Alter the DSL slightly to use explicit DslEntry types for settings.

* Create new DSLEntry type for settings so we can categorize what we parse
* Use DSLEntry to help solve the Setting[_] vs. Seq[Setting[_]] implicit fun.
* Hack away any non-Setting[_] DSLEntry for now.
* Add test in build.sbt to make sure the new DSL works.

main/src/main/scala/sbt/BuildUtil.scala

@@ -68,7 +68,8 @@ object BuildUtil {
       deps(proj)(_.aggregate)
     }
   }
-  def baseImports: Seq[String] = "import sbt._, Keys._" :: Nil
+
+  def baseImports: Seq[String] = "import sbt._, Keys._, dsl._" :: Nil
 
   def getImports(unit: BuildUnit): Seq[String] = unit.plugins.detected.imports
 

main/src/main/scala/sbt/EvaluateConfigurations.scala

@@ -122,8 +122,8 @@ object EvaluateConfigurations {
    * The name of the class we cast DSL "setting" (vs. definition) lines to.
    */
   val SettingsDefinitionName = {
-    val _ = classOf[sbt.Def.SettingsDefinition] // this line exists to try to provide a compile-time error when the following line needs to be changed
-    "sbt.Def.SettingsDefinition"
+    val _ = classOf[sbt.internals.DslEntry] // this line exists to try to provide a compile-time error when the following line needs to be changed
+    "sbt.internals.DslEntry"
   }
   /**
    * This actually compiles a scala expression which represents a Seq[Setting[_]], although the
@@ -147,7 +147,10 @@ object EvaluateConfigurations {
       }
       loader => {
         val pos = RangePosition(name, range shift 1)
-        result.getValue(loader).asInstanceOf[SettingsDefinition].settings map (_ withPos pos)
+        (result.getValue(loader).asInstanceOf[internals.DslEntry] match {
+          case internals.DslSetting(value) => value.settings
+          case _                           => Nil
+        }) map (_ withPos pos)
       }
     }
   private[this] def isSpace = (c: Char) => Character isWhitespace c

main/src/main/scala/sbt/dsl/package.scala

@@ -0,0 +1,13 @@
+package sbt
+
+import internals.{
+  DslEntry,
+  DslSetting,
+  DslEnablePlugins,
+  DslDisablePlugins
+}
+
+package object dsl {
+  def enablePlugins(ps: AutoPlugin*): DslEntry = DslEnablePlugins(ps)
+  def disablePlugins(ps: AutoPlugin*): DslEntry = DslDisablePlugins(ps)
+}

main/src/main/scala/sbt/internals/DslAst.scala

@@ -0,0 +1,20 @@
+package sbt
+package internals
+
+import Def._
+
+/** This reprsents a `Setting` expression configured by the sbt DSL. */
+sealed trait DslEntry
+object DslEntry {
+  implicit def fromSettingsDef(inc: SettingsDefinition): DslEntry =
+    DslSetting(inc)
+  implicit def fromSettingsDef(inc: Seq[Setting[_]]): DslEntry =
+    DslSetting(inc)
+}
+/** this represents an actually Setting[_] or Seq[Setting[_]] configured by the sbt DSL. */
+case class DslSetting(settings: SettingsDefinition) extends DslEntry
+/** this represents an `enablePlugins()` in the sbt DSL */
+case class DslEnablePlugins(plugins: Seq[AutoPlugin]) extends DslEntry
+/** this represents an `disablePlugins()` in the sbt DSL */
+case class DslDisablePlugins(plugins: Seq[AutoPlugin]) extends DslEntry
+

sbt/src/sbt-test/project/auto-plugins/build.sbt

@@ -13,6 +13,8 @@ lazy val projD = project
 // with S selected, Q is loaded automatically, which in turn selects R
 lazy val projE = project.enablePlugins(S)
 
+disablePlugins(plugins.IvyPlugin)
+
 check := {
 	val adel = (del in projA).?.value // should be None
 	same(adel, None, "del in projA")