Allow root project to be explicitly specified in Build.rootProject

main/Build.scala

@@ -14,6 +14,9 @@ trait Build
 	def projects: Seq[Project] = ReflectUtilities.allVals[Project](this).values.toSeq
 	def settings: Seq[Setting[_]] = Defaults.buildCore
 	def buildLoaders: Seq[BuildLoader.Components] = Nil
+	/** Explicitly defines the root project.
+	* If None, the root project is the first project in the build's root directory or just the first project if none are in the root directory.*/
+	def rootProject: Option[Project] = None
 }
 trait Plugin
 {

main/Load.scala

@@ -294,9 +294,10 @@ object Load
 		def isRoot(p: Project) = p.base == unit.localBase
 
 		val externals = referenced(defined).toList
-		val projectsInRoot = defined.filter(isRoot).map(_.id)
-		val rootProjects = if(projectsInRoot.isEmpty) defined.head.id :: Nil else projectsInRoot
-		(new PartBuildUnit(unit, defined.map(d => (d.id, d)).toMap, rootProjects, buildSettings(unit)), externals)
+		val explicitRoots = unit.definitions.builds.flatMap(_.rootProject)
+		val projectsInRoot = if(explicitRoots.isEmpty) defined.filter(isRoot) else explicitRoots
+		val rootProjects = if(projectsInRoot.isEmpty) defined.head :: Nil else projectsInRoot
+		(new PartBuildUnit(unit, defined.map(d => (d.id, d)).toMap, rootProjects.map(_.id), buildSettings(unit)), externals)
 	}
 	def buildSettings(unit: BuildUnit): Seq[Setting[_]] =
 	{

sbt/src/sbt-test/project/root-project/changes/RootA.scala

@@ -0,0 +1,14 @@
+import sbt._
+import Keys._
+
+object B extends Build
+{
+	override def rootProject = Some(a)
+
+	lazy val a = Project("a", file("a")) settings(
+		TaskKey[Unit]("taskA") := {}
+	)
+	lazy val b = Project("b", file("b")) settings(
+		TaskKey[Unit]("taskB") := {}
+	)
+}

sbt/src/sbt-test/project/root-project/changes/RootD.scala

@@ -0,0 +1,14 @@
+import sbt._
+import Keys._
+
+object B extends Build
+{
+	override def rootProject = Some(d)
+
+	lazy val c = Project("c", file("c")) settings(
+		TaskKey[Unit]("taskC") := {}
+	)
+	lazy val d = Project("d", file("d")) settings(
+		TaskKey[Unit]("taskD") := {}
+	)
+}

sbt/src/sbt-test/project/root-project/test

@@ -0,0 +1,11 @@
+$ copy-file changes/RootA.scala project/P.scala
+> reload
+> taskA
+-> taskB
+> b/taskB
+
+$ copy-file changes/RootD.scala project/P.scala
+> reload
+> taskD
+-> taskC
+> c/taskC