trees to record wildcard export as dependencies by inheritance in incremental compilation.
• Restrict wildcard export from a package prefix

[smarter]

I've pushed a commit showing how things are broken.

[smarter]

To get this to work, ExtractDependencies will need to handle wildcard exports from a class in the same way it handles inheriting from that type: by recording a DependencyByInheritance, wildcard exports from a package can never be handled correctly and should be disallowed.

[bishabosha]

Thanks, right so the best way to break stuff is to add a new definition

[bishabosha]

I believe this is ready now

[smarter]

Can you clean the history of this PR to remove some of the back-and-forth ? I think having everything in one commit would be fine.

[smarter]

otherwise lgtm

[smarter]

Oh, one more thing: we should disallow wildcard export from a package because (unless I'm mistaken) we can't do incremental compilation for that for the same reason we can't do it for imports: sbt/zinc#226

[bishabosha]

@smarter do you mean just check in ExtractDependencies for if the prefix is a package?, or reject it with an error in typer e.g.

[smarter]

In typer yes

[bishabosha]

added a error when wildcard exports come from a package prefix

[smarter]

Otherwise LGTM, thanks for working on this!

[bishabosha]

@smarter I tested the following (commenting out the package prefix check) and confirmed that sbt/zinc#226 does prevent new members being seen:

// A.scala
export p1._
export p2._

class A {
  val b = new B()
}

// p1.scala
package p1

class B

// p2.scala
package p2

class Ignored // needed so that A.scala compiles

// If after p2 is compiled we add `class B` here, then `A.scala` is not recompiled
// class B