Translate errors from logic system to Natures system.

main/src/main/scala/sbt/AutoPlugin.scala

@@ -1,6 +1,7 @@
 package sbt
 
-	import logic.{Atom, Clause, Clauses, Formula, Literal, Logic}
+	import logic.{Atom, Clause, Clauses, Formula, Literal, Logic, Negated}
+	import Logic.{CyclicNegation, InitialContradictions, InitialOverlap, LogicException}
 	import Def.Setting
 	import Natures._
 
@@ -75,6 +76,11 @@ abstract class AutoPlugin
 	// TODO?: def commands: Seq[Command]
 }
 
+final class AutoPluginException(val origin: LogicException, prefix: String) extends RuntimeException(prefix + Natures.translateMessage(origin)) {
+	def withPrefix(p: String) = new AutoPluginException(origin, p)
+}
+
+
 /** An expression that matches `Nature`s. */
 sealed trait Natures {
 	def && (o: Basic): Natures
@@ -101,14 +107,24 @@ object Natures
 		{
 			val byAtom = defined.map(x => (Atom(x.provides.label), x)).toMap
 			val clauses = Clauses( defined.map(d => asClause(d)) )
-			requestedNatures => {
+			requestedNatures =>
-				val results = Logic.reduce(clauses, flatten(requestedNatures).toSet)
+				Logic.reduce(clauses, flatten(requestedNatures).toSet) match {
-				// results includes the originally requested (positive) atoms,
+					case Left(problem) => throw new AutoPluginException(problem, "")
-				//   which won't have a corresponding AutoPlugin to map back to
+					case Right(results) =>
-				results.ordered.flatMap(a => byAtom.get(a).toList)
+						// results includes the originally requested (positive) atoms,
-			}
+						//   which won't have a corresponding AutoPlugin to map back to
+						results.ordered.flatMap(a => byAtom.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(", ")
+
 	/** [[Natures]] instance that doesn't require any [[Nature]]s. */
 	def empty: Natures = Empty
 	private[sbt] final object Empty extends Natures {

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

@@ -6,11 +6,10 @@ package sbt
 	import java.io.File
 	import java.net.{URI,URL}
 	import compiler.{Eval,EvalImports}
-	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}
@@ -463,7 +462,9 @@ object Load
 		def loadSbtFiles(auto: AddSettings, base: File, autoPlugins: Seq[AutoPlugin]): LoadedSbtFile =
 			loadSettings(auto, base, plugins, eval, injectSettings, memoSettings, autoPlugins)
 		def loadForProjects = newProjects map { project =>
-			val autoPlugins = plugins.detected.compileNatures(project.natures)
+			val autoPlugins =
+				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
@@ -485,6 +486,8 @@ object Load
 		else
 			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, loadedPlugins: sbt.LoadedPlugins, eval: ()=>Eval, injectSettings: InjectSettings, memoSettings: mutable.Map[File, LoadedSbtFile], autoPlugins: Seq[AutoPlugin]): LoadedSbtFile =
 	{

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

@@ -88,6 +88,7 @@ object Dag
 		def visit(nodes: List[B], stack: List[B]): List[B] = nodes match {
 			case Nil => Nil
 			case node :: tail =>
+				def indent = "\t" * stack.size
 				val atom = toA(node)
 				if(!visited(atom))
 				{
@@ -102,7 +103,7 @@ object Dag
 				else if(!finished(atom))
 				{
 					// cycle. If negation is involved, it is an error.
-					val between = stack.takeWhile(f => toA(f) != atom)
+					val between = node :: stack.takeWhile(f => toA(f) != atom)
 					if(between exists isNegated)
 						between
 					else

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

@@ -82,22 +82,26 @@ object Formula {
 
 object Logic
 {
-	def reduceAll(clauses: List[Clause], initialFacts: Set[Literal]): Matched = reduce(Clauses(clauses), initialFacts)
+	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]): Matched =
+	def reduce(clauses: Clauses, initialFacts: Set[Literal]): Either[LogicException, Matched] =
 	{
 		val (posSeq, negSeq) = separate(initialFacts.toSeq)
 		val (pos, neg) = (posSeq.toSet, negSeq.toSet)
 
-		checkContradictions(pos, neg)
+		val problem =
-		checkOverlap(clauses, pos)
+			checkContradictions(pos, neg) orElse
-		checkAcyclic(clauses)
+			checkOverlap(clauses, pos) orElse
+			checkAcyclic(clauses)
 
-		reduce0(clauses, initialFacts, Matched.empty)
+		problem.toLeft(
+			reduce0(clauses, initialFacts, Matched.empty)
+		)
 	}
 
 
@@ -105,22 +109,21 @@ object Logic
 	* 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. */
-	def checkOverlap(clauses: Clauses, initialFacts: Set[Atom]) {
+	private[this] def checkOverlap(clauses: Clauses, initialFacts: Set[Atom]): Option[InitialOverlap] = {
 		val as = atoms(clauses)
 		val initialOverlap = initialFacts.filter(as.inHead)
-		if(initialOverlap.nonEmpty) throw new InitialOverlap(initialOverlap)
+		if(initialOverlap.nonEmpty) Some(new InitialOverlap(initialOverlap)) else None
 	}
 
-	private[this] def checkContradictions(pos: Set[Atom], neg: Set[Atom]) {
+	private[this] def checkContradictions(pos: Set[Atom], neg: Set[Atom]): Option[InitialContradictions] = {
 		val contradictions = pos intersect neg
-		if(contradictions.nonEmpty) throw new InitialContradictions(contradictions)
+		if(contradictions.nonEmpty) Some(new InitialContradictions(contradictions)) else None
 	}
 
-	def checkAcyclic(clauses: Clauses) {
+	private[this] def checkAcyclic(clauses: Clauses): Option[CyclicNegation] = {
 		val deps = dependencyMap(clauses)
 		val cycle = Dag.findNegativeCycle(system(deps))(deps.keys.toList)
-		if(cycle.nonEmpty)
+		if(cycle.nonEmpty) Some(new CyclicNegation(cycle)) else None
-			throw new CyclicNegation(cycle)
 	}
 	private[this] def system(deps: Map[Atom, Set[Literal]]) = new Dag.System[Atom] {
 		type B = Literal
@@ -139,9 +142,10 @@ object Logic
 				(m /: heads) { (n, head) => n.updated(head, n.getOrElse(head, Set.empty) ++ deps) }
 		}
 
-	final class InitialContradictions(val literals: Set[Atom]) extends RuntimeException("Initial facts cannot be both true and false:\n\t" + literals.mkString("\n\t"))
+	sealed abstract class LogicException(override val toString: String)
-	final class InitialOverlap(val literals: Set[Atom]) extends RuntimeException("Initial positive facts cannot be implied by any clauses:\n\t" + literals.mkString("\n\t"))
+	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 CyclicNegation(val cycle: List[Literal]) extends RuntimeException("Negation may not be involved in a cycle:\n\t" + cycle.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]) {