Test chars safely when highlighting

The arrow test at lastOffset - 3 is out-of-range when highlighting,
which parses a snippet of source.

Author: som-snytt

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -641,7 +641,7 @@ object Parsers {
     def inBracesOrIndented[T](body: => T, rewriteWithColon: Boolean = false): T =
       if in.token == INDENT then
         val rewriteToBraces = in.rewriteNoIndent
-          && !testChars(in.lastOffset - 3, " =>") // braces are always optional after `=>` so none should be inserted
+          && !testCharsSafe(in.lastOffset - 3, " =>") // braces are optional after `=>` so none should be inserted
         if rewriteToBraces then indentedToBraces(body)
         else enclosed(INDENT, body)
       else
@@ -744,6 +744,9 @@ object Parsers {
       str.isEmpty ||
       testChar(from, str.head) && testChars(from + 1, str.tail)
 
+    def testCharsSafe(from: Int, str: String): Boolean =
+      from >= 0 && testChars(from, str)
+
     def skipBlanks(idx: Int, step: Int = 1): Int =
       if (testChar(idx, c => c == ' ' || c == '\t' || c == Chars.CR)) skipBlanks(idx + step, step)
       else idx
@@ -860,7 +863,7 @@ object Parsers {
         case _ => false
       }
       var canRewrite = allBraces(in.currentRegion) && // test (1)
-        !testChars(in.lastOffset - 3, " =>") // test(6)
+        !testCharsSafe(in.lastOffset - 3, " =>") // test(6)
 
       def isStartOfSymbolicFunction: Boolean =
         opStack.headOption.exists { x =>

compiler/test/dotty/tools/utils.scala

@@ -124,6 +124,7 @@ private val toolArg = raw"(?://|/\*| \*) ?(?i:(${ToolName.values.mkString("|")})
 private val directiveOptionsArg = raw"//> using options (.*)".r.unanchored
 private val directiveJavacOptions = raw"//> using javacOpt (.*)".r.unanchored
 private val directiveTargetOptions = raw"//> using target.platform (jvm|scala-js)".r.unanchored
+private val directiveUnsupported = raw"//> using (scala) (.*)".r.unanchored
 private val directiveUnknown = raw"//> using (.*)".r.unanchored
 
 // Inspect the lines for compiler options of the form
@@ -141,6 +142,7 @@ def toolArgsParse(lines: List[String], filename: Option[String]): List[(String,S
     case directiveOptionsArg(args) => List(("scalac", args))
     case directiveJavacOptions(args) => List(("javac", args))
     case directiveTargetOptions(platform) => List(("target", platform))
+    case directiveUnsupported(name, args) => Nil
     case directiveUnknown(rest) => sys.error(s"Unknown directive: `//> using ${CommandLineParser.tokenize(rest).headOption.getOrElse("''")}`${filename.fold("")(f => s" in file $f")}")
     case _ => Nil
   }

tests/neg/i22906.check

@@ -0,0 +1,6 @@
+Flag -indent set repeatedly
+-- Error: tests/neg/i22906.scala:5:15 ----------------------------------------------------------------------------------
+5 |    {`1`: Int  =>  5} // error
+  |               ^
+  |               parentheses are required around the parameter of a lambda
+  |               This construct can be rewritten automatically under -rewrite -source 3.0-migration.

tests/neg/i22906.scala

@@ -0,0 +1,6 @@
+//> using options -rewrite -indent
+//> nominally using scala 3.7.0-RC1
+// does not reproduce under "vulpix" test rig, which enforces certain flag sets?
+
+def program: Int => Int =
+    {`1`: Int  =>  5} // error