Enable cross-version support for Scala sources.

This allows attaching unmanaged sources on `per-ScalaVersion` basis.

Sources from `src/{main,test}/scala-<scalaBinaryVersion>` is collected
in addition to `src/{main,test}/scala` by default.

Note that the treatment of `scalaBinaryVersion` would vary depending on
`scalaVersion` pre-2.10 and 2.10 onwards.

For example:
- with `scalaVersion` `2.9.3`, the `scalaBinaryVersion` is set to `2.9.3`
  thus the files in `src/{main,test}/scala-2.9.3` as well
 `src/{main,test}/scala` would be available in the `sources` list.

- with `scalaVersion` `2.10.1`, the `scalaBinaryVersion` is set to `2.10`
  thus the files in `src/{main,test}/scala-2.10` as well
 `src/{main,test}/scala` would be available in the `sources` list.

main/src/main/scala/sbt/Defaults.scala

@@ -181,7 +181,7 @@ object Defaults extends BuildCommon {
     sourceManaged <<= configSrcSub(sourceManaged),
     scalaSource := sourceDirectory.value / "scala",
     javaSource := sourceDirectory.value / "java",
-    unmanagedSourceDirectories := Seq(scalaSource.value, javaSource.value),
+    unmanagedSourceDirectories := makeCrossSources(scalaSource.value, javaSource.value, scalaBinaryVersion.value, crossPaths.value),
     unmanagedSources <<= collectFiles(unmanagedSourceDirectories, includeFilter in unmanagedSources, excludeFilter in unmanagedSources),
     watchSources in ConfigGlobal <++= unmanagedSources,
     managedSourceDirectories := Seq(sourceManaged.value),
@@ -241,6 +241,14 @@ object Defaults extends BuildCommon {
     derive(compilersSetting),
     derive(scalaBinaryVersion := binaryScalaVersion(scalaVersion.value))
   ))
+
+  def makeCrossSources(scalaSrcDir: File, javaSrcDir: File, sv: String, cross: Boolean): Seq[File] = {
+    if (cross)
+      Seq(scalaSrcDir.getParentFile / s"${scalaSrcDir.name}-$sv", scalaSrcDir, javaSrcDir)
+    else
+      Seq(scalaSrcDir, javaSrcDir)
+  }
+
   def makeCrossTarget(t: File, sv: String, sbtv: String, plugin: Boolean, cross: Boolean): File =
     {
       val scalaBase = if (cross) t / ("scala-" + sv) else t
@@ -948,7 +956,7 @@ object Defaults extends BuildCommon {
   @deprecated("Settings now split into AutoPlugins.", "0.13.2")
   lazy val projectBaseSettings: Seq[Setting[_]] = projectCore ++ runnerSettings ++ paths ++ baseClasspaths ++ baseTasks ++ compileBase ++ disableAggregation
 
-  // These are project level settings that MUST be on every project.  
+  // These are project level settings that MUST be on every project.
   lazy val coreDefaultSettings: Seq[Setting[_]] =
     projectCore ++ disableAggregation ++ Seq(
       // Missing but core settings
@@ -1111,8 +1119,8 @@ object Classpaths {
     projectResolver <<= projectResolverTask,
     projectDependencies <<= projectDependenciesTask,
     // TODO - Is this the appropriate split?  Ivy defines this simply as
-    //        just project + library, while the JVM plugin will define it as 
-    //        having the additional sbtPlugin + autoScala magikz. 
+    //        just project + library, while the JVM plugin will define it as
+    //        having the additional sbtPlugin + autoScala magikz.
     allDependencies := {
       projectDependencies.value ++ libraryDependencies.value
     },

main/src/main/scala/sbt/Keys.scala

@@ -106,7 +106,7 @@ object Keys {
   val cacheDirectory = SettingKey[File]("cache-directory", "Directory used for caching task data.", BMinusSetting)
   val cleanFiles = SettingKey[Seq[File]]("clean-files", "The files to recursively delete during a clean.", BSetting)
   val cleanKeepFiles = SettingKey[Seq[File]]("clean-keep-files", "Files to keep during a clean.", CSetting)
-  val crossPaths = SettingKey[Boolean]("cross-paths", "If true, enables cross paths, which distinguish output directories for cross-building.", ASetting)
+  val crossPaths = SettingKey[Boolean]("cross-paths", "If true, enables cross paths, which distinguish input and output directories for cross-building.", ASetting)
   val taskTemporaryDirectory = SettingKey[File]("task-temporary-directory", "Directory used for temporary files for tasks that is deleted after each task execution.", DSetting)
 
   // Generators

sbt/src/sbt-test/source-dependencies/cross-source/src/main/scala-2.10/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = s"String interpolation is ${what.toUpperCase}!"
+}

sbt/src/sbt-test/source-dependencies/cross-source/src/main/scala-2.9.3/B.scala

@@ -0,0 +1,3 @@
+object B {
+  def show(what: String): String = "String interpolation is " + what.toUpperCase
+}

sbt/src/sbt-test/source-dependencies/cross-source/src/main/scala/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def show(what: String): String = B.show(what)
+}

sbt/src/sbt-test/source-dependencies/cross-source/test

@@ -0,0 +1,16 @@
+# A.scala needs B.scala, it won't be in source list
+> ++2.11.4
+-> compile
+
+# A.scala needs B.scala, it would be in source list
+> ++2.10.4
+> compile
+
+# A.scala needs B.scala, it would be in source list
+> ++2.9.3
+> compile
+
+# Injecting the wrong B.scala in source list
+$ copy-file src/main/scala-2.10/B.scala src/main/scala-2.9.3/B.scala
+> ++2.9.3
+-> compile