Remove some heads and tails

main-command/src/main/scala/sbt/State.scala

@@ -166,7 +166,7 @@ object State {
     /** Changes the maximum number of commands kept, adjusting the current history if necessary.*/
     def setMaxSize(size: Int): History =
       new History(if (size <= 0) executed else executed.take(size), size)
-    def current: String = executed.head
+    def currentOption: Option[String] = executed.headOption
     def previous: Option[String] = executed.drop(1).headOption
   }
   /** Constructs an empty command History with a default, finite command limit.*/

main/src/main/scala/sbt/internal/BuildStructure.scala

@@ -41,12 +41,14 @@ final class StructureIndex(
  *                The first root project is used as the default in several situations where a project is not otherwise selected.
  */
 final class LoadedBuildUnit(val unit: BuildUnit, val defined: Map[String, ResolvedProject], val rootProjects: Seq[String], val buildSettings: Seq[Setting[_]]) extends BuildUnitBase {
-  assert(rootProjects.nonEmpty, "No root projects defined for build unit " + unit)
   /**
    * The project to use as the default when one is not otherwise selected.
    * [[LocalRootProject]] resolves to this from within the same build.
    */
-  val root = rootProjects.head
+  val root = rootProjects match {
+    case Nil           => throw new java.lang.AssertionError("assertion failed: No root projects defined for build unit " + unit)
+    case Seq(root, _*) => root
+  }
 
   /** The base directory of the build unit (not the build definition).*/
   def localBase = unit.localBase

main/src/main/scala/sbt/internal/Load.scala

@@ -312,7 +312,10 @@ private[sbt] object Load {
   def loaded(unit: BuildUnit): (PartBuildUnit, List[ProjectReference]) =
     {
       val defined = projects(unit)
-      if (defined.isEmpty) sys.error("No projects defined in build unit " + unit)
+      val firstDefined = defined match {
+        case Nil            => sys.error("No projects defined in build unit " + unit)
+        case Seq(first, _*) => first
+      }
 
       // since base directories are resolved at this point (after 'projects'),
       //   we can compare Files instead of converting to URIs
@@ -321,7 +324,7 @@ private[sbt] object Load {
       val externals = referenced(defined).toList
       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
+      val rootProjects = if (projectsInRoot.isEmpty) firstDefined :: 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[_]] =

main/src/main/scala/sbt/internal/SettingGraph.scala

@@ -77,19 +77,21 @@ object Graph {
           }) +
           s.slice(at + 1, s.length)
       else s
-    def toAsciiLines(node: A, level: Int): Vector[String] = {
+    def toAsciiLines(node: A, level: Int): (String, Vector[String]) = {
       val line = limitLine((twoSpaces * level) + (if (level == 0) "" else "+-") + display(node))
       val cs = Vector(children(node): _*)
       val childLines = cs map { toAsciiLines(_, level + 1) }
       val withBar = childLines.zipWithIndex flatMap {
-        case (lines, pos) if pos < (cs.size - 1) => lines map { insertBar(_, 2 * (level + 1)) }
-        case (lines, pos) =>
-          if (lines.last.trim != "") lines ++ Vector(twoSpaces * (level + 1))
-          else lines
+        case ((line, withBar), pos) if pos < (cs.size - 1) =>
+          (line +: withBar) map { insertBar(_, 2 * (level + 1)) }
+        case ((line, withBar), pos) if withBar.lastOption.getOrElse(line).trim != "" =>
+          (line +: withBar) ++ Vector(twoSpaces * (level + 1))
+        case ((line, withBar), _) => line +: withBar
       }
-      line +: withBar
+      (line, withBar)
     }
 
-    toAsciiLines(top, 0).mkString("\n")
+    val (line, withBar) = toAsciiLines(top, 0)
+    (line +: withBar).mkString("\n")
   }
 }

main/src/test/scala/sbt/internal/TestBuild.scala

@@ -99,7 +99,7 @@ object TestBuild {
     def inheritConfig(ref: ResolvedReference, config: ConfigKey) = projectFor(ref).confMap(config.name).extended map toConfigKey
     def inheritTask(task: AttributeKey[_]) = taskMap.get(task) match { case None => Nil; case Some(t) => t.delegates map getKey }
     def inheritProject(ref: ProjectRef) = project(ref).delegates
-    def resolve(ref: Reference) = Scope.resolveReference(builds.head.uri, rootProject, ref)
+    def resolve(ref: Reference) = Scope.resolveReference(root.uri, rootProject, ref)
     lazy val delegates: Scope => Seq[Scope] =
       Scope.delegates(
         allProjects,