Introduce option to treat colon at eol as indentation start

A `:` at the end of a line starting an indentation region is now
allowed only if option `-Yindent-colon` is set. Rewriting to colons
is also subject to that setting.

Author: odersky

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -52,6 +52,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val oldSyntax: Setting[Boolean] = BooleanSetting("-old-syntax", "Require `(...)` around conditions")
   val indent: Setting[Boolean] = BooleanSetting("-indent", "allow significant indentation")
   val noindent: Setting[Boolean] = BooleanSetting("-noindent", "require classical {...} syntax, indentation is not significant")
+  val YindentColons: Setting[Boolean] = BooleanSetting("-Yindent-colons", "allow colons at ends-of-lines to start indentation blocks")
 
   /** Decompiler settings */
   val printTasty: Setting[Boolean] = BooleanSetting("-print-tasty", "Prints the raw tasty.") withAbbreviation "--print-tasty"

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

@@ -717,6 +717,7 @@ object Parsers {
      */
     def bracesToIndented[T](body: => T): T = {
       val colonRequired = possibleColonOffset == in.lastOffset
+      if colonRequired && !in.colonSyntax then return body
       val (startOpening, endOpening) = startingElimRegion(colonRequired)
       val isOutermost = in.currentRegion.isOutermost
       def allBraces(r: Region): Boolean = r match {

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

@@ -152,12 +152,15 @@ object Scanners {
     val rewriteNoIndent = ctx.settings.noindent.value && rewrite
 
     val noindentSyntax =
-      ctx.settings.noindent.value ||
-      ctx.settings.oldSyntax.value ||
-      isScala2Mode
+      ctx.settings.noindent.value
+      || ctx.settings.oldSyntax.value
+      || isScala2Mode
     val indentSyntax =
-      (if (Config.defaultIndent) !noindentSyntax else ctx.settings.indent.value) ||
-      rewriteNoIndent
+      (if (Config.defaultIndent) !noindentSyntax else ctx.settings.indent.value)
+      || rewriteNoIndent
+    val colonSyntax =
+      ctx.settings.YindentColons.value
+      || rewriteNoIndent
 
     if (rewrite) {
       val s = ctx.settings
@@ -590,7 +593,7 @@ object Scanners {
           lookahead()
           val atEOL = isAfterLineEnd
           reset()
-          if (atEOL) token = COLONEOL
+          if colonSyntax && atEOL then token = COLONEOL
         case EOF | RBRACE =>
           currentRegion match {
             case r: Indented if !r.isOutermost =>

compiler/src/dotty/tools/dotc/transform/Mixin.scala

@@ -263,7 +263,6 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
         transformFollowing(polyDefDef(mkForwarderSym(meth.asTerm, Bridge), forwarderRhsFn(meth)))
       }
 
-
     cpy.Template(impl)(
       constr =
         if (cls.is(Trait)) cpy.DefDef(impl.constr)(vparamss = Nil :: Nil)