Debugging Scalac in five minutes flat.

0. Install idea-sbt

$ sbt

 > *sbtIdeaRepo at http://mpeltonen.github.com/maven/
 > *idea is com.github.mpeltonen sbt-idea-processor 0.3.0
 > update

debug-scalac.log

~/code/scratch/scalac-debug: head /Users/jason/sbt.boot.properties 
[scala]
  version: 2.7.7
  classifiers: sources

[app]
  org: org.scala-tools.sbt
  name: sbt
  version: read(sbt.version)
  class: sbt.xMain
  components: xsbti
 ~/code/scratch/scalac-debug: sbt
Project does not exist, create new project? (y/N/s) y
Name: scalac-debug
Organization: scalac-debug
Version [1.0]: 
Scala version [2.7.7]: 2.9.0-SNAPSHOT
sbt version [0.7.3]: 0.7.4    
Getting Scala 2.7.7 ...
downloading http://repo1.maven.org/maven2/org/scala-lang/scala-compiler/2.7.7/scala-compiler-2.7.7-sources.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-compiler;2.7.7!scala-compiler.jar(src) (5048ms)
downloading http://repo1.maven.org/maven2/org/scala-lang/scala-library/2.7.7/scala-library-2.7.7-sources.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-library;2.7.7!scala-library.jar(src) (4361ms)
:: retrieving :: org.scala-tools.sbt#boot-scala
	confs: [default]
	4 artifacts copied, 0 already retrieved (11409kB/181ms)
Getting org.scala-tools.sbt sbt_2.7.7 0.7.4 ...
:: retrieving :: org.scala-tools.sbt#boot-app
	confs: [default]
	15 artifacts copied, 0 already retrieved (4096kB/68ms)
[success] Successfully initialized directory structure.
Getting Scala 2.9.0-SNAPSHOT ...
downloading http://scala-tools.org/repo-snapshots/org/scala-lang/scala-compiler/2.9.0-SNAPSHOT/scala-compiler-2.9.0-20110310.025207-153-sources.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-compiler;2.9.0-SNAPSHOT!scala-compiler.jar(src) (7324ms)
downloading http://scala-tools.org/repo-snapshots/org/scala-lang/scala-compiler/2.9.0-SNAPSHOT/scala-compiler-2.9.0-20110310.025207-153.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-compiler;2.9.0-SNAPSHOT!scala-compiler.jar (53548ms)
downloading http://scala-tools.org/repo-snapshots/org/scala-lang/scala-library/2.9.0-SNAPSHOT/scala-library-2.9.0-20110310.025207-153.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-library;2.9.0-SNAPSHOT!scala-library.jar (44185ms)
downloading http://scala-tools.org/repo-snapshots/org/scala-lang/scala-library/2.9.0-SNAPSHOT/scala-library-2.9.0-20110310.025207-153-sources.jar ...
	[SUCCESSFUL ] org.scala-lang#scala-library;2.9.0-SNAPSHOT!scala-library.jar(src) (5632ms)
:: retrieving :: org.scala-tools.sbt#boot-scala
	confs: [default]
	4 artifacts copied, 0 already retrieved (22047kB/183ms)
[info] Building project scalac-debug 1.0 against Scala 2.9.0-SNAPSHOT
[info]    using sbt.DefaultProject with sbt 0.7.4 and Scala 2.7.7
> idea
[info] :: retrieving :: org.scala-tools.sbt#retrieve-processor [sync]
[info] 	confs: [default]
[info] 	4 artifacts copied, 0 already retrieved (76kB/20ms)
[info] Created /Users/jason/code/scratch/scalac-debug/.idea
[info] Created /Users/jason/code/scratch/scalac-debug/project/project.iml
[info] Created /Users/jason/code/scratch/scalac-debug/scalac-debug.iml
> [info] 
[info] Total session time: 154 s, completed Mar 10, 2011 11:06:26 AM
[success] Build completed successfully.

~/code/scratch/scalac-debug: mkdir src/script
~/code/scratch/scalac-debug: echo 'println("Hello, Debugger!")' > src/script/Script.scala
~/code/scratch/scalac-debug: type ij
 ij is aliased to `open -b com.jetbrains.intellij'
~/code/scratch/scalac-debug: ij .
  

• Set a breakpoint in the compiler
• Debug the script: http://twitpic.com/482xiq/full
• Wait for the breakpoint to hit: http://twitpic.com/482xxb/full

To debug a local copy of Scalac, File, Project Settings, Libraries, 'buildScala', detach jars, attach instead directories or jars from, say, scala/build/pack