Fix indent insertion at arrow eol

Author: som-snytt

Diff for: compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -34,6 +34,7 @@ import config.Feature.{sourceVersion, migrateTo3}
 import config.SourceVersion.*
 import config.SourceVersion
 import dotty.tools.dotc.config.MigrationVersion
+import dotty.tools.dotc.util.chaining.*
 
 object Parsers {
 
@@ -1087,11 +1088,11 @@ object Parsers {
      */
     def followingIsLambdaAfterColon(): Boolean =
       val lookahead = in.LookaheadScanner(allowIndent = true)
+                      .tap(_.currentRegion.knownWidth = in.currentRegion.indentWidth)
       def isArrowIndent() =
         lookahead.isArrow
         && {
-          lookahead.observeArrowEOL()
-          lookahead.nextToken()
+          lookahead.observeArrowIndented()
           lookahead.token == INDENT || lookahead.token == EOF
         }
       lookahead.nextToken()
@@ -2655,14 +2656,10 @@ object Parsers {
 
     def closureRest(start: Int, location: Location, params: List[Tree]): Tree =
       atSpan(start, in.offset) {
-        if location == Location.InColonArg then
-          in.observeArrowEOL()
         if in.token == CTXARROW then
           if params.isEmpty then
             syntaxError(em"context function literals require at least one formal parameter", Span(start, in.lastOffset))
           in.nextToken()
-        else if in.token == ARROWeol then
-          in.nextToken()
         else
           accept(ARROW)
         val body =

Diff for: compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -92,7 +92,7 @@ object Scanners {
       || token == IDENTIFIER && isOperatorPart(name(name.length - 1))
 
     def isArrow =
-      token == ARROW || token == CTXARROW || token == ARROWeol
+      token == ARROW || token == CTXARROW
   }
 
   abstract class ScannerCommon(source: SourceFile)(using Context) extends CharArrayReader with TokenData {
@@ -646,7 +646,6 @@ object Scanners {
              || lastWidth == nextWidth
                && lastToken.match
                   case MATCH | CATCH => token == CASE
-                  case ARROWeol => true
                   case _ => false
         then
           if canStartIndentTokens.contains(lastToken) then
@@ -680,12 +679,19 @@ object Scanners {
         reset()
         if atEOL then token = COLONeol
 
-    def observeArrowEOL(): Unit =
-      if indentSyntax && token == ARROW then
+    // consume => and insert <indent> if applicable
+    def observeArrowIndented(): Unit =
+      if isArrow && indentSyntax then
         peekAhead()
         val atEOL = isAfterLineEnd || token == EOF
         reset()
-        if atEOL then token = ARROWeol
+        if atEOL then
+          val nextWidth = indentWidth(next.offset)
+          val lastWidth = currentRegion.indentWidth
+          if lastWidth < nextWidth then
+            currentRegion = Indented(nextWidth, COLONeol, currentRegion)
+            offset = next.offset
+            token = INDENT
 
     def observeIndented(): Unit =
       if indentSyntax && isNewLine then

Diff for: compiler/src/dotty/tools/dotc/parsing/Tokens.scala

@@ -202,10 +202,9 @@ object Tokens extends TokensCommon {
   inline val COLONeol = 89;         enter(COLONeol, ":", ": at eol")
     // A `:` recognized as starting an indentation block
   inline val SELFARROW = 90;        enter(SELFARROW, "=>") // reclassified ARROW following self-type
-  inline val ARROWeol = 99;         enter(ARROWeol, "=>", "=> at eol") // lambda ARROW at eol followed by indent
 
   /** XML mode */
-  inline val XMLSTART = 100;        enter(XMLSTART, "$XMLSTART$<") // TODO: deprecate
+  inline val XMLSTART = 99;         enter(XMLSTART, "$XMLSTART$<") // TODO: deprecate
 
   final val alphaKeywords: TokenSet = tokenRange(IF, END)
   final val symbolicKeywords: TokenSet = tokenRange(USCORE, CTXARROW)
@@ -283,7 +282,7 @@ object Tokens extends TokensCommon {
   final val closingRegionTokens = BitSet(RBRACE, RPAREN, RBRACKET, CASE) | statCtdTokens
 
   final val canStartIndentTokens: BitSet =
-    statCtdTokens | BitSet(COLONeol, WITH, EQUALS, ARROWeol, ARROW, CTXARROW, LARROW, WHILE, TRY, FOR, IF, THROW, RETURN)
+    statCtdTokens | BitSet(COLONeol, WITH, EQUALS, ARROW, CTXARROW, LARROW, WHILE, TRY, FOR, IF, THROW, RETURN)
 
   /** Faced with the choice between a type and a formal parameter, the following
    *  tokens determine it's a formal parameter.

Diff for: tests/neg/i22193.scala

@@ -5,6 +5,7 @@ def fn3(arg: String, arg2: String)(f: => Unit): Unit = f
 
 def test1() =
 
+  // ok baseline
   fn2(arg = "blue sleeps faster than tuesday", arg2 = "the quick brown fox jumped over the lazy dog"): env =>
     val x = env
     println(x)
@@ -15,12 +16,6 @@ def test1() =
   val x = env // error
   println(x)
 
-  fn2( // error
-      arg = "blue sleeps faster than tuesday",
-      arg2 = "the quick brown fox jumped over the lazy dog"): env =>
-    val x = env // error
-    println(x)
-
   fn2(
       arg = "blue sleeps faster than tuesday",
       arg2 = "the quick brown fox jumped over the lazy dog"):
@@ -30,13 +25,6 @@ def test1() =
 
 def test2() =
 
-  fn2(
-      arg = "blue sleeps faster than tuesday",
-      arg2 = "the quick brown fox jumped over the lazy dog"
-  ): env =>
-    val x = env
-    println(x)
-
   fn3( // error missing argument list for value of type (=> Unit) => Unit
     arg = "blue sleeps faster than tuesday",
     arg2 = "the quick brown fox jumped over the lazy dog"):

Diff for: tests/pos/i22193.scala

@@ -22,6 +22,12 @@ def test() =
        val x = env
        println(x)
 
+  fn2(
+      arg = "blue sleeps faster than tuesday",
+      arg2 = "the quick brown fox jumped over the lazy dog"): env =>
+    val x = env
+    println(x)
+
   // does compile
   fn2(
       arg = "blue sleeps faster than tuesday",
@@ -38,6 +44,13 @@ def test() =
       val x = env
       println(x)
 
+  fn2(
+      arg = "blue sleeps faster than tuesday",
+      arg2 = "the quick brown fox jumped over the lazy dog"
+  ): env =>
+    val x = env
+    println(x)
+
   fn3(
     arg = "blue sleeps faster than tuesday",
     arg2 = "the quick brown fox jumped over the lazy dog"):
@@ -55,3 +68,11 @@ def regress(x: Int) =
   x match
   case 42 =>
   case _ =>
+
+// previously lookahead calculated indent width at the colon
+def k(xs: List[Int]) =
+  xs.foldLeft(
+      0)
+                                            : (acc, x) =>
+        acc + x
+