Implemented general eviction warning. #1200

a> update
    [info] Updating {file:/foo/}a...
    [info] Resolving org.fusesource.jansi#jansi;1.4 ...
    [info] Done updating.
    [warn] Some dependencies were evicted:
    [warn]  * org.scala-lang:scala-library (2.10.1) -> 2.10.3

ivy/src/main/scala/sbt/EvictionWarning.scala

@@ -0,0 +1,44 @@
+package sbt
+
+import collection.mutable
+
+final class EvictionWarningOptions(
+    val configurations: Seq[String],
+    val level: Level.Value) {
+}
+object EvictionWarningOptions {
+  def apply(): EvictionWarningOptions =
+    new EvictionWarningOptions(Vector("compile"), Level.Warn)
+}
+
+object EvictionWarning {
+  def apply(options: EvictionWarningOptions, report: UpdateReport, log: Logger): Unit = {
+    val evictions = buildEvictions(options, report)
+    processEvictions(evictions, log)
+  }
+
+  private[sbt] def buildEvictions(options: EvictionWarningOptions, report: UpdateReport): Seq[ModuleDetailReport] = {
+    val buffer: mutable.ListBuffer[ModuleDetailReport] = mutable.ListBuffer()
+    val confs = report.configurations filter { x => options.configurations contains x.configuration }
+    confs flatMap { confReport =>
+      confReport.details map { detail =>
+        if ((detail.modules exists { _.evicted }) &&
+          !(buffer exists { x => (x.organization == detail.organization) && (x.name == detail.name) })) {
+          buffer += detail
+        }
+      }
+    }
+    buffer.toList.toVector
+  }
+
+  private[sbt] def processEvictions(evictions: Seq[ModuleDetailReport], log: Logger): Unit = {
+    if (!evictions.isEmpty) {
+      log.warn("Some dependencies were evicted:")
+      evictions foreach { detail =>
+        val revs = detail.modules filter { _.evicted } map { _.module.revision }
+        val winner = (detail.modules filterNot { _.evicted } map { _.module.revision }).headOption map { " -> " + _ } getOrElse ""
+        log.warn(s"\t* ${detail.organization}:${detail.name} (${revs.mkString(", ")})$winner")
+      }
+    }
+  }
+}