Fixes scala#7051 Detect typos in neg tests and give a warning

Author: ashwinbhaskar

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -631,7 +631,10 @@ trait ParallelTesting extends RunnerOrchestration { self =>
 
       if (compilerCrashed) Some(s"Compiler crashed when compiling: ${testSource.title}"                                                             )
       else if (actualErrors == 0) Some(s"\nNo errors found when compiling neg test $testSource"                                                            )
-      else if (expectedErrors != actualErrors) Some(s"\nWrong number of errors encountered when compiling $testSource, expected: $expectedErrors, actual: $actualErrors")
+      else if (expectedErrors != actualErrors) {
+        getWarnings(testSource.sourceFiles) foreach echo
+        Some(s"\nWrong number of errors encountered when compiling $testSource, expected: $expectedErrors, actual: $actualErrors")
+      }
       else if (hasMissingAnnotations) Some(s"\nErrors found on incorrect row numbers when compiling $testSource"                                               )
       else if (!errorMap.isEmpty) Some(s"\nExpected error(s) have {<error position>=<unreported error>}: $errorMap"                                        )
       else None
@@ -672,6 +675,35 @@ trait ParallelTesting extends RunnerOrchestration { self =>
       (errorMap, expectedErrors)
     }
 
+    /**
+     * We want to warn the developer about some of the possible typos when trying to point "error" and "nopos-error"
+     * For example - A warning could be emitted if the developer mistakenly types in "//error" instead of "// error"
+     * or "//nopos-error" instead of "// nopos-error"
+     *
+     * @param files test files
+     * @return Sequence of warning messages
+     */
+    def getWarnings(files : Seq[JFile]) : Seq[String] = {
+
+      def hasTypo(line :  String) : Boolean = line.contains("//error") || line.contains("//nopos-error")
+
+      def getWarning(line: String, fileName: String): String = if (line.contains("//error")) {
+        s"Wrong   : //error\n Correct : // error"
+      } else if (line.contains("//nopos-error")) {
+        s"Wrong : //nopos-error\n Correct : // nopos-error"
+      } else ""
+
+      val fileNamesAndLineNumber: Seq[(String, String, Int)] = for {
+        file <- files.filter(_.getName.endsWith(".scala"))
+        (line, lineNbr) <- Source.fromFile(file, "UTF-8").getLines().zipWithIndex.filter({ case (line, _) => hasTypo(line) })
+      } yield {
+        (file.getName, line, lineNbr)
+      }
+      fileNamesAndLineNumber.map({
+        case (fileName, line, lineNbr) => s"Possible typo in $fileName at line number $lineNbr \n " +
+          getWarning(line, fileName)})
+    }
+
     def getMissingExpectedErrors(errorMap: HashMap[String, Integer], reporterErrors: Iterator[MessageContainer]) = !reporterErrors.forall { error =>
       val key = if (error.pos.exists) {
         def toRelative(path: String): String =  // For some reason, absolute paths leak from the compiler itself...