implicitly check for unknown files (not part of a Mercurial repository);

--- a/src/Pure/General/mercurial.scala	Sun May 14 17:19:46 2017 +0200
+++ b/src/Pure/General/mercurial.scala	Sun May 14 20:16:13 2017 +0200
@@ -10,6 +10,9 @@
 
 import java.io.{File => JFile}
 
+import scala.annotation.tailrec
+import scala.collection.mutable
+
 
 object Mercurial
 {
@@ -155,4 +158,32 @@
       }
     }
   }
+
+
+  /* unknown files */
+
+  def unknown_files(files: List[Path], ssh: Option[SSH.Session] = None): List[Path] =
+  {
+    val unknown = new mutable.ListBuffer[Path]
+
+    @tailrec def check(paths: List[Path])
+    {
+      paths match {
+        case path :: rest =>
+          find_repository(path, ssh) match {
+            case None => unknown += path; check(rest)
+            case Some(hg) =>
+              val known =
+                hg.known_files().iterator.map(name =>
+                  (hg.root + Path.explode(name)).file.getCanonicalFile).toSet
+              if (!known(path.file.getCanonicalFile)) unknown += path
+              check(rest.filterNot(p => known(p.file.getCanonicalFile)))
+          }
+        case Nil =>
+      }
+    }
+
+    check(files)
+    unknown.toList
+  }
 }

--- a/src/Pure/Tools/build.scala	Sun May 14 17:19:46 2017 +0200
+++ b/src/Pure/Tools/build.scala	Sun May 14 20:16:13 2017 +0200
@@ -338,6 +338,7 @@
   def build(
     options: Options,
     progress: Progress = No_Progress,
+    check_unknown_files: Boolean = false,
     build_heap: Boolean = false,
     clean_build: Boolean = false,
     dirs: List[Path] = Nil,
@@ -377,6 +378,19 @@
     def sources_stamp(name: String): List[String] =
       (selected_sessions(name).meta_digest :: deps.sources(name)).map(_.toString).sorted
 
+    if (check_unknown_files) {
+      val source_files =
+        (for {
+          (_, base) <- deps.session_bases.iterator
+          (path, _) <- base.sources.iterator
+        } yield path).toList
+      val unknown_files = Mercurial.unknown_files(source_files)
+      if (unknown_files.nonEmpty) {
+        progress.echo_warning("Unknown files (not part of a Mercurial repository):" +
+          unknown_files.map(path => path.expand.implode).sorted.mkString("\n  ", "\n  ", ""))
+      }
+    }
+
 
     /* main build process */
 
@@ -680,6 +694,7 @@
     val results =
       progress.interrupt_handler {
         build(options, progress,
+          check_unknown_files = Mercurial.is_repository(Path.explode("~~")),
           build_heap = build_heap,
           clean_build = clean_build,
           dirs = dirs,