Improve skipping on syntax errors

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

@@ -37,14 +37,6 @@ object Parsers {
 
   case class OpInfo(operand: Tree, operator: Ident, offset: Offset)
 
-  class ParensCounters {
-    private var parCounts = new Array[Int](lastParen - firstParen)
-
-    def count(tok: Token): Int = parCounts(tok - firstParen)
-    def change(tok: Token, delta: Int): Unit = parCounts(tok - firstParen) += delta
-    def nonePositive: Boolean = parCounts forall (_ <= 0)
-  }
-
   enum Location(val inParens: Boolean, val inPattern: Boolean, val inArgs: Boolean):
     case InParens      extends Location(true, false, false)
     case InArgs        extends Location(true, false, true)
@@ -173,8 +165,6 @@ object Parsers {
 
     val in: Scanner = new Scanner(source)
 
-    val openParens: ParensCounters = new ParensCounters
-
     /** This is the general parse entry point.
      *  Overridden by ScriptParser
      */
@@ -261,55 +251,14 @@ object Parsers {
     }
 
     /** Skip on error to next safe point.
-     *  Safe points are:
-     *   - Closing braces, provided they match an opening brace before the error point.
-     *   - Closing parens and brackets, provided they match an opening parent or bracket
-     *     before the error point and there are no intervening other kinds of parens.
-     *   - Semicolons and newlines, provided there are no intervening braces.
-     *   - Definite statement starts on new lines, provided they are not more indented
-     *     than the last known statement start before the error point.
-     */
-    protected def skip(): Unit = {
-      val skippedParens = new ParensCounters
-      while (true) {
-        (in.token: @switch) match {
-          case EOF =>
-            return
-          case SEMI | NEWLINE | NEWLINES =>
-            if (skippedParens.count(LBRACE) == 0) return
-          case RBRACE =>
-            if (openParens.count(LBRACE) > 0 && skippedParens.count(LBRACE) == 0)
-              return
-            skippedParens.change(LBRACE, -1)
-          case RPAREN =>
-            if (openParens.count(LPAREN) > 0 && skippedParens.nonePositive)
-              return
-            skippedParens.change(LPAREN, -1)
-          case RBRACKET =>
-            if (openParens.count(LBRACKET) > 0 && skippedParens.nonePositive)
-              return
-            skippedParens.change(LBRACKET, -1)
-          case OUTDENT =>
-            if (openParens.count(INDENT) > 0 && skippedParens.count(INDENT) == 0)
-              return
-            skippedParens.change(INDENT, -1)
-          case LBRACE =>
-            skippedParens.change(LBRACE, +1)
-          case LPAREN =>
-            skippedParens.change(LPAREN, +1)
-          case LBRACKET=>
-            skippedParens.change(LBRACKET, +1)
-          case INDENT =>
-            skippedParens.change(INDENT, +1)
-          case _ =>
-            if (mustStartStat &&
-                in.isAfterLineEnd &&
-                isLeqIndented(in.offset, lastStatOffset max 0))
-              return
-        }
+     */
+    protected def skip(): Unit =
+      val lastRegion = in.currentRegion
+      def atStop =
+        in.token == EOF
+        || skipStopTokens.contains(in.token) && (in.currentRegion eq lastRegion)
+      while !atStop do
         in.nextToken()
-      }
-    }
 
     def warning(msg: Message, sourcePos: SourcePosition): Unit =
       report.warning(msg, sourcePos)
@@ -557,15 +506,9 @@ object Parsers {
 
 /* -------- COMBINATORS -------------------------------------------------------- */
 
-    def enclosed[T](tok: Token, body: => T): T = {
+    def enclosed[T](tok: Token, body: => T): T =
       accept(tok)
-      openParens.change(tok, 1)
-      try body
-      finally {
-        accept(tok + 1)
-        openParens.change(tok, -1)
-      }
-    }
+      try body finally accept(tok + 1)
 
     def inParens[T](body: => T): T = enclosed(LPAREN, body)
     def inBraces[T](body: => T): T = enclosed(LBRACE, body)
@@ -1429,7 +1372,6 @@ object Parsers {
             functionRest(Nil)
           }
           else {
-            openParens.change(LPAREN, 1)
             imods = modifiers(funTypeArgMods)
             val paramStart = in.offset
             val ts = funArgType() match {
@@ -1441,7 +1383,6 @@ object Parsers {
               case t =>
                 funArgTypesRest(t, funArgType)
             }
-            openParens.change(LPAREN, -1)
             accept(RPAREN)
             if isValParamList || in.token == ARROW || in.token == CTXARROW then
               functionRest(ts)
@@ -2149,14 +2090,12 @@ object Parsers {
         if in.token == RPAREN then
           Nil
         else
-          openParens.change(LPAREN, 1)
           var mods1 = mods
           if in.token == ERASED then mods1 = addModifier(mods1)
           try
             commaSeparated(() => binding(mods1))
           finally
             accept(RPAREN)
-            openParens.change(LPAREN, -1)
       else {
         val start = in.offset
         val name = bindingName()
@@ -2504,7 +2443,6 @@ object Parsers {
         val enums =
           if (leading == LBRACE || leading == LPAREN && followingIsEnclosedGenerators()) {
             in.nextToken()
-            openParens.change(leading, 1)
             val res =
               if (leading == LBRACE || in.token == CASE)
                 enumerators()
@@ -2514,7 +2452,6 @@ object Parsers {
                   if (in.token == RPAREN || pats.length > 1) {
                     wrappedEnums = false
                     accept(RPAREN)
-                    openParens.change(LPAREN, -1)
                     atSpan(start) { makeTupleOrParens(pats) } // note: alternatives `|' need to be weeded out by typer.
                   }
                   else pats.head
@@ -2523,7 +2460,6 @@ object Parsers {
             if (wrappedEnums) {
               val closingOnNewLine = in.isAfterLineEnd
               accept(leading + 1)
-              openParens.change(leading, -1)
               def hasMultiLineEnum =
                 res.exists { t =>
                   val pos = t.sourcePos

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

@@ -1383,6 +1383,22 @@ object Scanners {
     private def useOuterWidth(): Unit =
       if enclosing.knownWidth == null then enclosing.useOuterWidth()
       knownWidth = enclosing.knownWidth
+
+    private def delimiter = this match
+      case _: InString => "}(in string)"
+      case InParens(LPAREN, _) => ")"
+      case InParens(LBRACKET, _) => "]"
+      case _: InBraces => "}"
+      case _: InCase => "=>"
+      case _: Indented => "UNDENT"
+
+    /** Show open regions as list of lines with decreasing indentations */
+    def visualize: String =
+      indentWidth.toPrefix
+      + delimiter
+      + outer.match
+          case null => ""
+          case next: Region => "\n" + next.visualize
   end Region
 
   case class InString(multiLine: Boolean, outer: Region) extends Region

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

@@ -287,5 +287,7 @@ object Tokens extends TokensCommon {
 
   final val endMarkerTokens = identifierTokens | BitSet(IF, WHILE, FOR, MATCH, TRY, NEW, THROW, GIVEN, VAL, THIS)
 
+  final val skipStopTokens = BitSet(SEMI, NEWLINE, NEWLINES, RBRACE, RPAREN, RBRACKET, OUTDENT)
+
   final val softModifierNames = Set(nme.inline, nme.opaque, nme.open, nme.transparent, nme.infix)
 }

tests/neg/errpos.scala

@@ -1,11 +1,11 @@
 object Test {
   val x =     // error: expression expected
-  val y = 2   // error: ';' expected
+  val y = 2
 
   val z =     // error: expression expected
 
   // ...
-  val a = 3   // error: ';' expected
+  val a = 3
 
   val b = type // error: expression expected (on "type")
 

tests/neg/extensions-can-only-have-defs.scala

@@ -1,5 +1,5 @@
 object Test {
   extension [T] (t: T) {
-    val v: T = ??? // error // error
+    val v: T = ??? // error
   }
 }

tests/neg/i10546.scala

@@ -2,4 +2,4 @@ object test:
   def times(num : Int)(block : => Unit) : Unit = ()
     times(10): println("ah") // error: end of statement expected but '(' found // error
 
-  def foo: Set(Int) = Set(1) // error: end of statement expected but '(' found // error // error
+  def foo: Set(Int) = Set(1)

tests/neg/i4373.scala

@@ -16,7 +16,7 @@ class A4 extends _ with Base // error
 object Test {
   type T1 = _ // error
   type T2 = _[Int] // error // error
-  type T3 = _ { type S } // error
+  type T3 = _ { type S } // error // error
   type T4 = [X] =>> _ // error
 
   // Open questions:

tests/neg/i4934.scala

@@ -8,4 +8,4 @@ object Main {
 
 object Foo {
   val a = ""); // error: `}` expected but `)` found
-} // error: eof expected
+}

tests/neg/i5004.scala

@@ -1,6 +1,6 @@
 object i0 {
 1 match {
-def this(): Int  // error // error
-  def this()     // error
-}                // error
+def this(): Int  // error
+  def this()
+}
 }

tests/neg/i5032b.scala

@@ -1,2 +1,2 @@
 class a@
-class b@ // error // error
+class b@ // error

tests/neg/i7751.scala

@@ -1,2 +1,2 @@
 val a = Some(a=a,)=> // error // error
-val a = Some(x=y,)=> // error
+val a = Some(x=y,)=>

tests/neg/i8731.scala

@@ -3,27 +3,17 @@ object test:
   3 match
   case 3 => ???
   end match
-  case _ => () // error: missing parameter type
+  case _ => () // error: expected start of definition
   end match
 
   if 3 == 3 then
     ()
   end if
   else     // error: illegal start of definition
     ()
-  end if
+  end if   // error: misaligned end marker
 
   class Test {
     val test = 3
-  end Test   // error: misaligned end marker
-  }
-
-  while
-    3 == 3
-  end while  // error: `do` expected
-  do ()
-
-  for
-    a <- Seq()
-  end for    // error: `yield` or `do` expected
-  do ()
+  end Test // error: misaligned end marker
+  }  // error: eof expected, but unindent found

tests/neg/i8731a.scala

@@ -0,0 +1,10 @@
+object test:
+  while
+    3 == 3
+  end while  // error: `do` expected
+  do ()
+
+  for
+    a <- Seq()
+  end for    // error: `yield` or `do` expected
+  do ()            // error: expression expected but eof found

tests/neg/i9328.scala

@@ -14,8 +14,8 @@ case class Foo()
 
 case class Bar(
 } { ;  // error
-object Bar { // error
+object Bar {
   class Foo(a: Int) extends AnyVal
   Foo()
 }
-type Bar // error
+type Bar

tests/neg/indent.scala

@@ -4,6 +4,6 @@ object Test {
   val y3 =
     if (1) max 10 gt 0  // error: end of statement expected but integer literal found // error // error // error
       1
-    else  // error: end of statement expected but 'else' found
-      2   // error
-}
+    else
+      2
+} // error