Always check lower bound in testChar

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
-          && !testCharsSafe(in.lastOffset - 3, " =>") // braces are optional after `=>` so none should be inserted
+          && !testChars(in.lastOffset - 3, " =>") // braces are optional after `=>` so none should be inserted
         if rewriteToBraces then indentedToBraces(body)
         else enclosed(INDENT, body)
       else
@@ -732,21 +732,19 @@ object Parsers {
 
     def testChar(idx: Int, p: Char => Boolean): Boolean = {
       val txt = source.content
-      idx < txt.length && p(txt(idx))
+      idx >= 0 && idx < txt.length && p(txt(idx))
     }
 
     def testChar(idx: Int, c: Char): Boolean = {
       val txt = source.content
-      idx < txt.length && txt(idx) == c
+      idx >= 0 && idx < txt.length && txt(idx) == c
     }
 
     def testChars(from: Int, str: String): Boolean =
-      str.isEmpty ||
+      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
@@ -863,7 +861,7 @@ object Parsers {
         case _ => false
       }
       var canRewrite = allBraces(in.currentRegion) && // test (1)
-        !testCharsSafe(in.lastOffset - 3, " =>") // test(6)
+        !testChars(in.lastOffset - 3, " =>") // test(6)
 
       def isStartOfSymbolicFunction: Boolean =
         opStack.headOption.exists { x =>