Replace -Yindent-colons with language import

Replace -Yindent-colons with language import

   import language.experimental.fewerBraces

Author: odersky

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

@@ -25,7 +25,8 @@ object Feature:
 
   val dependent = experimental("dependent")
   val erasedDefinitions = experimental("erasedDefinitions")
-  val symbolLiterals: TermName = deprecated("symbolLiterals")
+  val symbolLiterals = deprecated("symbolLiterals")
+  val fewerBraces = experimental("fewerBraces")
 
 /** Is `feature` enabled by by a command-line setting? The enabling setting is
    *

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

@@ -121,7 +121,7 @@ class ScalaSettings extends Settings.SettingGroup with CommonScalaSettings {
   val oldSyntax: Setting[Boolean] = BooleanSetting("-old-syntax", "Require `(...)` around conditions.")
   val indent: Setting[Boolean] = BooleanSetting("-indent", "Together with -rewrite, remove {...} syntax when possible due to significant indentation.")
   val noindent: Setting[Boolean] = BooleanSetting("-no-indent", "Require classical {...} syntax, indentation is not significant.", aliases = List("-noindent"))
-  val YindentColons: Setting[Boolean] = BooleanSetting("-Yindent-colons", "Allow colons at ends-of-lines to start indentation blocks.")
+  val YindentColons: Setting[Boolean] = BooleanSetting("-Yindent-colons", "(disabled: use -language:experimental.fewerBraces instead)")
 
   /** Decompiler settings */
   val printTasty: Setting[Boolean] = BooleanSetting("-print-tasty", "Prints the raw tasty.", aliases = List("--print-tasty"))

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

@@ -743,7 +743,7 @@ object Parsers {
         }
       })
       canRewrite &= (in.isAfterLineEnd || statCtdTokens.contains(in.token)) // test (5)
-      if (canRewrite && (!underColonSyntax || in.colonSyntax)) {
+      if (canRewrite && (!underColonSyntax || in.fewerBracesEnabled)) {
         val openingPatchStr =
           if !colonRequired then ""
           else if testChar(startOpening - 1, Chars.isOperatorPart(_)) then " :"

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

@@ -182,9 +182,6 @@ object Scanners {
       ((if (Config.defaultIndent) !noindentSyntax else ctx.settings.indent.value)
        || rewriteNoIndent)
       && !isInstanceOf[LookaheadScanner]
-    val colonSyntax =
-      ctx.settings.YindentColons.value
-      || rewriteNoIndent
 
     if (rewrite) {
       val s = ctx.settings
@@ -201,6 +198,15 @@ object Scanners {
     def featureEnabled(name: TermName) = Feature.enabled(name)(using languageImportContext)
     def erasedEnabled = featureEnabled(Feature.erasedDefinitions) || ctx.settings.YerasedTerms.value
 
+    private var fewerBracesEnabledCache = false
+    private var fewerBracesEnabledCtx: Context = NoContext
+
+    def fewerBracesEnabled =
+      if fewerBracesEnabledCtx ne myLanguageImportContext then
+        fewerBracesEnabledCache = featureEnabled(Feature.fewerBraces)
+        fewerBracesEnabledCtx = myLanguageImportContext
+      fewerBracesEnabledCache
+
     /** All doc comments kept by their end position in a `Map`.
       *
       * Note: the map is necessary since the comments are looked up after an
@@ -635,7 +641,7 @@ object Scanners {
               else
                 reset()
         case COLON =>
-          if colonSyntax then observeColonEOL()
+          if fewerBracesEnabled then observeColonEOL()
         case RBRACE | RPAREN | RBRACKET =>
           closeIndented()
         case EOF =>

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -49,7 +49,6 @@ class CompilationTests {
         defaultOptions.and("-nowarn", "-Xfatal-warnings")
       ),
       compileFile("tests/pos-special/typeclass-scaling.scala", defaultOptions.and("-Xmax-inlines", "40")),
-      compileFile("tests/pos-special/indent-colons.scala", defaultOptions.and("-Yindent-colons")),
       compileFile("tests/pos-special/i7296.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileFile("tests/pos-special/notNull.scala", defaultOptions.and("-Yexplicit-nulls")),
       compileDir("tests/pos-special/adhoc-extension", defaultOptions.and("-source", "future", "-feature", "-Xfatal-warnings")),

docs/docs/reference/other-new-features/indentation.md

@@ -391,8 +391,11 @@ The `-indent` option only works on [new-style syntax](./control-syntax.md). So t
 
 Generally, the possible indentation regions coincide with those regions where braces `{...}` are also legal, no matter whether the braces enclose an expression or a set of definitions. There is one exception, though: Arguments to function can be enclosed in braces but they cannot be simply indented instead. Making indentation always significant for function arguments would be too restrictive and fragile.
 
-To allow such arguments to be written without braces, a variant of the indentation scheme is implemented under
-option `-Yindent-colons`. This variant is more contentious and less stable than the rest of the significant indentation scheme. In this variant, a colon `:` at the end of a line is also one of the possible tokens that opens an indentation region. Examples:
+To allow such arguments to be written without braces, a variant of the indentation scheme is implemented under language import
+```scala
+import language.experimental.fewerBraces
+```
+This variant is more contentious and less stable than the rest of the significant indentation scheme. In this variant, a colon `:` at the end of a line is also one of the possible tokens that opens an indentation region. Examples:
 
 ```scala
 times(10):

library/src/scala/runtime/stdLibPatches/language.scala

@@ -43,6 +43,9 @@ object language:
      */
     object erasedDefinitions
 
+    /** Experimental support for using indentation for arguments
+     */
+    object fewerBraces
   end experimental
 
   /** The deprecated object contains features that are no longer officially suypported in Scala.

tests/pos-special/indent-colons.scala renamed to tests/pos/indent-colons.scala

@@ -1,3 +1,4 @@
+import language.experimental.fewerBraces
 object Test:
 
   locally: