Add sbt incremental compilation support

To test this with sbt, see
https://github.com/lampepfl/dotty/wiki/Using-Dotty-with-sbt

The following flags are added:
 - -Yforce-sbt-phases: Run the phases used by sbt for incremental compilation
   (ExtractDependencies and ExtractAPI) even if the compiler is ran outside of
   sbt, for debugging.
 - -Ydump-sbt-inc: For every compiled foo.scala, output the API
   representation and dependencies used for sbt incremental compilation
   in foo.inc, implies -Yforce-sbt-phases.

This commit introduces two new phases which do not transform trees:
- `ExtractDependencies` which extracts the dependency information of the current
  compilation unit and sends it to sbt via callbacks
- `ExtractAPI` which creates a representation of the API of the current compilation
  unit and sends it to sbt via callbacks

Briefly, when a file changes sbt will recompile it, if its API has
changed (determined by what `ExtractAPI` sent) then sbt will determine
which reverse-dependencies (determined by what `ExtractDependencies`
sent) of the API have to be recompiled depending on what changed.
See http://www.scala-sbt.org/0.13/docs/Understanding-Recompilation.html for
more information on how sbt incremental compilation works.

This phase was originally based on
https://github.com/adriaanm/scala/tree/sbt-api-consolidate/src/compiler/scala/tools/sbt
which attempts to integrate the sbt phases into scalac (and is itself based
on https://github.com/sbt/sbt/tree/0.13/compile/interface/src/main/scala/xsbt),
but it has been heavily refactored and adapted to Dotty. The main
functional differences are:
- ExtractDependencies runs right after Frontend (so that we don't lose
  dependency informations because of the simplifications done by PostTyper),
  but ExtractAPI runs right after PostTyper (so that SuperAccessors are
  part of the API).
- `ExtractAPI` only extract types as they are defined and never "as seen
  from" some some specific prefix, see its documentation for more details.
- `ExtractDependenciesTraverser` and `ExtractUsedNames` have been fused into
  one tree traversal in `ExtractDependenciesCollector`.

TODO: Try to run these phases in parallel with the rest of the compiler
pipeline since they're independent (except for the sbt callbacks in `GenBCode`) ?

1 files changed, 10 insertions, 1 deletions

diff --git a/AUTHORS.md b/AUTHORS.md
index 4d54d2ee3..0ba845dc4 100644
--- a/AUTHORS.md
+++ b/AUTHORS.md
@@ -42,7 +42,7 @@ The majority of the dotty codebase is new code, with the exception of the compon
 > The utilities package is a mix of new and adapted components. The files in  [scala/scala](https://github.com/scala/scala) were originally authored by many people,
 > including Paul Phillips, Martin Odersky, Sean McDirmid, and others.
   
-`dotty.tools.io`   
+`dotty.tools.io`
 
 > The I/O support library was adapted from current Scala compiler. Original authors were Paul Phillips and others.
 
@@ -52,3 +52,12 @@ The majority of the dotty codebase is new code, with the exception of the compon
 > [scala/scala](https://github.com/scala/scala). It has been reworked to fit
 > the needs of dotty. Original authors include: Adrian Moors, Lukas Rytz,
 > Grzegorz Kossakowski, Paul Phillips
+
+`dotty.tools.dotc.sbt`
+
+> The sbt compiler phases are based on
+> https://github.com/adriaanm/scala/tree/sbt-api-consolidate/src/compiler/scala/tools/sbt
+> which attempts to integrate the sbt phases into scalac and is itself based on
+> the [compiler bridge in sbt 0.13](https://github.com/sbt/sbt/tree/0.13/compile/interface/src/main/scala/xsbt),
+> but has been heavily adapted and refactored.
+> Original authors were Mark Harrah, Grzegorz Kossakowski, Martin Duhemm, Adriaan Moors and others.