Merge pull request #1130 from sbt/wip/auto-plugins

AUTO PLUGIN FEATURE!!!!

main/src/main/scala/sbt/Build.scala

@@ -18,6 +18,7 @@ trait Build
 	* 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
 }
+// TODO 0.14.0: decide if Plugin should be deprecated in favor of AutoPlugin
 trait Plugin
 {
 	@deprecated("Override projectSettings or buildSettings instead.", "0.12.0")

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

@@ -30,23 +30,108 @@ final class StructureIndex(
 	val keyIndex: KeyIndex,
 	val aggregateKeyIndex: KeyIndex
 )
+
+/** A resolved build unit.  (`ResolvedBuildUnit` would be a better name to distinguish it from the loaded, but unresolved `BuildUnit`.)
+* @param unit The loaded, but unresolved [[BuildUnit]] this was resolved from.
+* @param defined The definitive map from project IDs to resolved projects.
+*                These projects have had [[Reference]]s resolved and [[AutoPlugin]]s evaluated.
+* @param rootProjects The list of project IDs for the projects considered roots of this build.
+*                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.isEmpty, "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
+
+	/** The base directory of the build unit (not the build definition).*/
 	def localBase = unit.localBase
+
+	/** The classpath to use when compiling against this build unit's publicly visible code.
+	* It includes build definition and plugin classes, but not classes for .sbt file statements and expressions. */
 	def classpath: Seq[File] = unit.definitions.target ++ unit.plugins.classpath
+
+	/** The class loader to use for this build unit's publicly visible code.
+	* It includes build definition and plugin classes, but not classes for .sbt file statements and expressions. */
 	def loader = unit.definitions.loader
+
+	/** The imports to use for .sbt files, `consoleProject` and other contexts that use code from the build definition. */
 	def imports = BuildUtil.getImports(unit)
 	override def toString = unit.toString
 }
 
+// TODO: figure out how to deprecate and drop buildNames
+/** The built and loaded build definition, including loaded but unresolved [[Project]]s, for a build unit (for a single URI).
+*
+* @param base The base directory of the build definition, typically `<build base>/project/`.
+* @param loader The ClassLoader containing all classes and plugins for the build definition project.
+*               Note that this does not include classes for .sbt files.
+* @param builds The list of [[Build]]s for the build unit.
+*               In addition to auto-discovered [[Build]]s, this includes any auto-generated default [[Build]]s.
+* @param projects The list of all [[Project]]s from all [[Build]]s.
+*                 These projects have not yet been resolved, but they have had auto-plugins applied.
+*                 In particular, each [[Project]]'s `autoPlugins` field is populated according to their configured `natures`
+*                 and their `settings` and `configurations` updated as appropriate.
+* @param buildNames No longer used and will be deprecated once feasible.
+*/
 final class LoadedDefinitions(val base: File, val target: Seq[File], val loader: ClassLoader, val builds: Seq[Build], val projects: Seq[Project], val buildNames: Seq[String])
-final class LoadedPlugins(val base: File, val pluginData: PluginData, val loader: ClassLoader, val plugins: Seq[Plugin], val pluginNames: Seq[String])
+
+/** Auto-detected top-level modules (as in `object X`) of type `T` paired with their source names. */
+final class DetectedModules[T](val modules: Seq[(String, T)])
 {
+	/** The source names of the modules.  This is "X" in `object X`, as opposed to the implementation class name "X$".
+	* The names are returned in a stable order such that `names zip values` pairs a name with the actual module. */
+	def names: Seq[String] = modules.map(_._1)
+
+	/** The singleton value of the module.
+	* The values are returned in a stable order such that `names zip values` pairs a name with the actual module. */
+	def values: Seq[T] = modules.map(_._2)
+}
+
+/** Auto-discovered modules for the build definition project.  These include modules defined in build definition sources
+* as well as modules in binary dependencies.
+*
+* @param builds The [[Build]]s detected in the build definition.  This does not include the default [[Build]] that sbt creates if none is defined.
+*/
+final class DetectedPlugins(val plugins: DetectedModules[Plugin], val autoImports: DetectedModules[AutoImport], val autoPlugins: DetectedModules[AutoPlugin], val builds: DetectedModules[Build])
+{
+	/** Sequence of import expressions for the build definition.  This includes the names of the [[Plugin]], [[Build]], and [[AutoImport]] modules, but not the [[AutoPlugin]] modules. */
+	lazy val imports: Seq[String] = BuildUtil.getImports(plugins.names ++ builds.names ++ autoImports.names)
+
+	/** A function to select the right [[AutoPlugin]]s from [[autoPlugins]] given the defined [[Natures]] for a [[Project]]. */
+	lazy val compileNatures: Natures => Seq[AutoPlugin] = Natures.compile(autoPlugins.values.toList)
+}
+
+/** The built and loaded build definition project.
+* @param base The base directory for the build definition project (not the base of the project itself).
+* @param pluginData Evaluated tasks/settings from the build definition for later use.
+*                   This is necessary because the build definition project is discarded.
+* @param loader The class loader for the build definition project, notably excluding classes used for .sbt files.
+* @param detected Auto-detected modules in the build definition.
+*/
+final class LoadedPlugins(val base: File, val pluginData: PluginData, val loader: ClassLoader, val detected: DetectedPlugins)
+{
+	@deprecated("Use the primary constructor.", "0.13.2")
+	def this(base: File, pluginData: PluginData, loader: ClassLoader, plugins: Seq[Plugin], pluginNames: Seq[String]) =
+		this(base, pluginData, loader,
+			new DetectedPlugins(new DetectedModules(pluginNames zip plugins), new DetectedModules(Nil), new DetectedModules(Nil), new DetectedModules(Nil))
+		)
+
+	@deprecated("Use detected.plugins.values.", "0.13.2")
+	val plugins: Seq[Plugin] = detected.plugins.values
+	@deprecated("Use detected.plugins.names.", "0.13.2")
+	val pluginNames: Seq[String] = detected.plugins.names
+
 	def fullClasspath: Seq[Attributed[File]] = pluginData.classpath
 	def classpath = data(fullClasspath)
+
 }
+/** The loaded, but unresolved build unit.
+* @param uri The uniquely identifying URI for the build.
+* @param localBase The working location of the build on the filesystem.
+*        For local URIs, this is the same as `uri`, but for remote URIs, this is the local copy or workspace allocated for the build.
+*/
 final class BuildUnit(val uri: URI, val localBase: File, val definitions: LoadedDefinitions, val plugins: LoadedPlugins)
 {
 	override def toString = if(uri.getScheme == "file") localBase.toString else (uri + " (locally: " + localBase +")")
@@ -57,6 +142,8 @@ final class LoadedBuild(val root: URI, val units: Map[URI, LoadedBuildUnit])
 	BuildUtil.checkCycles(units)
 	def allProjectRefs: Seq[(ProjectRef, ResolvedProject)] = for( (uri, unit) <- units.toSeq; (id, proj) <- unit.defined ) yield ProjectRef(uri, id) -> proj
 	def extra(data: Settings[Scope])(keyIndex: KeyIndex): BuildUtil[ResolvedProject] = BuildUtil(root, units, keyIndex, data)
+
+	private[sbt] def autos = GroupedAutoPlugins(units)
 }
 final class PartBuild(val root: URI, val units: Map[URI, PartBuildUnit])
 sealed trait BuildUnitBase { def rootProjects: Seq[String]; def buildSettings: Seq[Setting[_]] }

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

@@ -35,7 +35,7 @@ final class BuildUtil[Proj](
 		case _ => None
 	}
 
-	val configurationsForAxis: Option[ResolvedReference] => Seq[String] = 
+	val configurationsForAxis: Option[ResolvedReference] => Seq[String] =
 		refOpt => configurations(projectForAxis(refOpt)).map(_.name)
 }
 object BuildUtil
@@ -48,6 +48,20 @@ object BuildUtil
 		new BuildUtil(keyIndex, data, root, Load getRootProject units, getp, configs, aggregates)
 	}
 
+	def dependencies(units: Map[URI, LoadedBuildUnit]): BuildDependencies =
+	{
+			import collection.mutable.HashMap
+		val agg = new HashMap[ProjectRef, Seq[ProjectRef]]
+		val cp = new HashMap[ProjectRef, Seq[ClasspathDep[ProjectRef]]]
+		for(lbu <- units.values; rp <- lbu.defined.values)
+		{
+			val ref = ProjectRef(lbu.unit.uri, rp.id)
+			cp(ref) = rp.dependencies
+			agg(ref) = rp.aggregate
+		}
+		BuildDependencies(cp.toMap, agg.toMap)
+	}
+
 	def checkCycles(units: Map[URI, LoadedBuildUnit])
 	{
 		def getRef(pref: ProjectRef) = units(pref.build).defined(pref.project)
@@ -60,8 +74,14 @@ object BuildUtil
 		}
 	}
 	def baseImports: Seq[String] = "import sbt._, Keys._" :: Nil
-	def getImports(unit: BuildUnit): Seq[String] = getImports(unit.plugins.pluginNames, unit.definitions.buildNames)
+
-	def getImports(pluginNames: Seq[String], buildNames: Seq[String]): Seq[String] = baseImports ++ importAllRoot(pluginNames ++ buildNames)
+	def getImports(unit: BuildUnit): Seq[String] = unit.plugins.detected.imports
+
+	@deprecated("Use getImports(Seq[String]).", "0.13.2")
+	def getImports(pluginNames: Seq[String], buildNames: Seq[String]): Seq[String] = getImports(pluginNames ++ buildNames)
+
+	def getImports(names: Seq[String]): Seq[String] = baseImports ++ importAllRoot(names)
+
 	def importAll(values: Seq[String]): Seq[String] = if(values.isEmpty) Nil else values.map( _ + "._" ).mkString("import ", ", ", "") :: Nil
 	def importAllRoot(values: Seq[String]): Seq[String] = importAll(values map rootedName)
 	def rootedName(s: String): String = if(s contains '.') "_root_." + s else s

main/src/main/scala/sbt/CommandStrings.scala

@@ -49,6 +49,11 @@ $ShowCommand <task>
 
 	Evaluates the specified task and display the value returned by the task."""
 
+	val PluginsCommand = "plugins"
+	val PluginCommand = "plugin"
+	def pluginsBrief = "Lists currently available plugins."
+	def pluginsDetailed = pluginsBrief // TODO: expand
+
 	val LastCommand = "last"
 	val LastGrepCommand = "last-grep"
 	val ExportCommand = "export"

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

@@ -179,7 +179,7 @@ object Defaults extends BuildCommon
 		unmanagedResources <<= collectFiles(unmanagedResourceDirectories, includeFilter in unmanagedResources, excludeFilter in unmanagedResources),
 		watchSources in ConfigGlobal ++= unmanagedResources.value,
 		resourceGenerators :== Nil,
-		resourceGenerators <+= (definedSbtPlugins, resourceManaged) map writePluginsDescriptor,
+		resourceGenerators <+= (discoveredSbtPlugins, resourceManaged) map PluginDiscovery.writeDescriptors,
 		managedResources <<= generate(resourceGenerators),
 		resources <<= Classpaths.concat(managedResources, unmanagedResources)
 	)
@@ -233,6 +233,7 @@ object Defaults extends BuildCommon
 		consoleQuick <<= consoleQuickTask,
 		discoveredMainClasses <<= compile map discoverMainClasses storeAs discoveredMainClasses triggeredBy compile,
 		definedSbtPlugins <<= discoverPlugins,
+		discoveredSbtPlugins <<= discoverSbtPluginNames,
 		inTask(run)(runnerTask :: Nil).head,
 		selectMainClass := mainClass.value orElse selectRunMain(discoveredMainClasses.value),
 		mainClass in run := (selectMainClass in run).value,
@@ -764,27 +765,21 @@ object Defaults extends BuildCommon
 
 	def sbtPluginExtra(m: ModuleID, sbtV: String, scalaV: String): ModuleID =
 		m.extra(CustomPomParser.SbtVersionKey -> sbtV, CustomPomParser.ScalaVersionKey -> scalaV).copy(crossVersion = CrossVersion.Disabled)
+
+	@deprecated("Use PluginDiscovery.writeDescriptor.", "0.13.2")
 	def writePluginsDescriptor(plugins: Set[String], dir: File): Seq[File] =
-	{
+		PluginDiscovery.writeDescriptor(plugins.toSeq, dir, PluginDiscovery.Paths.Plugins).toList
-		val descriptor: File = dir / "sbt" / "sbt.plugins"
+
-		if(plugins.isEmpty)
+	def discoverSbtPluginNames: Initialize[Task[PluginDiscovery.DiscoveredNames]] = Def.task {
-		{
+		if(sbtPlugin.value) PluginDiscovery.discoverSourceAll(compile.value) else PluginDiscovery.emptyDiscoveredNames
-			IO.delete(descriptor)
-			Nil
-		}
-		else
-		{
-			IO.writeLines(descriptor, plugins.toSeq.sorted)
-			descriptor :: Nil
-		}
 	}
+
+	@deprecated("Use discoverSbtPluginNames.", "0.13.2")
 	def discoverPlugins: Initialize[Task[Set[String]]] = (compile, sbtPlugin, streams) map { (analysis, isPlugin, s) => if(isPlugin) discoverSbtPlugins(analysis, s.log) else Set.empty }
+
+	@deprecated("Use PluginDiscovery.sourceModuleNames[Plugin].", "0.13.2")
 	def discoverSbtPlugins(analysis: inc.Analysis, log: Logger): Set[String] =
-	{
+		PluginDiscovery.sourceModuleNames(analysis, classOf[Plugin].getName).toSet
-		val pluginClass = classOf[Plugin].getName
-		val discovery = Discovery(Set(pluginClass), Set.empty)( Tests allDefs analysis )
-		discovery collect { case (df, disc) if (disc.baseClasses contains pluginClass) && disc.isModule => df.name } toSet;
-	}
 
 	def copyResourcesTask =
 	(classDirectory, resources, resourceDirectories, streams) map { (target, resrcs, dirs, s) =>
@@ -1262,19 +1257,8 @@ object Classpaths
 			if(useJars) Seq(pkgTask).join else psTask
 		}
 
-	def constructBuildDependencies: Initialize[BuildDependencies] =
+	def constructBuildDependencies: Initialize[BuildDependencies] = loadedBuild(lb => BuildUtil.dependencies(lb.units))
-		loadedBuild { lb =>
+
-				import collection.mutable.HashMap
-			val agg = new HashMap[ProjectRef, Seq[ProjectRef]]
-			val cp = new HashMap[ProjectRef, Seq[ClasspathDep[ProjectRef]]]
-			for(lbu <- lb.units.values; rp <- lbu.defined.values)
-			{
-				val ref = ProjectRef(lbu.unit.uri, rp.id)
-				cp(ref) = rp.dependencies
-				agg(ref) = rp.aggregate
-			}
-			BuildDependencies(cp.toMap, agg.toMap)
-		}
 	def internalDependencies: Initialize[Task[Classpath]] =
 		(thisProjectRef, classpathConfiguration, configuration, settingsData, buildDependencies) flatMap internalDependencies0
 	def unmanagedDependencies: Initialize[Task[Classpath]] =

main/src/main/scala/sbt/GroupedAutoPlugins.scala

@@ -0,0 +1,20 @@
+package sbt
+
+	import Def.Setting
+	import java.net.URI
+
+private[sbt] final class GroupedAutoPlugins(val all: Seq[AutoPlugin], val byBuild: Map[URI, Seq[AutoPlugin]])
+{
+	def globalSettings: Seq[Setting[_]] = all.flatMap(_.globalSettings)
+	def buildSettings(uri: URI): Seq[Setting[_]] = byBuild.getOrElse(uri, Nil).flatMap(_.buildSettings)
+}
+
+private[sbt] object GroupedAutoPlugins
+{
+	private[sbt] def apply(units: Map[URI, LoadedBuildUnit]): GroupedAutoPlugins =
+	{
+		val byBuild: Map[URI, Seq[AutoPlugin]] = units.mapValues(unit => unit.defined.values.flatMap(_.autoPlugins).toSeq.distinct).toMap
+		val all: Seq[AutoPlugin] = byBuild.values.toSeq.flatten.distinct
+		new GroupedAutoPlugins(all, byBuild)
+	}
+}

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

@@ -131,6 +131,7 @@ object Keys
 	val crossVersion = SettingKey[CrossVersion]("cross-version", "Configures handling of the Scala version when cross-building.", CSetting)
 	val classpathOptions = SettingKey[ClasspathOptions]("classpath-options", "Configures handling of Scala classpaths.", DSetting)
 	val definedSbtPlugins = TaskKey[Set[String]]("defined-sbt-plugins", "The set of names of Plugin implementations defined by this project.", CTask)
+	val discoveredSbtPlugins = TaskKey[PluginDiscovery.DiscoveredNames]("discovered-sbt-plugins", "The names of sbt plugin-related modules (modules that extend Build, Plugin, AutoImport, AutoPlugin) defined by this project.", CTask)
 	val sbtPlugin = SettingKey[Boolean]("sbt-plugin", "If true, enables adding sbt as a dependency and auto-generation of the plugin descriptor file.", BMinusSetting)
 	val printWarnings = TaskKey[Unit]("print-warnings", "Shows warnings from compilation, including ones that weren't printed initially.", BPlusTask)
 	val fileInputOptions = SettingKey[Seq[String]]("file-input-options", "Options that take file input, which may invalidate the cache.", CSetting)
@@ -348,7 +349,7 @@ object Keys
 
 	// Experimental in sbt 0.13.2 to enable grabing semantic compile failures.
 	private[sbt] val compilerReporter = TaskKey[Option[xsbti.Reporter]]("compilerReporter", "Experimental hook to listen (or send) compilation failure messages.", DTask)
-	
+
 	val triggeredBy = Def.triggeredBy
 	val runBefore = Def.runBefore
 

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

@@ -6,19 +6,16 @@ package sbt
 	import java.io.File
 	import java.net.{URI,URL}
 	import compiler.{Eval,EvalImports}
-	import xsbt.api.{Discovered,Discovery}
-	import xsbti.compile.CompileOrder
 	import classpath.ClasspathUtilities
 	import scala.annotation.tailrec
 	import collection.mutable
-	import Compiler.{Compilers,Inputs}
+	import Compiler.Compilers
 	import inc.{FileValueCache, Locate}
 	import Project.{inScope,makeSettings}
 	import Def.{isDummy, ScopedKey, ScopeLocal, Setting}
 	import Keys.{appConfiguration, baseDirectory, configuration, fullResolvers, fullClasspath, pluginData, streams, thisProject, thisProjectRef, update}
 	import Keys.{exportedProducts, loadedBuild, onLoadMessage, resolvedScoped, sbtPlugin, scalacOptions, taskDefinitionKey}
 	import tools.nsc.reporters.ConsoleReporter
-	import Build.analyzed
 	import Attributed.data
 	import Scope.{GlobalScope, ThisScope}
 	import Types.const
@@ -180,7 +177,7 @@ object Load
 		val keys = Index.allKeys(settings)
 		val attributeKeys = Index.attributeKeys(data) ++ keys.map(_.key)
 		val scopedKeys = keys ++ data.allKeys( (s,k) => ScopedKey(s,k))
-		val projectsMap = projects.mapValues(_.defined.keySet)
+		val projectsMap = projects.mapValues(_.defined.keySet).toMap
 		val keyIndex = KeyIndex(scopedKeys, projectsMap)
 		val aggIndex = KeyIndex.aggregate(scopedKeys, extra(keyIndex), projectsMap)
 		new sbt.StructureIndex(Index.stringToKeyMap(attributeKeys), Index.taskToKeyMap(data), Index.triggers(data), keyIndex, aggIndex)
@@ -201,10 +198,10 @@ object Load
 	{
 		((loadedBuild in GlobalScope :== loaded) +:
 		transformProjectOnly(loaded.root, rootProject, injectSettings.global)) ++
-		inScope(GlobalScope)( pluginGlobalSettings(loaded) ) ++
+		inScope(GlobalScope)( pluginGlobalSettings(loaded) ++ loaded.autos.globalSettings ) ++
 		loaded.units.toSeq.flatMap { case (uri, build) =>
-			val plugins = build.unit.plugins.plugins
+			val plugins = build.unit.plugins.detected.plugins.values
-			val pluginBuildSettings = plugins.flatMap(_.buildSettings)
+			val pluginBuildSettings = plugins.flatMap(_.buildSettings) ++ loaded.autos.buildSettings(uri)
 			val pluginNotThis = plugins.flatMap(_.settings) filterNot isProjectThis
 			val projectSettings = build.defined flatMap { case (id, project) =>
 				val ref = ProjectRef(uri, id)
@@ -220,9 +217,10 @@ object Load
 			buildSettings ++ projectSettings
 		}
 	}
+	@deprecated("Does not account for AutoPlugins and will be made private.", "0.13.2")
 	def pluginGlobalSettings(loaded: sbt.LoadedBuild): Seq[Setting[_]] =
 		loaded.units.toSeq flatMap { case (_, build) =>
-			build.unit.plugins.plugins flatMap { _.globalSettings }
+			build.unit.plugins.detected.plugins.values flatMap { _.globalSettings }
 		}
 
 	@deprecated("No longer used.", "0.13.0")
@@ -368,10 +366,11 @@ object Load
 	def resolveProjects(loaded: sbt.PartBuild): sbt.LoadedBuild =
 	{
 		val rootProject = getRootProject(loaded.units)
-		new sbt.LoadedBuild(loaded.root, loaded.units map { case (uri, unit) =>
+		val units = loaded.units map { case (uri, unit) =>
 			IO.assertAbsolute(uri)
 			(uri, resolveProjects(uri, unit, rootProject))
-		})
+		}
+		new sbt.LoadedBuild(loaded.root, units)
 	}
 	def resolveProjects(uri: URI, unit: sbt.PartBuildUnit, rootProject: URI => String): sbt.LoadedBuildUnit =
 	{
@@ -399,10 +398,10 @@ object Load
 	def getBuild[T](map: Map[URI, T], uri: URI): T =
 		map.getOrElse(uri, noBuild(uri))
 
-	def emptyBuild(uri: URI) = sys.error("No root project defined for build unit '" + uri + "'")
+	def emptyBuild(uri: URI) = sys.error(s"No root project defined for build unit '$uri'")
-	def noBuild(uri: URI) = sys.error("Build unit '" + uri + "' not defined.")
+	def noBuild(uri: URI) = sys.error(s"Build unit '$uri' not defined.")
-	def noProject(uri: URI, id: String) = sys.error("No project '" + id + "' defined in '" + uri + "'.")
+	def noProject(uri: URI, id: String) = sys.error(s"No project '$id' defined in '$uri'.")
-	def noConfiguration(uri: URI, id: String, conf: String) = sys.error("No configuration '" + conf + "' defined in project '" + id + "' in '" + uri +"'")
+	def noConfiguration(uri: URI, id: String, conf: String) = sys.error(s"No configuration '$conf' defined in project '$id' in '$uri'")
 
 	def loadUnit(uri: URI, localBase: File, s: State, config: sbt.LoadBuildConfiguration): sbt.BuildUnit =
 	{
@@ -410,15 +409,13 @@ object Load
 		val defDir = projectStandard(normBase)
 
 		val plugs = plugins(defDir, s, config.copy(pluginManagement = config.pluginManagement.forPlugin))
-		val defNames = analyzed(plugs.fullClasspath) flatMap findDefinitions
+		val defsScala = plugs.detected.builds.values
-		val defsScala = if(defNames.isEmpty) Nil else loadDefinitions(plugs.loader, defNames)
-		val imports = BuildUtil.getImports(plugs.pluginNames, defNames)
 
 		lazy val eval = mkEval(plugs.classpath, defDir, plugs.pluginData.scalacOptions)
 		val initialProjects = defsScala.flatMap(b => projectsFromBuild(b, normBase))
 
 		val memoSettings = new mutable.HashMap[File, LoadedSbtFile]
-		def loadProjects(ps: Seq[Project]) = loadTransitive(ps, normBase, imports, plugs, () => eval, config.injectSettings, Nil, memoSettings)
+		def loadProjects(ps: Seq[Project]) = loadTransitive(ps, normBase, plugs, () => eval, config.injectSettings, Nil, memoSettings)
 		val loadedProjectsRaw = loadProjects(initialProjects)
 		val hasRoot = loadedProjectsRaw.exists(_.base == normBase) || defsScala.exists(_.rootProject.isDefined)
 		val (loadedProjects, defaultBuildIfNone) =
@@ -434,7 +431,7 @@ object Load
 			}
 
 		val defs = if(defsScala.isEmpty) defaultBuildIfNone :: Nil else defsScala
-		val loadedDefs = new sbt.LoadedDefinitions(defDir, Nil, plugs.loader, defs, loadedProjects, defNames)
+		val loadedDefs = new sbt.LoadedDefinitions(defDir, Nil, plugs.loader, defs, loadedProjects, plugs.detected.builds.names)
 		new sbt.BuildUnit(uri, normBase, loadedDefs, plugs)
 	}
 
@@ -460,16 +457,22 @@ object Load
 	private[this] def projectsFromBuild(b: Build, base: File): Seq[Project] =
 		b.projectDefinitions(base).map(resolveBase(base))
 
-	private[this] def loadTransitive(newProjects: Seq[Project], buildBase: File, imports: Seq[String], plugins: sbt.LoadedPlugins, eval: () => Eval, injectSettings: InjectSettings, acc: Seq[Project], memoSettings: mutable.Map[File, LoadedSbtFile]): Seq[Project] =
+	private[this] def loadTransitive(newProjects: Seq[Project], buildBase: File, plugins: sbt.LoadedPlugins, eval: () => Eval, injectSettings: InjectSettings, acc: Seq[Project], memoSettings: mutable.Map[File, LoadedSbtFile]): Seq[Project] =
 	{
-		def loadSbtFiles(auto: AddSettings, base: File): LoadedSbtFile =
+		def loadSbtFiles(auto: AddSettings, base: File, autoPlugins: Seq[AutoPlugin]): LoadedSbtFile =
-			loadSettings(auto, base, imports, plugins, eval, injectSettings, memoSettings)
+			loadSettings(auto, base, plugins, eval, injectSettings, memoSettings, autoPlugins)
 		def loadForProjects = newProjects map { project =>
-			val loadedSbtFiles = loadSbtFiles(project.auto, project.base)
+			val autoPlugins =
-			val transformed = project.copy(settings = (project.settings: Seq[Setting[_]]) ++ loadedSbtFiles.settings)
+				try plugins.detected.compileNatures(project.natures)
+				catch { case e: AutoPluginException => throw translateAutoPluginException(e, project) }
+			val autoConfigs = autoPlugins.flatMap(_.projectConfigurations)
+			val loadedSbtFiles = loadSbtFiles(project.auto, project.base, autoPlugins)
+			val newSettings = (project.settings: Seq[Setting[_]]) ++ loadedSbtFiles.settings
+			// add the automatically selected settings, record the selected AutoPlugins, and register the automatically selected configurations
+			val transformed = project.copy(settings = newSettings).setAutoPlugins(autoPlugins).overrideConfigs(autoConfigs : _*)
 			(transformed, loadedSbtFiles.projects)
 		}
-		def defaultLoad = loadSbtFiles(AddSettings.defaultSbtFiles, buildBase).projects
+		def defaultLoad = loadSbtFiles(AddSettings.defaultSbtFiles, buildBase, Nil).projects
 		val (nextProjects, loadedProjects) =
 			if(newProjects.isEmpty) // load the .sbt files in the root directory to look for Projects
 				(defaultLoad, acc)
@@ -481,10 +484,12 @@ object Load
 		if(nextProjects.isEmpty)
 			loadedProjects
 		else
-			loadTransitive(nextProjects, buildBase, imports, plugins, eval, injectSettings, loadedProjects, memoSettings)
+			loadTransitive(nextProjects, buildBase, plugins, eval, injectSettings, loadedProjects, memoSettings)
 	}
+	private[this] def translateAutoPluginException(e: AutoPluginException, project: Project): AutoPluginException =
+		e.withPrefix(s"Error determining plugins for project '${project.id}' in ${project.base}:\n")
 
-	private[this] def loadSettings(auto: AddSettings, projectBase: File, buildImports: Seq[String], loadedPlugins: sbt.LoadedPlugins, eval: ()=>Eval, injectSettings: InjectSettings, memoSettings: mutable.Map[File, LoadedSbtFile]): LoadedSbtFile =
+	private[this] def loadSettings(auto: AddSettings, projectBase: File, loadedPlugins: sbt.LoadedPlugins, eval: ()=>Eval, injectSettings: InjectSettings, memoSettings: mutable.Map[File, LoadedSbtFile], autoPlugins: Seq[AutoPlugin]): LoadedSbtFile =
 	{
 		lazy val defaultSbtFiles = configurationSources(projectBase)
 		def settings(ss: Seq[Setting[_]]) = new LoadedSbtFile(ss, Nil, Nil)
@@ -499,14 +504,20 @@ object Load
 			lf
 		}
 		def loadSettingsFile(src: File): LoadedSbtFile =
-			EvaluateConfigurations.evaluateSbtFile(eval(), src, IO.readLines(src), buildImports, 0)(loader)
+			EvaluateConfigurations.evaluateSbtFile(eval(), src, IO.readLines(src), loadedPlugins.detected.imports, 0)(loader)
 
 			import AddSettings.{User,SbtFiles,DefaultSbtFiles,Plugins,Sequence}
+		def pluginSettings(f: Plugins) = {
+			val included = loadedPlugins.detected.plugins.values.filter(f.include) // don't apply the filter to AutoPlugins, only Plugins
+			val oldStyle = included.flatMap(p => p.settings.filter(isProjectThis) ++ p.projectSettings)
+			val autoStyle = autoPlugins.flatMap(_.projectSettings)
+			oldStyle ++ autoStyle
+		}
 		def expand(auto: AddSettings): LoadedSbtFile = auto match {
 			case User => settings(injectSettings.projectLoaded(loader))
 			case sf: SbtFiles => loadSettings( sf.files.map(f => IO.resolve(projectBase, f)))
 			case sf: DefaultSbtFiles => loadSettings( defaultSbtFiles.filter(sf.include))
-			case f: Plugins => settings(loadedPlugins.plugins.filter(f.include).flatMap(p => p.settings.filter(isProjectThis) ++ p.projectSettings))
+			case p: Plugins => settings(pluginSettings(p))
 			case q: Sequence => (LoadedSbtFile.empty /: q.sequence) { (b,add) => b.merge( expand(add) ) }
 		}
 		expand(auto)
@@ -599,67 +610,46 @@ object Load
 		config.evalPluginDef(pluginDef, pluginState)
 	}
 
+	@deprecated("Use ModuleUtilities.getCheckedObjects[Build].", "0.13.2")
 	def loadDefinitions(loader: ClassLoader, defs: Seq[String]): Seq[Build] =
 		defs map { definition => loadDefinition(loader, definition) }
+
+	@deprecated("Use ModuleUtilities.getCheckedObject[Build].", "0.13.2")
 	def loadDefinition(loader: ClassLoader, definition: String): Build =
 		ModuleUtilities.getObject(definition, loader).asInstanceOf[Build]
 
 	def loadPlugins(dir: File, data: PluginData, loader: ClassLoader): sbt.LoadedPlugins =
-	{
+		new sbt.LoadedPlugins(dir, data, loader, PluginDiscovery.discoverAll(data, loader))
-		val (pluginNames, plugins) = if(data.classpath.isEmpty) (Nil, Nil) else {
+
-			val names = getPluginNames(data.classpath, loader)
+	@deprecated("Replaced by the more general PluginDiscovery.binarySourceModuleNames and will be made private.", "0.13.2")
-			val loaded =
-				try loadPlugins(loader, names)
-				catch {
-					case e: ExceptionInInitializerError =>
-						val cause = e.getCause
-						if(cause eq null) throw e else throw cause
-					case e: LinkageError => incompatiblePlugins(data, e)
-				}
-			(names, loaded)
-		}
-		new sbt.LoadedPlugins(dir, data, loader, plugins, pluginNames)
-	}
-	private[this] def incompatiblePlugins(data: PluginData, t: LinkageError): Nothing =
-	{
-		val evicted = data.report.toList.flatMap(_.configurations.flatMap(_.evicted))
-		val evictedModules = evicted map { id => (id.organization, id.name) } distinct ;
-		val evictedStrings = evictedModules map { case (o,n) => o + ":" + n }
-		val msgBase = "Binary incompatibility in plugins detected."
-		val msgExtra = if(evictedStrings.isEmpty) "" else "\nNote that conflicts were resolved for some dependencies:\n\t" + evictedStrings.mkString("\n\t")
-		throw new IncompatiblePluginsException(msgBase + msgExtra, t)
-	}
 	def getPluginNames(classpath: Seq[Attributed[File]], loader: ClassLoader): Seq[String] =
-		 ( binaryPlugins(data(classpath), loader) ++ (analyzed(classpath) flatMap findPlugins) ).distinct
+		PluginDiscovery.binarySourceModuleNames(classpath, loader, PluginDiscovery.Paths.Plugins, classOf[Plugin].getName)
 
+	@deprecated("Use PluginDiscovery.binaryModuleNames.", "0.13.2")
 	def binaryPlugins(classpath: Seq[File], loader: ClassLoader): Seq[String] =
-	{
+		PluginDiscovery.binaryModuleNames(classpath, loader, PluginDiscovery.Paths.Plugins)
-		import collection.JavaConversions._
+
-		loader.getResources("sbt/sbt.plugins").toSeq.filter(onClasspath(classpath)) flatMap { u =>
+	@deprecated("Use PluginDiscovery.onClasspath", "0.13.2")
-			IO.readLinesURL(u).map( _.trim).filter(!_.isEmpty)
-		}
-	}
 	def onClasspath(classpath: Seq[File])(url: URL): Boolean =
-		IO.urlAsFile(url) exists (classpath.contains _)
+		PluginDiscovery.onClasspath(classpath)(url)
 
+	@deprecated("Use ModuleUtilities.getCheckedObjects[Plugin].", "0.13.2")
 	def loadPlugins(loader: ClassLoader, pluginNames: Seq[String]): Seq[Plugin] =
-		pluginNames.map(pluginName => loadPlugin(pluginName, loader))
+		ModuleUtilities.getCheckedObjects[Plugin](pluginNames, loader).map(_._2)
 
+	@deprecated("Use ModuleUtilities.getCheckedObject[Plugin].", "0.13.2")
 	def loadPlugin(pluginName: String, loader: ClassLoader): Plugin =
-		ModuleUtilities.getObject(pluginName, loader).asInstanceOf[Plugin]
+		ModuleUtilities.getCheckedObject[Plugin](pluginName, loader)
 
+	@deprecated("No longer used.", "0.13.2")
 	def findPlugins(analysis: inc.Analysis): Seq[String]  =  discover(analysis, "sbt.Plugin")
+
+	@deprecated("No longer used.", "0.13.2")
 	def findDefinitions(analysis: inc.Analysis): Seq[String]  =  discover(analysis, "sbt.Build")
+
+	@deprecated("Use PluginDiscovery.sourceModuleNames", "0.13.2")
 	def discover(analysis: inc.Analysis, subclasses: String*): Seq[String] =
-	{
+		PluginDiscovery.sourceModuleNames(analysis, subclasses : _*)
-		val subclassSet = subclasses.toSet
-		val ds = Discovery(subclassSet, Set.empty)(Tests.allDefs(analysis))
-		ds.flatMap {
-			case (definition, Discovered(subs,_,_,true)) =>
-				if((subs & subclassSet).isEmpty) Nil else definition.name :: Nil
-			case _ => Nil
-		}
-	}
 
 	def initialSession(structure: sbt.BuildStructure, rootEval: () => Eval, s: State): SessionSettings = {
 		val session = s get Keys.sessionSettings

main/src/main/scala/sbt/Main.scala

@@ -89,7 +89,7 @@ object BuiltinCommands
 	def ScriptCommands: Seq[Command] = Seq(ignore, exit, Script.command, setLogLevel, early, act, nop)
 	def DefaultCommands: Seq[Command] = Seq(ignore, help, completionsCommand, about, tasks, settingsCommand, loadProject,
 		projects, project, reboot, read, history, set, sessionCommand, inspect, loadProjectImpl, loadFailed, Cross.crossBuild, Cross.switchVersion,
-		setOnFailure, clearOnFailure, stashOnFailure, popOnFailure, setLogLevel,
+		setOnFailure, clearOnFailure, stashOnFailure, popOnFailure, setLogLevel, plugin, plugins,
 		ifLast, multi, shell, continuous, eval, alias, append, last, lastGrep, export, boot, nop, call, exit, early, initialize, act) ++
 		compatCommands
 	def DefaultBootCommands: Seq[String] = LoadProject :: (IfLast + " " + Shell) :: Nil
@@ -125,7 +125,8 @@ object BuiltinCommands
 
 	def aboutPlugins(e: Extracted): String =
 	{
-		val allPluginNames = e.structure.units.values.flatMap(_.unit.plugins.pluginNames).toSeq.distinct
+		def list(b: BuildUnit) = b.plugins.detected.autoPlugins.values.map(_.label) ++ b.plugins.detected.plugins.names
+		val allPluginNames = e.structure.units.values.flatMap(u => list(u.unit)).toSeq.distinct
 		if(allPluginNames.isEmpty) "" else allPluginNames.mkString("Available Plugins: ", ", ", "")
 	}
 	def aboutScala(s: State, e: Extracted): String =
@@ -374,6 +375,20 @@ object BuiltinCommands
 			Help.detailOnly(taskDetail(allTaskAndSettingKeys(s)))
 		else
 			Help.empty
+	def plugins = Command.command(PluginsCommand, pluginsBrief, pluginsDetailed) { s =>
+		val helpString = NaturesDebug.helpAll(s)
+		System.out.println(helpString)
+		s
+	}
+	val pluginParser: State => Parser[AutoPlugin] = s => {
+		val autoPlugins: Map[String, AutoPlugin] = NaturesDebug.autoPluginMap(s)
+		token(Space) ~> Act.knownIDParser(autoPlugins, "plugin")
+	}
+	def plugin = Command(PluginCommand)(pluginParser) { (s, plugin) =>
+		val helpString = NaturesDebug.help(plugin, s)
+		System.out.println(helpString)
+		s
+	}
 
 	def projects = Command(ProjectsCommand, (ProjectsCommand, projectsBrief), projectsDetailed )(s => projectsParser(s).?) {
 		case (s, Some(modifyBuilds)) => transformExtraBuilds(s, modifyBuilds)

main/src/main/scala/sbt/Natures.scala

@@ -0,0 +1,207 @@
+package sbt
+/*
+TODO:
+- index all available AutoPlugins to get the tasks that will be added
+- error message when a task doesn't exist that it would be provided by plugin x, enabled by natures y,z, blocked by a, b
+*/
+
+	import logic.{Atom, Clause, Clauses, Formula, Literal, Logic, Negated}
+	import Logic.{CyclicNegation, InitialContradictions, InitialOverlap, LogicException}
+	import Def.Setting
+	import Natures._
+
+/** Marks a top-level object so that sbt will wildcard import it for .sbt files, `consoleProject`, and `set`. */
+trait AutoImport
+
+/**
+An AutoPlugin defines a group of settings and the conditions where the settings are automatically added to a build (called "activation").
+The `select` method defines the conditions and a method like `projectSettings` defines the settings to add.
+
+Steps for plugin authors:
+1. Determine the [[Nature]]s that, when present (or absent), activate the AutoPlugin.
+2. Determine the settings/configurations to automatically inject when activated.
+
+For example, the following will automatically add the settings in `projectSettings`
+  to a project that has both the `Web` and `Javascript` natures enabled.
+
+    object MyPlugin extends AutoPlugin {
+        def select = Web && Javascript
+        override def projectSettings = Seq(...)
+    }
+
+Steps for users:
+1. Add dependencies on plugins as usual with addSbtPlugin
+2. Add Natures to Projects, which will automatically select the plugin settings to add for those Projects.
+3. Exclude plugins, if desired.
+
+For example, given natures Web and Javascript (perhaps provided by plugins added with addSbtPlugin),
+
+  <Project>.natures( Web && Javascript )
+
+will activate `MyPlugin` defined above and have its settings automatically added.  If the user instead defines
+
+  <Project>.natures( Web && Javascript && !MyPlugin)
+
+then the `MyPlugin` settings (and anything that activates only when `MyPlugin` is activated) will not be added.
+*/
+abstract class AutoPlugin extends Natures.Basic
+{
+	/** This AutoPlugin will be activated for a project when the [[Natures]] matcher returned by this method matches that project's natures
+	* AND the user does not explicitly exclude the Nature returned by `provides`.
+	*
+	* For example, if this method returns `Web && Javascript`, this plugin instance will only be added
+	* if the `Web` and `Javascript` natures are enabled. */
+	def select: Natures
+
+	val label: String = getClass.getName.stripSuffix("$")
+
+	/** The [[Configuration]]s to add to each project that activates this AutoPlugin.*/
+	def projectConfigurations: Seq[Configuration] = Nil
+
+	/** The [[Setting]]s to add in the scope of each project that activates this AutoPlugin. */
+	def projectSettings: Seq[Setting[_]] = Nil
+
+	/** The [[Setting]]s to add to the build scope for each project that activates this AutoPlugin.
+	* The settings returned here are guaranteed to be added to a given build scope only once
+	* regardless of how many projects for that build activate this AutoPlugin. */
+	def buildSettings: Seq[Setting[_]] = Nil
+
+	/** The [[Setting]]s to add to the global scope exactly once if any project activates this AutoPlugin. */
+	def globalSettings: Seq[Setting[_]] = Nil
+
+	// TODO?: def commands: Seq[Command]
+
+	def unary_! : Exclude = Exclude(this)
+}
+
+/** An error that occurs when auto-plugins aren't configured properly.
+* It translates the error from the underlying logic system to be targeted at end users. */
+final class AutoPluginException private(val message: String, val origin: Option[LogicException]) extends RuntimeException(message)
+{
+	/** Prepends `p` to the error message derived from `origin`. */
+	def withPrefix(p: String) = new AutoPluginException(p + message, origin)
+}
+object AutoPluginException
+{
+	def apply(msg: String): AutoPluginException = new AutoPluginException(msg, None)
+	def apply(origin: LogicException): AutoPluginException = new AutoPluginException(Natures.translateMessage(origin), Some(origin))
+}
+
+/** An expression that matches `Nature`s. */
+sealed trait Natures {
+	def && (o: Basic): Natures
+}
+
+/** Represents a feature or conceptual group of settings.
+* `label` is the unique ID for this nature. */
+final case class Nature(label: String) extends Basic {
+	/** Constructs a Natures matcher that excludes this Nature. */
+	override def toString = label
+}
+
+object Natures
+{
+	/** Given the available auto plugins `defined`, returns a function that selects [[AutoPlugin]]s for the provided [[Nature]]s.
+	* The [[AutoPlugin]]s are topologically sorted so that a selected [[AutoPlugin]] comes before its selecting [[AutoPlugin]].*/
+	def compile(defined: List[AutoPlugin]): Natures => Seq[AutoPlugin] =
+		if(defined.isEmpty)
+			Types.const(Nil)
+		else
+		{
+			val byAtom = defined.map(x => (Atom(x.label), x))
+			val byAtomMap = byAtom.toMap
+			if(byAtom.size != byAtomMap.size) duplicateProvidesError(byAtom)
+			val clauses = Clauses( defined.map(d => asClause(d)) )
+			requestedNatures =>
+				Logic.reduce(clauses, flattenConvert(requestedNatures).toSet) match {
+					case Left(problem) => throw AutoPluginException(problem)
+					case Right(results) =>
+						// results includes the originally requested (positive) atoms,
+						//   which won't have a corresponding AutoPlugin to map back to
+						results.ordered.flatMap(a => byAtomMap.get(a).toList)
+				}
+		}
+
+	private[sbt] def translateMessage(e: LogicException) = e match {
+		case ic: InitialContradictions => s"Contradiction in selected natures.  These natures were both included and excluded: ${literalsString(ic.literals.toSeq)}"
+		case io: InitialOverlap => s"Cannot directly enable plugins.  Plugins are enabled when their required natures are satisifed.  The directly selected plugins were: ${literalsString(io.literals.toSeq)}"
+		case cn: CyclicNegation => s"Cycles in plugin requirements cannot involve excludes.  The problematic cycle is: ${literalsString(cn.cycle)}"
+	}
+	private[this] def literalsString(lits: Seq[Literal]): String =
+		lits map { case Atom(l) => l; case Negated(Atom(l)) => l } mkString(", ")
+
+	private[this] def duplicateProvidesError(byAtom: Seq[(Atom, AutoPlugin)]) {
+		val dupsByAtom = byAtom.groupBy(_._1).mapValues(_.map(_._2))
+		val dupStrings = for( (atom, dups) <- dupsByAtom if dups.size > 1 ) yield
+			s"${atom.label} by ${dups.mkString(", ")}"
+		val (ns, nl) = if(dupStrings.size > 1) ("s", "\n\t") else ("", " ")
+		val message = s"Nature$ns provided by multiple AutoPlugins:$nl${dupStrings.mkString(nl)}"
+		throw AutoPluginException(message)
+	}
+
+	/** [[Natures]] instance that doesn't require any [[Nature]]s. */
+	def empty: Natures = Empty
+	private[sbt] final object Empty extends Natures {
+		def &&(o: Basic): Natures = o
+		override def toString = "<none>"
+	}
+
+	/** An included or excluded Nature.  TODO: better name than Basic. */
+	sealed abstract class Basic extends Natures {
+		def &&(o: Basic): Natures = And(this :: o :: Nil)
+	}
+	private[sbt] final case class Exclude(n: AutoPlugin) extends Basic  {
+		override def toString = s"!$n"
+	}
+	private[sbt] final case class And(natures: List[Basic]) extends Natures {
+		def &&(o: Basic): Natures = And(o :: natures)
+		override def toString = natures.mkString(", ")
+	}
+	private[sbt] def and(a: Natures, b: Natures) = b match {
+		case Empty => a
+		case And(ns) => (a /: ns)(_ && _)
+		case b: Basic => a && b
+	}
+	private[sbt] def remove(a: Natures, del: Set[Basic]): Natures = a match {
+		case b: Basic => if(del(b)) Empty else b
+		case Empty => Empty
+		case And(ns) =>
+			val removed = ns.filterNot(del)
+			if(removed.isEmpty) Empty else And(removed)
+	}
+
+	/** Defines a clause for `ap` such that the [[Nature]] provided by `ap` is the head and the selector for `ap` is the body. */
+	private[sbt] def asClause(ap: AutoPlugin): Clause =
+		Clause( convert(ap.select), Set(Atom(ap.label)) )
+
+	private[this] def flattenConvert(n: Natures): Seq[Literal] = n match {
+		case And(ns) => convertAll(ns)
+		case b: Basic => convertBasic(b) :: Nil
+		case Empty => Nil
+	}
+	private[sbt] def flatten(n: Natures): Seq[Basic] = n match {
+		case And(ns) => ns
+		case b: Basic => b :: Nil
+		case Empty => Nil
+	}
+
+	private[this] def convert(n: Natures): Formula = n match {
+		case And(ns) => convertAll(ns).reduce[Formula](_ && _)
+		case b: Basic => convertBasic(b)
+		case Empty => Formula.True
+	}
+	private[this] def convertBasic(b: Basic): Literal = b match {
+		case Exclude(n) => !convertBasic(n)
+		case Nature(s) => Atom(s)
+		case a: AutoPlugin => Atom(a.label)
+	}
+	private[this] def convertAll(ns: Seq[Basic]): Seq[Literal] = ns map convertBasic
+
+	/** True if the select clause `n` is satisifed by `model`. */
+	def satisfied(n: Natures, model: Set[AutoPlugin], natures: Set[Nature]): Boolean =
+		flatten(n) forall {
+			case Exclude(a) => !model(a)
+			case n: Nature => natures(n)
+			case ap: AutoPlugin => model(ap)
+		}
+}

main/src/main/scala/sbt/NaturesDebug.scala

@@ -0,0 +1,386 @@
+package sbt
+
+	import Def.Setting
+	import Natures._
+	import NaturesDebug._
+	import java.net.URI
+
+private[sbt] class NaturesDebug(val available: List[AutoPlugin], val nameToKey: Map[String, AttributeKey[_]], val provided: Relation[AutoPlugin, AttributeKey[_]])
+{
+	/** The set of [[AutoPlugin]]s that might define a key named `keyName`.
+	* Because plugins can define keys in different scopes, this should only be used as a guideline. */
+	def providers(keyName: String): Set[AutoPlugin] = nameToKey.get(keyName) match {
+		case None => Set.empty
+		case Some(key) => provided.reverse(key)
+	}
+	/** Describes alternative approaches for defining key [[keyName]] in [[context]].*/
+	def toEnable(keyName: String, context: Context): List[PluginEnable] =
+		providers(keyName).toList.map(plugin => pluginEnable(context, plugin))
+
+	/** Provides text to suggest how [[notFoundKey]] can be defined in [[context]]. */
+	def debug(notFoundKey: String, context: Context): String =
+	{
+		val (activated, deactivated) = Util.separate(toEnable(notFoundKey, context)) {
+			case pa: PluginActivated => Left(pa)
+			case pd: EnableDeactivated => Right(pd)
+		}
+		val activePrefix = if(activated.nonEmpty) s"Some already activated plugins define $notFoundKey: ${activated.mkString(", ")}\n" else ""
+		activePrefix + debugDeactivated(notFoundKey, deactivated)
+	}
+	private[this] def debugDeactivated(notFoundKey: String, deactivated: Seq[EnableDeactivated]): String =
+	{
+		val (impossible, possible) = Util.separate(deactivated) {
+			case pi: PluginImpossible => Left(pi)
+			case pr: PluginRequirements => Right(pr)
+		}
+		if(possible.nonEmpty) {
+			val explained = possible.map(explainPluginEnable)
+			val possibleString =
+				if(explained.size > 1) explained.zipWithIndex.map{case (s,i) => s"$i. $s"}.mkString("Multiple plugins are available that can provide $notFoundKey:\n", "\n", "")
+				else s"$notFoundKey is provided by an available (but not activated) plugin:\n${explained.mkString}"
+			def impossiblePlugins = impossible.map(_.plugin.label).mkString(", ")
+			val imPostfix = if(impossible.isEmpty) "" else s"\n\nThere are other available plugins that provide $notFoundKey, but they are impossible to add: $impossiblePlugins"
+			possibleString + imPostfix
+		}
+		else if(impossible.isEmpty)
+			s"No available plugin provides key $notFoundKey."
+		else {
+			val explanations = impossible.map(explainPluginEnable)
+			explanations.mkString(s"Plugins are available that could provide $notFoundKey, but they are impossible to add:\n\t", "\n\t", "")
+		}
+	}
+
+	/** Text that suggests how to activate [[plugin]] in [[context]] if possible and if it is not already activated.*/
+	def help(plugin: AutoPlugin, context: Context): String =
+		if(context.enabled.contains(plugin))
+			activatedHelp(plugin)
+		else
+			deactivatedHelp(plugin, context)
+	private def activatedHelp(plugin: AutoPlugin): String =
+	{
+		val prefix = s"${plugin.label} is activated."
+		val keys = provided.forward(plugin)
+		val keysString = if(keys.isEmpty) "" else s"\nIt may affect these keys: ${multi(keys.toList.map(_.label))}"
+		val configs = plugin.projectConfigurations
+		val confsString = if(configs.isEmpty) "" else s"\nIt defines these configurations: ${multi(configs.map(_.name))}"
+		prefix + keysString + confsString
+	}
+	private def deactivatedHelp(plugin: AutoPlugin, context: Context): String =
+	{
+		val prefix = s"${plugin.label} is NOT activated."
+		val keys = provided.forward(plugin)
+		val keysString = if(keys.isEmpty) "" else s"\nActivating it may affect these keys: ${multi(keys.toList.map(_.label))}"
+		val configs = plugin.projectConfigurations
+		val confsString = if(configs.isEmpty) "" else s"\nActivating it will define these configurations: ${multi(configs.map(_.name))}"
+		val toActivate = explainPluginEnable(pluginEnable(context, plugin))
+		s"$prefix$keysString$confsString\n$toActivate"
+	}
+
+	private[this] def multi(strs: Seq[String]): String = strs.mkString(if(strs.size > 4) "\n\t" else ", ")
+}
+
+private[sbt] object NaturesDebug
+{
+	def helpAll(s: State): String =
+		if(Project.isProjectLoaded(s))
+		{
+			val extracted = Project.extract(s)
+			import extracted._
+			def helpBuild(uri: URI, build: LoadedBuildUnit): String =
+			{
+				val pluginStrings = for(plugin <- availableAutoPlugins(build)) yield {
+					val activatedIn = build.defined.values.toList.filter(_.autoPlugins.contains(plugin)).map(_.id)
+					val actString = if(activatedIn.nonEmpty) activatedIn.mkString(": enabled in ", ", ", "") else "" // TODO: deal with large builds
+					s"\n\t${plugin.label}$actString"
+				}
+				s"In $uri${pluginStrings.mkString}"
+			}
+			val buildStrings = for((uri, build) <- structure.units) yield helpBuild(uri, build)
+			buildStrings.mkString("\n")
+		}
+		else
+			"No project is currently loaded."
+
+	def autoPluginMap(s: State): Map[String, AutoPlugin] =
+	{
+		val extracted = Project.extract(s)
+		import extracted._
+		structure.units.values.toList.flatMap(availableAutoPlugins).map(plugin => (plugin.label, plugin)).toMap
+	}
+	private[this] def availableAutoPlugins(build: LoadedBuildUnit): Seq[AutoPlugin] =
+		build.unit.plugins.detected.autoPlugins.values
+
+	def help(plugin: AutoPlugin, s: State): String =
+	{
+		val extracted = Project.extract(s)
+		import extracted._
+		def definesPlugin(p: ResolvedProject): Boolean = p.autoPlugins.contains(plugin)
+		def projectForRef(ref: ProjectRef): ResolvedProject = get(Keys.thisProject in ref)
+		val perBuild: Map[URI, Set[AutoPlugin]] = structure.units.mapValues(unit => availableAutoPlugins(unit).toSet)
+		val pluginsThisBuild = perBuild.getOrElse(currentRef.build, Set.empty).toList
+		lazy val context = Context(currentProject.natures, currentProject.autoPlugins, Natures.compile(pluginsThisBuild), pluginsThisBuild)
+		lazy val debug = NaturesDebug(context.available)
+		if(!pluginsThisBuild.contains(plugin)) {
+			val availableInBuilds: List[URI] = perBuild.toList.filter(_._2(plugin)).map(_._1)
+			s"Plugin ${plugin.label} is only available in builds:\n\t${availableInBuilds.mkString("\n\t")}\nSwitch to a project in one of those builds using `project` and rerun this command for more information."
+		} else if(definesPlugin(currentProject))
+			debug.activatedHelp(plugin)
+		else {
+			val thisAggregated = BuildUtil.dependencies(structure.units).aggregateTransitive.getOrElse(currentRef, Nil)
+			val definedInAggregated = thisAggregated.filter(ref => definesPlugin(projectForRef(ref)))
+			if(definedInAggregated.nonEmpty) {
+				val projectNames = definedInAggregated.map(_.project) // TODO: usually in this build, but could technically require the build to be qualified
+				s"Plugin ${plugin.label} is not activated on this project, but this project aggregates projects where it is activated:\n\t${projectNames.mkString("\n\t")}"
+			} else {
+				val base = debug.deactivatedHelp(plugin, context)
+				val aggNote = if(thisAggregated.nonEmpty) "Note: This project aggregates other projects and this" else "Note: This"
+				val common =  " information is for this project only."
+				val helpOther = "To see how to activate this plugin for another project, change to the project using `project <name>` and rerun this command."
+				s"$base\n$aggNote$common\n$helpOther"
+			}
+		}
+	}
+
+	/** Precomputes information for debugging natures and plugins. */
+	def apply(available: List[AutoPlugin]): NaturesDebug =
+	{
+		val keyR = definedKeys(available)
+		val nameToKey: Map[String, AttributeKey[_]] = keyR._2s.toList.map(key => (key.label, key)).toMap
+		new NaturesDebug(available, nameToKey, keyR)
+	}
+
+	/** The context for debugging a plugin (de)activation.
+	* @param initial The initially defined [[Nature]]s.
+	* @param enabled The resulting model.
+	* @param compile The function used to compute the model.
+	* @param available All [[AutoPlugin]]s available for consideration. */
+	final case class Context(initial: Natures, enabled: Seq[AutoPlugin], compile: Natures => Seq[AutoPlugin], available: List[AutoPlugin])
+
+	/** Describes the steps to activate a plugin in some context. */
+	sealed abstract class PluginEnable
+	/** Describes a [[plugin]] that is already activated in the [[context]].*/
+	final case class PluginActivated(plugin: AutoPlugin, context: Context) extends PluginEnable
+	sealed abstract class EnableDeactivated extends PluginEnable
+	/** Describes a [[plugin]] that cannot be activated in a [[context]] due to [[contradictions]] in requirements. */
+	final case class PluginImpossible(plugin: AutoPlugin, context: Context, contradictions: Set[AutoPlugin]) extends EnableDeactivated
+
+	/** Describes the requirements for activating [[plugin]] in [[context]].
+	* @param context The base natures, exclusions, and ultimately activated plugins
+	* @param blockingExcludes Existing exclusions that prevent [[plugin]] from being activated and must be dropped
+	* @param enablingNatures [[Nature]]s that are not currently enabled, but need to be enabled for [[plugin]] to activate
+	* @param extraEnabledPlugins Plugins that will be enabled as a result of [[plugin]] activating, but are not required for [[plugin]] to activate
+	* @param willRemove Plugins that will be deactivated as a result of [[plugin]] activating
+	* @param deactivate Describes plugins that must be deactivated for [[plugin]] to activate.  These require an explicit exclusion or dropping a transitive [[Nature]].*/
+	final case class PluginRequirements(plugin: AutoPlugin, context: Context, blockingExcludes: Set[AutoPlugin], enablingNatures: Set[Nature], extraEnabledPlugins: Set[AutoPlugin], willRemove: Set[AutoPlugin], deactivate: List[DeactivatePlugin]) extends EnableDeactivated
+
+	/** Describes a [[plugin]] that must be removed in order to activate another plugin in some context.
+	* The [[plugin]] can always be directly, explicitly excluded.
+	* @param removeOneOf If non-empty, removing one of these [[Nature]]s will deactivate [[plugin]] without affecting the other plugin.  If empty, a direct exclusion is required.
+	* @param newlySelected If false, this plugin was selected in the original context.  */
+	final case class DeactivatePlugin(plugin: AutoPlugin, removeOneOf: Set[Nature], newlySelected: Boolean)
+
+	/** Determines how to enable [[plugin]] in [[context]]. */
+	def pluginEnable(context: Context, plugin: AutoPlugin): PluginEnable =
+		if(context.enabled.contains(plugin))
+			PluginActivated(plugin, context)
+		else
+			enableDeactivated(context, plugin)
+
+	private[this] def enableDeactivated(context: Context, plugin: AutoPlugin): PluginEnable =
+	{
+		// deconstruct the context
+		val initialModel = context.enabled.toSet
+		val initial = flatten(context.initial)
+		val initialNatures = natures(initial)
+		val initialExcludes = excludes(initial)
+
+		val minModel = minimalModel(plugin)
+
+		/* example 1
+		A :- B, not C
+		C :- D, E
+		initial: B, D, E
+		propose: drop D or E
+
+		initial: B, not A
+		propose: drop 'not A'
+
+		example 2
+		A :- B, not C
+		C :- B
+		initial: <empty>
+		propose: B, exclude C
+		*/
+
+		// `plugin` will only be activated when all of these natures are activated
+		// Deactivating any one of these would deactivate `plugin`.
+		val minRequiredNatures = natures(minModel)
+
+		// `plugin` will only be activated when all of these plugins are activated
+		// Deactivating any one of these would deactivate `plugin`.
+		val minRequiredPlugins = minModel.collect{ case a: AutoPlugin => a }.toSet
+
+		// The presence of any one of these plugins would deactivate `plugin`
+		val minAbsentPlugins = excludes(minModel).toSet
+
+		// Plugins that must be both activated and deactivated for `plugin` to activate.
+		//  A non-empty list here cannot be satisfied and is an error.
+		val contradictions = minAbsentPlugins & minRequiredPlugins
+
+		if(contradictions.nonEmpty)
+			PluginImpossible(plugin, context, contradictions)
+		else
+		{
+			// Natures that the user has to add to the currently selected natures in order to enable `plugin`.
+			val addToExistingNatures = minRequiredNatures -- initialNatures
+
+			// Plugins that are currently excluded that need to be allowed.
+			val blockingExcludes = initialExcludes & minRequiredPlugins
+
+			// The model that results when the minimal natures are enabled and the minimal plugins are excluded.
+			//  This can include more plugins than just `minRequiredPlugins` because the natures required for `plugin`
+			//  might activate other plugins as well.
+			val modelForMin = context.compile(and(includeAll(minRequiredNatures), excludeAll(minAbsentPlugins)))
+
+			val incrementalInputs = and( includeAll(minRequiredNatures ++ initialNatures), excludeAll(minAbsentPlugins ++ initialExcludes -- minRequiredPlugins))
+			val incrementalModel = context.compile(incrementalInputs).toSet
+
+			// Plugins that are newly enabled as a result of selecting the natures needed for `plugin`, but aren't strictly required for `plugin`.
+			//   These could be excluded and `plugin` and the user's current plugins would still be activated.
+			val extraPlugins = incrementalModel.toSet -- minRequiredPlugins -- initialModel
+
+			// Plugins that will no longer be enabled as a result of enabling `plugin`.
+			val willRemove = initialModel -- incrementalModel
+
+			// Determine the plugins that must be independently deactivated.
+			// If both A and B must be deactivated, but A transitively depends on B, deactivating B will deactivate A.
+			// If A must be deactivated, but one if its (transitively) required natures isn't present, it won't be activated.
+			//   So, in either of these cases, A doesn't need to be considered further and won't be included in this set.
+			val minDeactivate = minAbsentPlugins.filter(p => Natures.satisfied(p.select, incrementalModel, natures(flatten(incrementalInputs))))
+
+			val deactivate = for(d <- minDeactivate.toList) yield {
+				// removing any one of these natures will deactivate `d`.  TODO: This is not an especially efficient implementation.
+				val removeToDeactivate = natures(minimalModel(d)) -- minRequiredNatures
+				val newlySelected = !initialModel(d)
+				// a. suggest removing a nature in removeOneToDeactivate to deactivate d
+				// b. suggest excluding `d` to directly deactivate it in any case
+				// c. note whether d was already activated (in context.enabled) or is newly selected
+				DeactivatePlugin(d, removeToDeactivate, newlySelected)
+			}
+
+			PluginRequirements(plugin, context, blockingExcludes, addToExistingNatures, extraPlugins, willRemove, deactivate)
+		}
+	}
+
+	private[this] def includeAll[T <: Basic](basic: Set[T]): Natures = And(basic.toList)
+	private[this] def excludeAll(plugins: Set[AutoPlugin]): Natures = And(plugins map (p => Exclude(p)) toList)
+
+	private[this] def excludes(bs: Seq[Basic]): Set[AutoPlugin] = bs.collect { case Exclude(b) => b }.toSet
+	private[this] def natures(bs: Seq[Basic]): Set[Nature] = bs.collect { case n: Nature => n }.toSet
+
+	// If there is a model that includes `plugin`, it includes at least what is returned by this method.
+	// This is the list of natures and plugins that must be included as well as list of plugins that must not be present.
+	// It might not be valid, such as if there are contradictions or if there are cycles that are unsatisfiable.
+	// The actual model might be larger, since other plugins might be enabled by the selected natures.
+	private[this] def minimalModel(plugin: AutoPlugin): Seq[Basic] = Dag.topologicalSortUnchecked(plugin: Basic) {
+		case _: Exclude | _: Nature => Nil
+		case ap: AutoPlugin => Natures.flatten(ap.select)
+	}
+
+	/** String representation of [[PluginEnable]], intended for end users. */
+	def explainPluginEnable(ps: PluginEnable): String =
+		ps match {
+			case PluginRequirements(plugin, context, blockingExcludes, enablingNatures, extraEnabledPlugins, toBeRemoved, deactivate) =>
+				def indent(str: String) = if(str.isEmpty) "" else s"\t$str"
+				def note(str: String) = if(str.isEmpty) "" else s"Note: $str"
+				val parts =
+					indent(excludedError(false /* TODO */, blockingExcludes.toList)) ::
+					indent(required(enablingNatures.toList)) ::
+					indent(needToDeactivate(deactivate)) ::
+					note(willAdd(plugin, extraEnabledPlugins.toList)) ::
+					note(willRemove(plugin, toBeRemoved.toList)) ::
+					Nil
+				parts.filterNot(_.isEmpty).mkString("\n")
+			case PluginImpossible(plugin, context, contradictions) => pluginImpossible(plugin, contradictions)
+			case PluginActivated(plugin, context) => s"Plugin ${plugin.label} already activated."
+		}
+
+	/** Provides a [[Relation]] between plugins and the keys they potentially define.
+	* Because plugins can define keys in different scopes and keys can be overridden, this is not definitive.*/
+	def definedKeys(available: List[AutoPlugin]): Relation[AutoPlugin, AttributeKey[_]] =
+	{
+		def extractDefinedKeys(ss: Seq[Setting[_]]): Seq[AttributeKey[_]] =
+			ss.map(_.key.key)
+		def allSettings(p: AutoPlugin): Seq[Setting[_]] = p.projectSettings ++ p.buildSettings ++ p.globalSettings
+		val empty = Relation.empty[AutoPlugin, AttributeKey[_]]
+		(empty /: available)( (r,p) => r + (p, extractDefinedKeys(allSettings(p))) )
+	}
+
+	private[this] def excludedError(transitive: Boolean, dependencies: List[AutoPlugin]): String =
+		str(dependencies)(excludedPluginError(transitive), excludedPluginsError(transitive))
+
+	private[this] def excludedPluginError(transitive: Boolean)(dependency: AutoPlugin) =
+		s"Required ${transitiveString(transitive)}dependency ${dependency.label} was excluded."
+	private[this] def excludedPluginsError(transitive: Boolean)(dependencies: List[AutoPlugin]) =
+		s"Required ${transitiveString(transitive)}dependencies were excluded:\n\t${labels(dependencies).mkString("\n\t")}"
+	private[this] def transitiveString(transitive: Boolean) =
+		if(transitive) "(transitive) " else ""
+
+	private[this] def required(natures: List[Nature]): String =
+		str(natures)(requiredNature, requiredNatures)
+
+	private[this] def requiredNature(nature: Nature) =
+		s"Required nature ${nature.label} not present."
+	private[this] def requiredNatures(natures: List[Nature]) =
+		s"Required natures not present:\n\t${natures.map(_.label).mkString("\n\t")}"
+
+	private[this] def str[A](list: List[A])(f: A => String, fs: List[A] => String): String = list match {
+		case Nil => ""
+		case single :: Nil => f(single)
+		case _ => fs(list)
+	}
+
+	private[this] def willAdd(base: AutoPlugin, plugins: List[AutoPlugin]): String =
+		str(plugins)(willAddPlugin(base), willAddPlugins(base))
+
+	private[this] def willAddPlugin(base: AutoPlugin)(plugin: AutoPlugin) =
+		s"Enabling ${base.label} will also enable ${plugin.label}"
+	private[this] def willAddPlugins(base: AutoPlugin)(plugins: List[AutoPlugin]) =
+		s"Enabling ${base.label} will also enable:\n\t${labels(plugins).mkString("\n\t")}"
+
+	private[this] def willRemove(base: AutoPlugin, plugins: List[AutoPlugin]): String =
+		str(plugins)(willRemovePlugin(base), willRemovePlugins(base))
+
+	private[this] def willRemovePlugin(base: AutoPlugin)(plugin: AutoPlugin) =
+		s"Enabling ${base.label} will disable ${plugin.label}"
+	private[this] def willRemovePlugins(base: AutoPlugin)(plugins: List[AutoPlugin]) =
+		s"Enabling ${base.label} will disable:\n\t${labels(plugins).mkString("\n\t")}"
+
+	private[this] def labels(plugins: List[AutoPlugin]): List[String] =
+		plugins.map(_.label)
+
+	private[this] def needToDeactivate(deactivate: List[DeactivatePlugin]): String =
+		str(deactivate)(deactivate1, deactivateN)
+	private[this] def deactivateN(plugins: List[DeactivatePlugin]): String =
+		plugins.map(deactivateString).mkString("These plugins need to be deactivated:\n\t", "\n\t", "")
+	private[this] def deactivate1(deactivate: DeactivatePlugin): String =
+		s"Need to deactivate ${deactivateString(deactivate)}"
+	private[this] def deactivateString(d: DeactivatePlugin): String =
+	{
+		val removeNaturesString: String =
+			d.removeOneOf.toList match {
+				case Nil => ""
+				case x :: Nil => s" or no longer include $x"
+				case xs => s" or remove one of ${xs.mkString(", ")}"
+			}
+		s"${d.plugin.label}: directly exclude it${removeNaturesString}"
+	}
+
+	private[this] def pluginImpossible(plugin: AutoPlugin, contradictions: Set[AutoPlugin]): String =
+		str(contradictions.toList)(pluginImpossible1(plugin), pluginImpossibleN(plugin))
+
+	private[this] def pluginImpossible1(plugin: AutoPlugin)(contradiction: AutoPlugin): String =
+		s"There is no way to enable plugin ${plugin.label}.  It (or its dependencies) requires plugin ${contradiction.label} to both be present and absent.  Please report the problem to the plugin's author."
+	private[this] def pluginImpossibleN(plugin: AutoPlugin)(contradictions: List[AutoPlugin]): String =
+		s"There is no way to enable plugin ${plugin.label}.  It (or its dependencies) requires these plugins to be both present and absent:\n\t${labels(contradictions).mkString("\n\t")}\nPlease report the problem to the plugin's author."
+}

main/src/main/scala/sbt/PluginDiscovery.scala

@@ -0,0 +1,135 @@
+package sbt
+
+	import java.io.File
+	import java.net.URL
+	import Attributed.data
+	import Build.analyzed
+	import xsbt.api.{Discovered,Discovery}
+
+object PluginDiscovery
+{
+	/** Relative paths of resources that list top-level modules that are available.
+	* Normally, the classes for those modules will be in the same classpath entry as the resource. */
+	object Paths
+	{
+		final val AutoPlugins = "sbt/sbt.autoplugins"
+		final val Plugins = "sbt/sbt.plugins"
+		final val Builds = "sbt/sbt.builds"
+		final val AutoImports = "sbt/sbt.autoimports"
+	}
+	/** Names of top-level modules that subclass sbt plugin-related classes: [[Plugin]], [[AutoImport]], [[AutoPlugin]], and [[Build]]. */
+	final class DiscoveredNames(val plugins: Seq[String], val autoImports: Seq[String], val autoPlugins: Seq[String], val builds: Seq[String])
+
+	def emptyDiscoveredNames: DiscoveredNames = new DiscoveredNames(Nil, Nil, Nil, Nil)
+
+	/** Discovers and loads the sbt-plugin-related top-level modules from the classpath and source analysis in `data` and using the provided class `loader`. */
+	def discoverAll(data: PluginData, loader: ClassLoader): DetectedPlugins =
+	{
+		def discover[T](resource: String)(implicit mf: reflect.ClassManifest[T]) =
+			binarySourceModules[T](data, loader, resource)
+		import Paths._
+		new DetectedPlugins(discover[Plugin](Plugins), discover[AutoImport](AutoImports), discover[AutoPlugin](AutoPlugins), discover[Build](Builds))
+	}
+
+	/** Discovers the sbt-plugin-related top-level modules from the provided source `analysis`. */
+	def discoverSourceAll(analysis: inc.Analysis): DiscoveredNames =
+	{
+		def discover[T](implicit mf: reflect.ClassManifest[T]): Seq[String] =
+			sourceModuleNames(analysis, mf.erasure.getName)
+		new DiscoveredNames(discover[Plugin], discover[AutoImport], discover[AutoPlugin], discover[Build])
+	}
+
+	// TODO: for 0.14.0, consider consolidating into a single file, which would make the classpath search 4x faster
+	/** Writes discovered module `names` to zero or more files in `dir` as per [[writeDescriptor]] and returns the list of files written. */
+	def writeDescriptors(names: DiscoveredNames, dir: File): Seq[File] =
+	{
+		import Paths._
+		val files =
+			writeDescriptor(names.plugins, dir, Plugins) ::
+			writeDescriptor(names.autoPlugins, dir, AutoPlugins) ::
+			writeDescriptor(names.builds, dir, Builds) ::
+			writeDescriptor(names.autoImports, dir, AutoImports) ::
+			Nil
+		files.flatMap(_.toList)
+	}
+
+	/** Stores the module `names` in `dir / path`, one per line, unless `names` is empty and then the file is deleted and `None` returned. */
+	def writeDescriptor(names: Seq[String], dir: File, path: String): Option[File] =
+	{
+		val descriptor: File = new File(dir, path)
+		if(names.isEmpty)
+		{
+			IO.delete(descriptor)
+			None
+		}
+		else
+		{
+			IO.writeLines(descriptor, names.distinct.sorted)
+			Some(descriptor)
+		}
+	}
+
+	/** Discovers the names of top-level modules listed in resources named `resourceName` as per [[binaryModuleNames]] or
+	* available as analyzed source and extending from any of `subclasses` as per [[sourceModuleNames]]. */
+	def binarySourceModuleNames(classpath: Seq[Attributed[File]], loader: ClassLoader, resourceName: String, subclasses: String*): Seq[String] =
+		(
+			binaryModuleNames(data(classpath), loader, resourceName) ++
+			(analyzed(classpath) flatMap ( a => sourceModuleNames(a, subclasses : _*) ))
+		).distinct
+
+	/** Discovers top-level modules in `analysis` that inherit from any of `subclasses`. */
+	def sourceModuleNames(analysis: inc.Analysis, subclasses: String*): Seq[String] =
+	{
+		val subclassSet = subclasses.toSet
+		val ds = Discovery(subclassSet, Set.empty)(Tests.allDefs(analysis))
+		ds.flatMap {
+			case (definition, Discovered(subs,_,_,true)) =>
+				if((subs & subclassSet).isEmpty) Nil else definition.name :: Nil
+			case _ => Nil
+		}
+	}
+
+	/** Obtains the list of modules identified in all resource files `resourceName` from `loader` that are on `classpath`.
+	* `classpath` and `loader` are both required to ensure that `loader`
+	* doesn't bring in any resources outside of the intended `classpath`, such as from parent loaders. */
+	def binaryModuleNames(classpath: Seq[File], loader: ClassLoader, resourceName: String): Seq[String] =
+	{
+		import collection.JavaConversions._
+		loader.getResources(resourceName).toSeq.filter(onClasspath(classpath)) flatMap { u =>
+			IO.readLinesURL(u).map( _.trim).filter(!_.isEmpty)
+		}
+	}
+
+	/** Returns `true` if `url` is an entry in `classpath`.*/
+	def onClasspath(classpath: Seq[File])(url: URL): Boolean =
+		IO.urlAsFile(url) exists (classpath.contains _)
+
+	private[sbt] def binarySourceModules[T](data: PluginData, loader: ClassLoader, resourceName: String)(implicit mf: reflect.ClassManifest[T]): DetectedModules[T] =
+	{
+		val classpath = data.classpath
+		val namesAndValues = if(classpath.isEmpty) Nil else {
+			val names = binarySourceModuleNames(classpath, loader, resourceName, mf.erasure.getName)
+			loadModules[T](data, names, loader)
+		}
+		new DetectedModules(namesAndValues)
+	}
+
+	private[this] def loadModules[T: ClassManifest](data: PluginData, names: Seq[String], loader: ClassLoader): Seq[(String,T)] =
+		try ModuleUtilities.getCheckedObjects[T](names, loader)
+		catch {
+			case e: ExceptionInInitializerError =>
+				val cause = e.getCause
+				if(cause eq null) throw e else throw cause
+			case e: LinkageError => incompatiblePlugins(data, e)
+		}
+
+	private[this] def incompatiblePlugins(data: PluginData, t: LinkageError): Nothing =
+	{
+		val evicted = data.report.toList.flatMap(_.configurations.flatMap(_.evicted))
+		val evictedModules = evicted map { id => (id.organization, id.name) } distinct ;
+		val evictedStrings = evictedModules map { case (o,n) => o + ":" + n }
+		val msgBase = "Binary incompatibility in plugins detected."
+		val msgExtra = if(evictedStrings.isEmpty) "" else "\nNote that conflicts were resolved for some dependencies:\n\t" + evictedStrings.mkString("\n\t")
+		throw new IncompatiblePluginsException(msgBase + msgExtra, t)
+	}
+}

main/src/main/scala/sbt/Project.scala

@@ -50,33 +50,52 @@ sealed trait ProjectDefinition[PR <: ProjectReference]
 	/** Configures the sources of automatically appended settings.*/
 	def auto: AddSettings
 
+	/** The [[Natures]] associated with this project.
+	A [[Nature]] is a common label that is used by plugins to determine what settings, if any, to add to a project. */
+	def natures: Natures
+
+	/** The [[AutoPlugin]]s enabled for this project.  This value is only available on a loaded Project. */
+	private[sbt] def autoPlugins: Seq[AutoPlugin]
+
 	override final def hashCode: Int = id.hashCode ^ base.hashCode ^ getClass.hashCode
 	override final def equals(o: Any) = o match {
 		case p: ProjectDefinition[_] => p.getClass == this.getClass && p.id == id && p.base == base
 		case _ => false
 	}
-	override def toString = "Project(id: " + id + ", base: " + base + ", aggregate: " + aggregate + ", dependencies: " + dependencies + ", configurations: " + configurations + ")"
+	override def toString =
+	{
+		val agg = ifNonEmpty("aggregate", aggregate)
+		val dep = ifNonEmpty("dependencies", dependencies)
+		val conf = ifNonEmpty("configurations", configurations)
+		val autos = ifNonEmpty("autoPlugins", autoPlugins.map(_.label))
+		val fields = s"id $id" :: s"base: $base" :: agg ::: dep ::: conf ::: (s"natures: List($natures)" :: autos)
+		s"Project(${fields.mkString(", ")})"
+	}
+	private[this] def ifNonEmpty[T](label: String, ts: Iterable[T]): List[String] = if(ts.isEmpty) Nil else s"$label: $ts" :: Nil
 }
 sealed trait Project extends ProjectDefinition[ProjectReference]
 {
+	// TODO: add parameters for natures and autoPlugins in 0.14.0 (not reasonable to do in a binary compatible way in 0.13)
 	def copy(id: String = id, base: File = base, aggregate: => Seq[ProjectReference] = aggregate, dependencies: => Seq[ClasspathDep[ProjectReference]] = dependencies,
 		delegates: => Seq[ProjectReference] = delegates, settings: => Seq[Setting[_]] = settings, configurations: Seq[Configuration] = configurations,
 		auto: AddSettings = auto): Project =
-			Project(id, base, aggregate = aggregate, dependencies = dependencies, delegates = delegates, settings, configurations, auto)
+			unresolved(id, base, aggregate = aggregate, dependencies = dependencies, delegates = delegates, settings, configurations, auto, natures, autoPlugins)
 
 	def resolve(resolveRef: ProjectReference => ProjectRef): ResolvedProject =
 	{
 		def resolveRefs(prs: Seq[ProjectReference]) = prs map resolveRef
 		def resolveDeps(ds: Seq[ClasspathDep[ProjectReference]]) = ds map resolveDep
 		def resolveDep(d: ClasspathDep[ProjectReference]) = ResolvedClasspathDependency(resolveRef(d.project), d.configuration)
-		resolved(id, base, aggregate = resolveRefs(aggregate), dependencies = resolveDeps(dependencies), delegates = resolveRefs(delegates), settings, configurations, auto)
+		resolved(id, base, aggregate = resolveRefs(aggregate), dependencies = resolveDeps(dependencies), delegates = resolveRefs(delegates),
+			settings, configurations, auto, natures, autoPlugins)
 	}
 	def resolveBuild(resolveRef: ProjectReference => ProjectReference): Project =
 	{
 		def resolveRefs(prs: Seq[ProjectReference]) = prs map resolveRef
 		def resolveDeps(ds: Seq[ClasspathDep[ProjectReference]]) = ds map resolveDep
 		def resolveDep(d: ClasspathDep[ProjectReference]) = ClasspathDependency(resolveRef(d.project), d.configuration)
-		apply(id, base, aggregate = resolveRefs(aggregate), dependencies = resolveDeps(dependencies), delegates = resolveRefs(delegates), settings, configurations, auto)
+		unresolved(id, base, aggregate = resolveRefs(aggregate), dependencies = resolveDeps(dependencies), delegates = resolveRefs(delegates),
+			settings, configurations, auto, natures, autoPlugins)
 	}
 
 	/** Applies the given functions to this Project.
@@ -116,8 +135,30 @@ sealed trait Project extends ProjectDefinition[ProjectReference]
 	/** Sets the list of .sbt files to parse for settings to be appended to this project's settings.
 	* Any configured .sbt files are removed from this project's list.*/
 	def setSbtFiles(files: File*): Project = copy(auto = AddSettings.append( AddSettings.clearSbtFiles(auto), AddSettings.sbtFiles(files: _*)) )
+
+	/** Sets the [[Nature]]s of this project.
+	A [[Nature]] is a common label that is used by plugins to determine what settings, if any, to add to a project. */
+	def addNatures(ns: Nature*): Project = setNatures(Natures.and(natures, Natures.And(ns.toList)))
+
+	/** Disable the given plugins on this project. */
+	def disablePlugins(plugins: AutoPlugin*): Project =
+		setNatures(Natures.and(natures, Natures.And(plugins.map(p => Natures.Exclude(p)).toList)))
+
+	private[this] def setNatures(ns: Natures): Project = {
+		// TODO: for 0.14.0, use copy when it has the additional `natures` parameter
+		unresolved(id, base, aggregate = aggregate, dependencies = dependencies, delegates = delegates, settings, configurations, auto, ns, autoPlugins)
+	}
+
+	/** Definitively set the [[AutoPlugin]]s for this project. */
+	private[sbt] def setAutoPlugins(autos: Seq[AutoPlugin]): Project = {
+		// TODO: for 0.14.0, use copy when it has the additional `autoPlugins` parameter
+		unresolved(id, base, aggregate = aggregate, dependencies = dependencies, delegates = delegates, settings, configurations, auto, natures, autos)
+	}
+}
+sealed trait ResolvedProject extends ProjectDefinition[ProjectRef] {
+	/** The [[AutoPlugin]]s enabled for this project as computed from [[natures]].*/
+	def autoPlugins: Seq[AutoPlugin]
 }
-sealed trait ResolvedProject extends ProjectDefinition[ProjectRef]
 
 sealed trait ClasspathDep[PR <: ProjectReference] { def project: PR; def configuration: Option[String] }
 final case class ResolvedClasspathDependency(project: ProjectRef, configuration: Option[String]) extends ClasspathDep[ProjectRef]
@@ -150,23 +191,22 @@ object Project extends ProjectExtra
 		Def.showRelativeKey( ProjectRef(loaded.root, loaded.units(loaded.root).rootProjects.head), loaded.allProjectRefs.size > 1, keyNameColor)
 
 	private abstract class ProjectDef[PR <: ProjectReference](val id: String, val base: File, aggregate0: => Seq[PR], dependencies0: => Seq[ClasspathDep[PR]],
-		delegates0: => Seq[PR], settings0: => Seq[Def.Setting[_]], val configurations: Seq[Configuration], val auto: AddSettings) extends ProjectDefinition[PR]
+		delegates0: => Seq[PR], settings0: => Seq[Def.Setting[_]], val configurations: Seq[Configuration], val auto: AddSettings,
+		val natures: Natures, val autoPlugins: Seq[AutoPlugin]) extends ProjectDefinition[PR]
 	{
 		lazy val aggregate = aggregate0
 		lazy val dependencies = dependencies0
 		lazy val delegates = delegates0
 		lazy val settings = settings0
-	
+
 		Dag.topologicalSort(configurations)(_.extendsConfigs) // checks for cyclic references here instead of having to do it in Scope.delegates
 	}
 
+	// TODO: add parameter for natures in 0.14.0
 	def apply(id: String, base: File, aggregate: => Seq[ProjectReference] = Nil, dependencies: => Seq[ClasspathDep[ProjectReference]] = Nil,
 		delegates: => Seq[ProjectReference] = Nil, settings: => Seq[Def.Setting[_]] = defaultSettings, configurations: Seq[Configuration] = Configurations.default,
 		auto: AddSettings = AddSettings.allDefaults): Project =
-	{
+			unresolved(id, base, aggregate, dependencies, delegates, settings, configurations, auto, Natures.empty, Nil)
-		validProjectID(id).foreach(errMsg => sys.error("Invalid project ID: " + errMsg))
-		new ProjectDef[ProjectReference](id, base, aggregate, dependencies, delegates, settings, configurations, auto) with Project
-	}
 
 	/** Returns None if `id` is a valid Project ID or Some containing the parser error message if it is not.*/
 	def validProjectID(id: String): Option[String] = DefaultParsers.parse(id, DefaultParsers.ID).left.toOption
@@ -185,9 +225,23 @@ object Project extends ProjectExtra
 	* This is a best effort implementation, since valid characters are not documented or consistent.*/
 	def normalizeModuleID(id: String): String = normalizeBase(id)
 
+	@deprecated("Will be removed.", "0.13.2")
 	def resolved(id: String, base: File, aggregate: => Seq[ProjectRef], dependencies: => Seq[ResolvedClasspathDependency], delegates: => Seq[ProjectRef],
 		settings: Seq[Def.Setting[_]], configurations: Seq[Configuration], auto: AddSettings): ResolvedProject =
-			new ProjectDef[ProjectRef](id, base, aggregate, dependencies, delegates, settings, configurations, auto) with ResolvedProject
+			resolved(id, base, aggregate, dependencies, delegates, settings, configurations, auto, Natures.empty, Nil)
+
+	private def resolved(id: String, base: File, aggregate: => Seq[ProjectRef], dependencies: => Seq[ClasspathDep[ProjectRef]],
+		delegates: => Seq[ProjectRef], settings: Seq[Def.Setting[_]], configurations: Seq[Configuration], auto: AddSettings,
+		natures: Natures, autoPlugins: Seq[AutoPlugin]): ResolvedProject =
+			new ProjectDef[ProjectRef](id, base, aggregate, dependencies, delegates, settings, configurations, auto, natures, autoPlugins) with ResolvedProject
+
+	private def unresolved(id: String, base: File, aggregate: => Seq[ProjectReference], dependencies: => Seq[ClasspathDep[ProjectReference]],
+		delegates: => Seq[ProjectReference], settings: => Seq[Def.Setting[_]], configurations: Seq[Configuration], auto: AddSettings,
+		natures: Natures, autoPlugins: Seq[AutoPlugin]): Project =
+	{
+		validProjectID(id).foreach(errMsg => sys.error("Invalid project ID: " + errMsg))
+		new ProjectDef[ProjectReference](id, base, aggregate, dependencies, delegates, settings, configurations, auto, natures, autoPlugins) with Project
+	}
 
 	def defaultSettings: Seq[Def.Setting[_]] = Defaults.defaultSettings
 
@@ -307,7 +361,7 @@ object Project extends ProjectExtra
 	def details(structure: BuildStructure, actual: Boolean, scope: Scope, key: AttributeKey[_])(implicit display: Show[ScopedKey[_]]): String =
 	{
 		val scoped = ScopedKey(scope,key)
-		
+
 		val data = scopedKeyData(structure, scope, key) map {_.description} getOrElse {"No entry for key."}
 		val description = key.description match { case Some(desc) => "Description:\n\t" + desc + "\n"; case None => "" }
 
@@ -413,7 +467,7 @@ object Project extends ProjectExtra
 	import DefaultParsers._
 
 	val loadActionParser = token(Space ~> ("plugins" ^^^ Plugins | "return" ^^^ Return)) ?? Current
-	
+
 	val ProjectReturn = AttributeKey[List[File]]("project-return", "Maintains a stack of builds visited using reload.")
 	def projectReturn(s: State): List[File] = getOrNil(s, ProjectReturn)
 	def inPluginProject(s: State): Boolean = projectReturn(s).toList.length > 1

project/Sbt.scala

@@ -73,6 +73,8 @@ object Sbt extends Build
 	lazy val datatypeSub = baseProject(utilPath /"datatype", "Datatype Generator") dependsOn(ioSub)
 		// cross versioning
 	lazy val crossSub = baseProject(utilPath / "cross", "Cross") settings(inConfig(Compile)(Transform.crossGenSettings): _*)
+		// A logic with restricted negation as failure for a unique, stable model
+	lazy val logicSub = testedBaseProject(utilPath / "logic", "Logic").dependsOn(collectionSub, relationSub)
 
 	/* **** Intermediate-level Modules **** */
 
@@ -130,7 +132,7 @@ object Sbt extends Build
 		completeSub, classpathSub, stdTaskSub, processSub) settings( sbinary )
 
 		// The main integration project for sbt.  It brings all of the subsystems together, configures them, and provides for overriding conventions.
-	lazy val mainSub = testedBaseProject(mainPath, "Main") dependsOn(actionsSub, mainSettingsSub, interfaceSub, ioSub, ivySub, launchInterfaceSub, logSub, processSub, runSub, commandSub) settings(scalaXml)
+	lazy val mainSub = testedBaseProject(mainPath, "Main") dependsOn(actionsSub, mainSettingsSub, interfaceSub, ioSub, ivySub, launchInterfaceSub, logSub, logicSub, processSub, runSub, commandSub) settings(scalaXml)
 
 		// Strictly for bringing implicits and aliases from subsystems into the top-level sbt namespace through a single package object
 		//  technically, we need a dependency on all of mainSub's dependencies, but we don't do that since this is strictly an integration project

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

@@ -0,0 +1,34 @@
+// excludePlugins(C) will prevent C, and thus D, from being auto-added
+lazy val a = project.addNatures(A, B).disablePlugins(Q)
+
+// without B, C is not added
+lazy val b = project.addNatures(A)
+
+// with both A and B, C is selected, which in turn selects D
+lazy val c = project.addNatures(A, B)
+
+// with no natures defined, nothing is auto-added
+lazy val d = project
+
+
+check := {
+	val ddel = (del in d).?.value // should be None
+	same(ddel, None, "del in d")
+	val bdel = (del in b).?.value // should be None
+	same(bdel, None, "del in b")
+	val adel = (del in a).?.value // should be None
+	same(adel, None, "del in a")
+//
+	val buildValue = (demo in ThisBuild).value
+	same(buildValue, "build 0", "demo in ThisBuild")
+	val globalValue = (demo in Global).value
+	same(globalValue, "global 0", "demo in Global")
+	val projValue = (demo in c).value
+	same(projValue, "project c Q R", "demo in c")
+	val qValue = (del in c in q).value
+	same(qValue, " Q R", "del in c in q")
+}
+
+def same[T](actual: T, expected: T, label: String) {
+	assert(actual == expected, s"Expected '$expected' for `$label`, got '$actual'")
+}

sbt/src/sbt-test/project/auto-plugins/project/Q.scala

@@ -0,0 +1,59 @@
+	import sbt._
+	import sbt.Keys.{name, resolvedScoped}
+	import java.util.concurrent.atomic.{AtomicInteger => AInt}
+
+object AI extends AutoImport
+{
+	lazy val A = Nature("A")
+	lazy val B = Nature("B")
+	lazy val D = Nature("D")
+
+	lazy val q = config("q")
+	lazy val p = config("p").extend(q)
+
+	lazy val demo = settingKey[String]("A demo setting.")
+	lazy val del = settingKey[String]("Another demo setting.")
+
+	lazy val check = settingKey[Unit]("Verifies settings are as they should be.")
+}
+
+	import AI._
+
+object Q extends AutoPlugin
+{
+	def select: Natures = A && B
+
+	override def projectConfigurations: Seq[Configuration] =
+		p ::
+		q ::
+		Nil
+
+   override def projectSettings: Seq[Setting[_]] =
+		(demo := s"project ${name.value}") ::
+		(del in q := " Q") ::
+		Nil
+
+   override def buildSettings: Seq[Setting[_]] =
+		(demo := s"build ${buildCount.getAndIncrement}") ::
+		Nil
+
+   override def globalSettings: Seq[Setting[_]] =
+		(demo := s"global ${globalCount.getAndIncrement}") ::
+		Nil
+
+	// used to ensure the build-level and global settings are only added once
+	private[this] val buildCount = new AInt(0)
+	private[this] val globalCount = new AInt(0)
+}
+
+object R extends AutoPlugin
+{
+	def select = Q && !D
+
+	override def projectSettings = Seq(
+		// tests proper ordering: R requires C, so C settings should come first
+		del in q += " R",
+		// tests that configurations are properly registered, enabling delegation from p to q
+		demo += (del in p).value
+	)
+}

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

@@ -0,0 +1 @@
+> check

sbt/src/sbt-test/project/binary-plugin/changes/define/A.scala

@@ -0,0 +1,21 @@
+import sbt._
+import Keys._
+
+
+object C extends AutoImport {
+	lazy val bN = Nature("B")
+	lazy val check = taskKey[Unit]("Checks that the AutoPlugin and Build are automatically added.")
+}
+
+	import C._
+
+object A extends AutoPlugin {
+	override def select = bN
+	override def projectSettings = Seq(
+		check := {}
+	)
+}
+
+object B extends Build {
+	lazy val extra = project.addNatures(bN)
+}

sbt/src/sbt-test/project/binary-plugin/changes/define/build.sbt

@@ -0,0 +1,3 @@
+sbtPlugin := true
+
+name := "demo-plugin"

sbt/src/sbt-test/project/binary-plugin/changes/use/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("org.example" % "demo-plugin" % "3.4")

sbt/src/sbt-test/project/binary-plugin/common.sbt

@@ -0,0 +1,7 @@
+organization in ThisBuild := "org.example"
+
+version in ThisBuild := "3.4"
+
+lazy val define = project
+
+lazy val use = project

sbt/src/sbt-test/project/binary-plugin/test

@@ -0,0 +1,10 @@
+$ copy-file changes/define/build.sbt build.sbt
+$ copy-file changes/define/A.scala A.scala
+
+# reload implied
+> publishLocal
+
+$ delete build.sbt A.scala
+$ copy-file changes/use/plugins.sbt project/plugins.sbt
+> reload
+> extra/check

util/classpath/src/main/scala/sbt/ModuleUtilities.scala

@@ -6,7 +6,7 @@ package sbt
 object ModuleUtilities
 {
 	/** Reflectively loads and returns the companion object for top-level class `className` from `loader`.
-	* The class name should not include the `$` that scalac appends to the underlying jvm class for 
+	* The class name should not include the `$` that scalac appends to the underlying jvm class for
 	* a companion object. */
 	def getObject(className: String, loader: ClassLoader): AnyRef =
 	{
@@ -14,4 +14,10 @@ object ModuleUtilities
 		val singletonField = obj.getField("MODULE$")
 		singletonField.get(null)
 	}
+
+	def getCheckedObject[T](className: String, loader: ClassLoader)(implicit mf: reflect.ClassManifest[T]): T =
+		mf.erasure.cast(getObject(className, loader)).asInstanceOf[T]
+
+	def getCheckedObjects[T](classNames: Seq[String], loader: ClassLoader)(implicit mf: reflect.ClassManifest[T]): Seq[(String,T)] =
+		classNames.map(name => (name, getCheckedObject(name, loader)))
 }

util/collection/src/main/scala/sbt/Dag.scala

@@ -15,7 +15,7 @@ object Dag
 	import JavaConverters.asScalaSetConverter
 
 	def topologicalSort[T](root: T)(dependencies: T => Iterable[T]): List[T] = topologicalSort(root :: Nil)(dependencies)
-	
+
 	def topologicalSort[T](nodes: Iterable[T])(dependencies: T => Iterable[T]): List[T] =
 	{
 		val discovered = new mutable.HashSet[T]
@@ -24,7 +24,7 @@ object Dag
 		def visitAll(nodes: Iterable[T]) = nodes foreach visit
 		def visit(node : T){
 			if (!discovered(node)) {
-				discovered(node) = true; 
+				discovered(node) = true;
 				try { visitAll(dependencies(node)); } catch { case c: Cyclic => throw node :: c }
 				finished += node;
 			}
@@ -33,11 +33,13 @@ object Dag
 		}
 
 		visitAll(nodes);
-	
+
 		finished.toList;
 	}
 	// doesn't check for cycles
-	def topologicalSortUnchecked[T](node: T)(dependencies: T => Iterable[T]): List[T] =
+	def topologicalSortUnchecked[T](node: T)(dependencies: T => Iterable[T]): List[T] = topologicalSortUnchecked(node :: Nil)(dependencies)
+
+	def topologicalSortUnchecked[T](nodes: Iterable[T])(dependencies: T => Iterable[T]): List[T] =
 	{
 		val discovered = new mutable.HashSet[T]
 		var finished: List[T] = Nil
@@ -45,23 +47,23 @@ object Dag
 		def visitAll(nodes: Iterable[T]) = nodes foreach visit
 		def visit(node : T){
 			if (!discovered(node)) {
-				discovered(node) = true; 
+				discovered(node) = true;
 				visitAll(dependencies(node))
 				finished ::= node;
 			}
 		}
 
-		visit(node);
+		visitAll(nodes);
 		finished;
 	}
 	final class Cyclic(val value: Any, val all: List[Any], val complete: Boolean)
 		extends Exception( "Cyclic reference involving " +
-			(if(complete) all.mkString("\n   ", "\n   ", "") else value) 
+			(if(complete) all.mkString("\n   ", "\n   ", "") else value)
 		)
 	{
 		def this(value: Any) = this(value, value :: Nil, false)
 		override def toString = getMessage
-		def ::(a: Any): Cyclic = 
+		def ::(a: Any): Cyclic =
 			if(complete)
 				this
 			else if(a == value)
@@ -69,5 +71,62 @@ object Dag
 			else
 				new Cyclic(value, a :: all, false)
 	}
-}
 
+	/** A directed graph with edges labeled positive or negative. */
+	private[sbt] trait DirectedSignedGraph[Node]
+	{
+		/** Directed edge type that tracks the sign and target (head) vertex.
+		* The sign can be obtained via [[isNegative]] and the target vertex via [[head]]. */
+		type Arrow
+		/** List of initial nodes. */
+		def nodes: List[Arrow]
+		/** Outgoing edges for `n`. */
+		def dependencies(n: Node): List[Arrow]
+		/** `true` if the edge `a` is "negative", false if it is "positive". */
+		def isNegative(a: Arrow): Boolean
+		/** The target of the directed edge `a`. */
+		def head(a: Arrow): Node
+	}
+
+	/** Traverses a directed graph defined by `graph` looking for a cycle that includes a "negative" edge.
+	* The directed edges are weighted by the caller as "positive" or "negative".
+	* If a cycle containing a "negative" edge is detected, its member edges are returned in order.
+	* Otherwise, the empty list is returned. */
+	private[sbt] def findNegativeCycle[Node](graph: DirectedSignedGraph[Node]): List[graph.Arrow] =
+	{
+			import scala.annotation.tailrec
+			import graph._
+		val finished = new mutable.HashSet[Node]
+		val visited = new mutable.HashSet[Node]
+
+		def visit(edges: List[Arrow], stack: List[Arrow]): List[Arrow] = edges match {
+			case Nil => Nil
+			case edge :: tail =>
+				val node = head(edge)
+				if(!visited(node))
+				{
+					visited += node
+					visit(dependencies(node), edge :: stack) match {
+						case Nil =>
+							finished += node
+							visit(tail, stack)
+						case cycle => cycle
+					}
+				}
+				else if(!finished(node))
+				{
+					// cycle. If a negative edge is involved, it is an error.
+					val between = edge :: stack.takeWhile(f => head(f) != node)
+					if(between exists isNegative)
+						between
+					else
+						visit(tail, stack)
+				}
+				else
+					visit(tail, stack)
+		}
+
+		visit(graph.nodes, Nil)
+	}
+
+}

util/logic/src/main/scala/sbt/logic/Logic.scala

@@ -0,0 +1,325 @@
+package sbt
+package logic
+
+	import scala.annotation.tailrec
+	import Formula.{And, True}
+
+/*
+Defines a propositional logic with negation as failure and only allows stratified rule sets (negation must be acyclic) in order to have a unique minimal model.
+
+For example, this is not allowed:
+  + p :- not q
+  + q :- not p
+but this is:
+  + p :- q
+  + q :- p
+as is this:
+  + p :- q
+  + q := not r
+
+
+ Some useful links:
+  + https://en.wikipedia.org/wiki/Nonmonotonic_logic
+  + https://en.wikipedia.org/wiki/Negation_as_failure
+  + https://en.wikipedia.org/wiki/Propositional_logic
+  + https://en.wikipedia.org/wiki/Stable_model_semantics
+  + http://www.w3.org/2005/rules/wg/wiki/negation
+*/
+
+
+/** Disjunction (or) of the list of clauses. */
+final case class Clauses(clauses: List[Clause]) {
+	assert(clauses.nonEmpty, "At least one clause is required.")
+}
+
+/** When the `body` Formula succeeds, atoms in `head` are true. */
+final case class Clause(body: Formula, head: Set[Atom])
+
+/** A literal is an [[Atom]] or its [[negation|Negated]]. */
+sealed abstract class Literal extends Formula {
+	/** The underlying (positive) atom. */
+	def atom: Atom
+	/** Negates this literal.*/
+	def unary_! : Literal
+}
+/** A variable with name `label`. */
+final case class Atom(label: String) extends Literal {
+	def atom = this
+	def unary_! : Negated = Negated(this)
+}
+/** A negated atom, in the sense of negation as failure, not logical negation.
+* That is, it is true if `atom` is not known/defined. */
+final case class Negated(atom: Atom) extends Literal {
+	def unary_! : Atom = atom
+}
+
+/** A formula consists of variables, negation, and conjunction (and).
+* (Disjunction is not currently included- it is modeled at the level of a sequence of clauses.
+*  This is less convenient when defining clauses, but is not less powerful.) */
+sealed abstract class Formula {
+	/** Constructs a clause that proves `atoms` when this formula is true. */
+	def proves(atom: Atom, atoms: Atom*): Clause = Clause(this, (atom +: atoms).toSet)
+
+	/** Constructs a formula that is true iff this formula and `f` are both true.*/
+	def && (f: Formula): Formula = (this, f) match {
+		case (True, x) => x
+		case (x, True) => x
+		case (And(as), And(bs)) => And(as ++ bs)
+		case (And(as), b: Literal) => And(as + b)
+		case (a: Literal, And(bs)) => And(bs + a)
+		case (a: Literal, b: Literal) => And( Set(a,b) )
+	}
+}
+
+
+object Formula {
+	/** A conjunction of literals. */
+   final case class And(literals: Set[Literal]) extends Formula {
+		assert(literals.nonEmpty, "'And' requires at least one literal.")
+	}
+	final case object True extends Formula
+}
+
+object Logic
+{
+	def reduceAll(clauses: List[Clause], initialFacts: Set[Literal]): Either[LogicException, Matched] =
+		reduce(Clauses(clauses), initialFacts)
+
+	/** Computes the variables in the unique stable model for the program represented by `clauses` and `initialFacts`.
+	* `clause` may not have any negative feedback (that is, negation is acyclic)
+	* and `initialFacts` cannot be in the head of any clauses in `clause`.
+	* These restrictions ensure that the logic program has a unique minimal model. */
+	def reduce(clauses: Clauses, initialFacts: Set[Literal]): Either[LogicException, Matched] =
+	{
+		val (posSeq, negSeq) = separate(initialFacts.toSeq)
+		val (pos, neg) = (posSeq.toSet, negSeq.toSet)
+
+		val problem =
+			checkContradictions(pos, neg) orElse
+			checkOverlap(clauses, pos) orElse
+			checkAcyclic(clauses)
+
+		problem.toLeft(
+			reduce0(clauses, initialFacts, Matched.empty)
+		)
+	}
+
+
+	/** Verifies `initialFacts` are not in the head of any `clauses`.
+	* This avoids the situation where an atom is proved but no clauses prove it.
+	* This isn't necessarily a problem, but the main sbt use cases expects
+	* a proven atom to have at least one clause satisfied. */
+	private[this] def checkOverlap(clauses: Clauses, initialFacts: Set[Atom]): Option[InitialOverlap] = {
+		val as = atoms(clauses)
+		val initialOverlap = initialFacts.filter(as.inHead)
+		if(initialOverlap.nonEmpty) Some(new InitialOverlap(initialOverlap)) else None
+	}
+
+	private[this] def checkContradictions(pos: Set[Atom], neg: Set[Atom]): Option[InitialContradictions] = {
+		val contradictions = pos intersect neg
+		if(contradictions.nonEmpty) Some(new InitialContradictions(contradictions)) else None
+	}
+
+	private[this] def checkAcyclic(clauses: Clauses): Option[CyclicNegation] = {
+		val deps = dependencyMap(clauses)
+		val cycle = Dag.findNegativeCycle(graph(deps))
+		if(cycle.nonEmpty) Some(new CyclicNegation(cycle)) else None
+	}
+	private[this] def graph(deps: Map[Atom, Set[Literal]]) = new Dag.DirectedSignedGraph[Atom] {
+		type Arrow = Literal
+		def nodes = deps.keys.toList
+		def dependencies(a: Atom) = deps.getOrElse(a, Set.empty).toList
+		def isNegative(b: Literal) = b match {
+			case Negated(_) => true
+			case Atom(_) => false
+		}
+		def head(b: Literal) = b.atom
+	}
+
+	private[this] def dependencyMap(clauses: Clauses): Map[Atom, Set[Literal]] =
+		(Map.empty[Atom, Set[Literal]] /: clauses.clauses) {
+			case (m, Clause(formula, heads)) =>
+				val deps = literals(formula)
+				(m /: heads) { (n, head) => n.updated(head, n.getOrElse(head, Set.empty) ++ deps) }
+		}
+
+	sealed abstract class LogicException(override val toString: String)
+	final class InitialContradictions(val literals: Set[Atom]) extends LogicException("Initial facts cannot be both true and false:\n\t" + literals.mkString("\n\t"))
+	final class InitialOverlap(val literals: Set[Atom]) extends LogicException("Initial positive facts cannot be implied by any clauses:\n\t" + literals.mkString("\n\t"))
+	final class CyclicNegation(val cycle: List[Literal]) extends LogicException("Negation may not be involved in a cycle:\n\t" + cycle.mkString("\n\t"))
+
+	/** Tracks proven atoms in the reverse order they were proved. */
+	final class Matched private(val provenSet: Set[Atom], reverseOrdered: List[Atom]) {
+		def add(atoms: Set[Atom]): Matched = add(atoms.toList)
+		def add(atoms: List[Atom]): Matched = {
+			val newOnly = atoms.filterNot(provenSet)
+			new Matched(provenSet ++ newOnly, newOnly ::: reverseOrdered)
+		}
+		def ordered: List[Atom] = reverseOrdered.reverse
+		override def toString = ordered.map(_.label).mkString("Matched(", ",", ")")
+	}
+	object Matched {
+		val empty = new Matched(Set.empty, Nil)
+	}
+
+	/** Separates a sequence of literals into `(pos, neg)` atom sequences. */
+	private[this] def separate(lits: Seq[Literal]): (Seq[Atom], Seq[Atom]) = Util.separate(lits) {
+		case a: Atom => Left(a)
+		case Negated(n) => Right(n)
+	}
+
+	/** Finds clauses that have no body and thus prove their head.
+	* Returns `(<proven atoms>, <remaining unproven clauses>)`. */
+	private[this] def findProven(c: Clauses): (Set[Atom], List[Clause]) =
+	{
+		val (proven, unproven) = c.clauses.partition(_.body == True)
+		(proven.flatMap(_.head).toSet, unproven)
+	}
+	private[this] def keepPositive(lits: Set[Literal]): Set[Atom] =
+		lits.collect{ case a: Atom => a}.toSet
+
+	// precondition: factsToProcess contains no contradictions
+	@tailrec
+	private[this] def reduce0(clauses: Clauses, factsToProcess: Set[Literal], state: Matched): Matched =
+		applyAll(clauses, factsToProcess) match {
+			case None => // all of the remaining clauses failed on the new facts
+				state
+			case Some(applied) =>
+				val (proven, unprovenClauses) = findProven(applied)
+				val processedFacts = state add keepPositive(factsToProcess)
+				val newlyProven = proven -- processedFacts.provenSet
+				val newState = processedFacts add newlyProven
+				if(unprovenClauses.isEmpty)
+					newState // no remaining clauses, done.
+				else {
+					val unproven = Clauses(unprovenClauses)
+					val nextFacts: Set[Literal] = if(newlyProven.nonEmpty) newlyProven.toSet else inferFailure(unproven)
+					reduce0(unproven, nextFacts, newState)
+				}
+		}
+
+	/** Finds negated atoms under the negation as failure rule and returns them.
+	* This should be called only after there are no more known atoms to be substituted. */
+	private[this] def inferFailure(clauses: Clauses): Set[Literal] =
+	{
+		/* At this point, there is at least one clause and one of the following is the case as the result of the acyclic negation rule:
+				i. there is at least one variable that occurs in a clause body but not in the head of a clause
+				ii. there is at least one variable that occurs in the head of a clause and does not transitively depend on a negated variable
+			In either case, each such variable x cannot be proven true and therefore proves 'not x' (negation as failure, !x in the code).
+		*/
+		val allAtoms = atoms(clauses)
+		val newFacts: Set[Literal] = negated(allAtoms.triviallyFalse)
+		if(newFacts.nonEmpty)
+			newFacts
+		else {
+			val possiblyTrue = hasNegatedDependency(clauses.clauses, Relation.empty, Relation.empty)
+			val newlyFalse: Set[Literal] = negated(allAtoms.inHead -- possiblyTrue)
+			if(newlyFalse.nonEmpty)
+				newlyFalse
+			else // should never happen due to the acyclic negation rule
+				error(s"No progress:\n\tclauses: $clauses\n\tpossibly true: $possiblyTrue")
+		}
+	}
+
+	private[this] def negated(atoms: Set[Atom]): Set[Literal] = atoms.map(a => Negated(a))
+
+	/** Computes the set of atoms in `clauses` that directly or transitively take a negated atom as input.
+	* For example, for the following clauses, this method would return `List(a, d)` :
+	*  a :- b, not c
+	*  d :- a
+	*/
+	@tailrec
+	def hasNegatedDependency(clauses: Seq[Clause], posDeps: Relation[Atom, Atom], negDeps: Relation[Atom, Atom]): List[Atom] =
+		clauses match {
+			case Seq() =>
+				// because cycles between positive literals are allowed, this isn't strictly a topological sort
+				Dag.topologicalSortUnchecked(negDeps._1s)(posDeps.reverse)
+			case Clause(formula, head) +: tail =>
+				// collect direct positive and negative literals and track them in separate graphs
+				val (pos, neg) = directDeps(formula)
+				val (newPos, newNeg) = ( (posDeps, negDeps) /: head) { case ( (pdeps, ndeps), d) =>
+					(pdeps + (d, pos), ndeps + (d, neg) )
+				}
+				hasNegatedDependency(tail, newPos, newNeg)
+		}
+
+	/** Computes the `(positive, negative)` literals in `formula`. */
+	private[this] def directDeps(formula: Formula): (Seq[Atom], Seq[Atom]) =
+		Util.separate(literals(formula).toSeq) {
+			case Negated(a) => Right(a)
+			case a: Atom => Left(a)
+		}
+	private[this] def literals(formula: Formula): Set[Literal] = formula match {
+		case And(lits) => lits
+		case l: Literal => Set(l)
+		case True => Set.empty
+	}
+
+	/** Computes the atoms in the heads and bodies of the clauses in `clause`. */
+	def atoms(cs: Clauses): Atoms = cs.clauses.map(c => Atoms(c.head, atoms(c.body))).reduce(_ ++ _)
+
+	/** Computes the set of all atoms in `formula`. */
+	def atoms(formula: Formula): Set[Atom] = formula match {
+		case And(lits) => lits.map(_.atom)
+		case Negated(lit) => Set(lit)
+		case a: Atom => Set(a)
+		case True => Set()
+	}
+
+	/** Represents the set of atoms in the heads of clauses and in the bodies (formulas) of clauses. */
+	final case class Atoms(val inHead: Set[Atom], val inFormula: Set[Atom]) {
+		/** Concatenates this with `as`. */
+		def ++ (as: Atoms): Atoms = Atoms(inHead ++ as.inHead, inFormula ++ as.inFormula)
+		/** Atoms that cannot be true because they do not occur in a head. */
+		def triviallyFalse: Set[Atom] = inFormula -- inHead
+	}
+
+	/** Applies known facts to `clause`s, deriving a new, possibly empty list of clauses.
+	* 1. If a fact is in the body of a clause, the derived clause has that fact removed from the body.
+	* 2. If the negation of a fact is in a body of a clause, that clause fails and is removed.
+	* 3. If a fact or its negation is in the head of a clause, the derived clause has that fact (or its negation) removed from the head.
+	* 4. If a head is empty, the clause proves nothing and is removed.
+	*
+	* NOTE: empty bodies do not cause a clause to succeed yet.
+	*       All known facts must be applied before this can be done in order to avoid inconsistencies.
+	* Precondition: no contradictions in `facts`
+	* Postcondition: no atom in `facts` is present in the result
+	* Postcondition: No clauses have an empty head
+	* */
+	def applyAll(cs: Clauses, facts: Set[Literal]): Option[Clauses] =
+	{
+		val newClauses =
+			if(facts.isEmpty)
+				cs.clauses.filter(_.head.nonEmpty) // still need to drop clauses with an empty head
+			else
+				cs.clauses.map(c => applyAll(c, facts)).flatMap(_.toList)
+		if(newClauses.isEmpty) None else Some(Clauses(newClauses))
+	}
+
+	def applyAll(c: Clause, facts: Set[Literal]): Option[Clause] =
+	{
+		val atoms = facts.map(_.atom)
+		val newHead = c.head -- atoms  // 3.
+		if(newHead.isEmpty)  // 4. empty head
+			None
+		else
+			substitute(c.body, facts).map( f => Clause(f, newHead) )  // 1, 2
+	}
+
+	/** Derives the formula that results from substituting `facts` into `formula`. */
+	@tailrec
+	def substitute(formula: Formula, facts: Set[Literal]): Option[Formula] = formula match {
+		case And(lits) =>
+			def negated(lits: Set[Literal]): Set[Literal] = lits.map(a => !a)
+			if( lits.exists( negated(facts) ) )  // 2.
+				None
+			else {
+				val newLits = lits -- facts
+				val newF = if(newLits.isEmpty) True else And(newLits)
+				Some(newF)  // 1.
+			}
+		case True => Some(True)
+		case lit: Literal => // define in terms of And
+			substitute(And(Set(lit)), facts)
+	}
+}

util/logic/src/test/scala/sbt/logic/Test.scala

@@ -0,0 +1,117 @@
+package sbt
+package logic
+
+	import org.scalacheck._
+	import Prop.secure
+	import Logic.{LogicException, Matched}
+
+object LogicTest extends Properties("Logic")
+{
+	import TestClauses._
+
+	property("Handles trivial resolution.") = secure( expect(trivial, Set(A) ) )
+	property("Handles less trivial resolution.") = secure( expect(lessTrivial, Set(B,A,D)) )
+	property("Handles cycles without negation") = secure( expect(cycles, Set(F,A,B)) )
+	property("Handles basic exclusion.") = secure( expect(excludedPos, Set()) )
+	property("Handles exclusion of head proved by negation.") = secure( expect(excludedNeg, Set()) )
+		// TODO: actually check ordering, probably as part of a check that dependencies are satisifed
+	property("Properly orders results.") = secure( expect(ordering, Set(B,A,C,E,F)))
+	property("Detects cyclic negation") = secure(
+		Logic.reduceAll(badClauses, Set()) match {
+			case Right(res) => false
+			case Left(err: Logic.CyclicNegation) => true
+			case Left(err) => error(s"Expected cyclic error, got: $err")
+		}
+	)
+
+	def expect(result: Either[LogicException, Matched], expected: Set[Atom]) = result match {
+		case Left(err) => false
+		case Right(res) =>
+			val actual = res.provenSet
+			(actual == expected) || error(s"Expected to prove $expected, but actually proved $actual")
+	}
+}
+
+object TestClauses
+{
+
+	val A = Atom("A")
+	val B = Atom("B")
+	val C = Atom("C")
+	val D = Atom("D")
+	val E = Atom("E")
+	val F = Atom("F")
+	val G = Atom("G")
+
+	val clauses =
+		A.proves(B) ::
+		A.proves(F) ::
+		B.proves(F) ::
+		F.proves(A) ::
+		(!C).proves(F) ::
+		D.proves(C) ::
+		C.proves(D) ::
+		Nil
+
+	val cycles = Logic.reduceAll(clauses, Set())
+
+	val badClauses =
+		A.proves(D) ::
+		clauses
+
+	val excludedNeg = {
+		val cs =
+			(!A).proves(B) ::
+			Nil
+		val init =
+			(!A) ::
+			(!B) ::
+			Nil
+		Logic.reduceAll(cs, init.toSet)
+	}
+
+	val excludedPos = {
+		val cs =
+			A.proves(B) ::
+			Nil
+		val init =
+			A ::
+			(!B) ::
+			Nil
+		Logic.reduceAll(cs, init.toSet)
+	}
+
+	val trivial = {
+		val cs =
+			Formula.True.proves(A) ::
+			Nil
+		Logic.reduceAll(cs, Set.empty)
+	}
+
+	val lessTrivial = {
+		val cs =
+			Formula.True.proves(A) ::
+			Formula.True.proves(B) ::
+			(A && B && (!C)).proves(D) ::
+			Nil
+		Logic.reduceAll(cs, Set())
+	}
+
+	val ordering = {
+		val cs =
+			E.proves(F) ::
+			(C && !D).proves(E) ::
+			(A && B).proves(C) ::
+			Nil
+		Logic.reduceAll(cs, Set(A,B))
+	}
+
+	def all {
+		println(s"Cycles: $cycles")
+		println(s"xNeg: $excludedNeg")
+		println(s"xPos: $excludedPos")
+		println(s"trivial: $trivial")
+		println(s"lessTrivial: $lessTrivial")
+		println(s"ordering: $ordering")
+	}
+}

util/relation/src/main/scala/sbt/Relation.scala

@@ -40,7 +40,7 @@ object Relation
 
 	private[sbt] def get[X,Y](map: M[X,Y], t: X): Set[Y] = map.getOrElse(t, Set.empty[Y])
 
-	private[sbt] type M[X,Y] = Map[X, Set[Y]]	
+	private[sbt] type M[X,Y] = Map[X, Set[Y]]
 }
 
 /** Binary relation between A and B.  It is a set of pairs (_1, _2) for _1 in A, _2 in B.  */
@@ -111,7 +111,7 @@ private final class MRelation[A,B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ext
 {
 	def forwardMap = fwd
 	def reverseMap = rev
-	
+
 	def forward(t: A) = get(fwd, t)
 	def reverse(t: B) = get(rev, t)
 
@@ -119,12 +119,12 @@ private final class MRelation[A,B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ext
 	def _2s = rev.keySet
 
 	def size = (fwd.valuesIterator map { _.size }).foldLeft(0)(_ + _)
-	
+
 	def all: Traversable[(A,B)] = fwd.iterator.flatMap { case (a, bs) => bs.iterator.map( b => (a,b) ) }.toTraversable
 
 	def +(pair: (A,B)) = this + (pair._1, Set(pair._2))
 	def +(from: A, to: B) = this + (from, to :: Nil)
-	def +(from: A, to: Traversable[B]) =
+	def +(from: A, to: Traversable[B]) = if(to.isEmpty) this else
 		new MRelation( add(fwd, from, to), (rev /: to) { (map, t) => add(map, t, from :: Nil) })
 
 	def ++(rs: Traversable[(A,B)]) = ((this: Relation[A,B]) /: rs) { _ + _ }