Make fewerBraces a standard feature

Enables fewerBraces as a standard feature from 3.3 on. No language
import is needed.

Author: odersky

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

@@ -101,7 +101,11 @@ object Feature:
       case Some(v) => v
       case none => sourceVersionSetting
 
-  def migrateTo3(using Context): Boolean = sourceVersion == `3.0-migration`
+  def migrateTo3(using Context): Boolean =
+    sourceVersion == `3.0-migration`
+
+  def fewerBracesEnabled(using Context) =
+    sourceVersion.isAtLeast(`3.3`) || enabled(fewerBraces)
 
   /** If current source migrates to `version`, issue given warning message
    *  and return `true`, otherwise return `false`.

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

@@ -64,7 +64,6 @@ trait AllScalaSettings extends CommonScalaSettings, PluginSettings, VerboseSetti
   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", "(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/config/SourceVersion.scala

@@ -8,6 +8,7 @@ import util.Property
 enum SourceVersion:
   case `3.0-migration`, `3.0`, `3.1` // Note: do not add `3.1-migration` here, 3.1 is the same language as 3.0.
   case `3.2-migration`, `3.2`
+  case `3.3-migration`, `3.3`
   case `future-migration`, `future`
 
   val isMigrating: Boolean = toString.endsWith("-migration")

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

@@ -778,15 +778,14 @@ object Parsers {
         }
       })
       canRewrite &= (in.isAfterLineEnd || statCtdTokens.contains(in.token)) // test (5)
-      if (canRewrite && (!underColonSyntax || in.fewerBracesEnabled)) {
+      if canRewrite && (!underColonSyntax || Feature.fewerBracesEnabled) then
         val openingPatchStr =
           if !colonRequired then ""
           else if testChar(startOpening - 1, Chars.isOperatorPart(_)) then " :"
           else ":"
         val (startClosing, endClosing) = closingElimRegion()
         patch(source, Span(startOpening, endOpening), openingPatchStr)
         patch(source, Span(startClosing, endClosing), "")
-      }
       t
     }
 
@@ -1025,7 +1024,7 @@ object Parsers {
      *      body
      */
     def isColonLambda =
-      in.fewerBracesEnabled && in.token == COLONfollow && followingIsLambdaAfterColon()
+      Feature.fewerBracesEnabled && in.token == COLONfollow && followingIsLambdaAfterColon()
 
     /**   operand { infixop operand | MatchClause } [postfixop],
      *
@@ -2370,7 +2369,7 @@ object Parsers {
     /** PostfixExpr   ::= InfixExpr [id [nl]]
      *  InfixExpr     ::= PrefixExpr
      *                  | InfixExpr id [nl] InfixExpr
-     *                  | InfixExpr id `:` IndentedExpr
+     *                  | InfixExpr id ColonArgument
      *                  | InfixExpr MatchClause
      */
     def postfixExpr(location: Location = Location.ElseWhere): Tree =
@@ -2414,10 +2413,11 @@ object Parsers {
      *                 |  SimpleExpr `.` MatchClause
      *                 |  SimpleExpr (TypeArgs | NamedTypeArgs)
      *                 |  SimpleExpr1 ArgumentExprs
-     *                 |  SimpleExpr1 `:` ColonArgument                   -- under language.experimental.fewerBraces
-     *  ColonArgument ::= indent (CaseClauses | Block) outdent
-     *                 |  FunParams (‘=>’ | ‘?=>’) ColonArgBody
-     *                 |  HkTypeParamClause ‘=>’ ColonArgBody
+     *                 |  SimpleExpr1 ColonArgument
+     *  ColonArgument ::= colon [LambdaStart]
+     *                    indent (CaseClauses | Block) outdent
+     *  LambdaStart   ::= FunParams (‘=>’ | ‘?=>’)
+     *                 |  HkTypeParamClause ‘=>’
      *  ColonArgBody  ::= indent (CaseClauses | Block) outdent
      *  Quoted        ::= ‘'’ ‘{’ Block ‘}’
      *                 |  ‘'’ ‘[’ Type ‘]’

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

@@ -17,7 +17,7 @@ import scala.collection.mutable
 import scala.collection.immutable.SortedMap
 import rewrites.Rewrites.patch
 import config.Feature
-import config.Feature.migrateTo3
+import config.Feature.{migrateTo3, fewerBracesEnabled}
 import config.SourceVersion.`3.0`
 import reporting.{NoProfile, Profile}
 
@@ -202,25 +202,6 @@ object Scanners {
     def featureEnabled(name: TermName) = Feature.enabled(name)(using languageImportContext)
     def erasedEnabled = featureEnabled(Feature.erasedDefinitions)
 
-    private inline val fewerBracesByDefault = false
-      // turn on to study impact on codebase if `fewerBraces` was the default
-
-    private var fewerBracesEnabledCache = false
-    private var fewerBracesEnabledCtx: Context = NoContext
-
-    def fewerBracesEnabled =
-      if fewerBracesEnabledCtx ne myLanguageImportContext then
-        fewerBracesEnabledCache =
-          featureEnabled(Feature.fewerBraces)
-          || fewerBracesByDefault && indentSyntax && !migrateTo3
-          		// ensure that fewer braces is not the default for 3.0-migration since
-          		//     { x: T =>
-          		//       expr
-          		//     }
-                // would be ambiguous
-        fewerBracesEnabledCtx = myLanguageImportContext
-      fewerBracesEnabledCache
-
     private var postfixOpsEnabledCache = false
     private var postfixOpsEnabledCtx: Context = NoContext
 

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

@@ -186,6 +186,60 @@ Refinement        ::=  :<<< [RefineDcl] {semi [RefineDcl]} >>>
 Packaging         ::=  ‘package’ QualId :<<< TopStats >>>
 ```
 
+## Optional Braces for Method Arguments
+
+Starting with Scala 3.3, a `<colon>` token is also recognized where a function argument would be expected. Examples:
+
+```scala
+times(10):
+  println("ah")
+  println("ha")
+```
+
+or
+
+```scala
+credentials `++`:
+  val file = Path.userHome / ".credentials"
+  if file.exists
+  then Seq(Credentials(file))
+  else Seq()
+```
+
+or
+
+```scala
+xs.map:
+  x =>
+    val y = x - 1
+    y * y
+```
+What's more, a `:` in these settings can also be followed on the same line by the parameter part and arrow of a lambda. So the last example could be compressed to this:
+
+```scala
+xs.map: x =>
+  val y = x - 1
+  y * y
+```
+and the following would also be legal:
+```scala
+xs.foldLeft(0): (x, y) =>
+  x + y
+```
+
+The grammar changes for optional braces around arguments are as follows.
+
+```
+SimpleExpr       ::=  ...
+                   |  SimpleExpr ColonArgument
+InfixExpr        ::=  ...
+                   |  InfixExpr id ColonArgument
+ColonArgument    ::=  colon [LambdaStart]
+                      indent (CaseClauses | Block) outdent
+LambdaStart      ::=  FunParams (‘=>’ | ‘?=>’)
+                   |  HkTypeParamClause ‘=>’
+```
+
 ## Spaces vs Tabs
 
 Indentation prefixes can consist of spaces and/or tabs. Indentation widths are the indentation prefixes themselves, ordered by the string prefix relation. So, so for instance "2 tabs, followed by 4 spaces" is strictly less than "2 tabs, followed by 5 spaces", but "2 tabs, followed by 4 spaces" is incomparable to "6 tabs" or to "4 spaces, followed by 2 tabs". It is an error if the indentation width of some line is incomparable with the indentation width of the region that's current at that point. To avoid such errors, it is a good idea not to mix spaces and tabs in the same source file.
@@ -448,62 +502,3 @@ indented regions where possible. When invoked with options `-rewrite -no-indent`
 The `-indent` option only works on [new-style syntax](./control-syntax.md). So to go from old-style syntax to new-style indented code one has to invoke the compiler twice, first with options `-rewrite -new-syntax`, then again with options
 `-rewrite -indent`. To go in the opposite direction, from indented code to old-style syntax, it's `-rewrite -no-indent`, followed by `-rewrite -old-syntax`.
 
-## Variant: Indentation Marker `:` for Arguments
-
-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 functions 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 language import
-```scala
-import language.experimental.fewerBraces
-```
-In this variant, a `<colon>` token is also recognized where function argument would be expected. Examples:
-
-```scala
-times(10):
-  println("ah")
-  println("ha")
-```
-
-or
-
-```scala
-credentials `++`:
-  val file = Path.userHome / ".credentials"
-  if file.exists
-  then Seq(Credentials(file))
-  else Seq()
-```
-
-or
-
-```scala
-xs.map:
-  x =>
-    val y = x - 1
-    y * y
-```
-What's more, a `:` in these settings can also be followed on the same line by the parameter part and arrow of a lambda. So the last example could be compressed to this:
-
-```scala
-xs.map: x =>
-  val y = x - 1
-  y * y
-```
-and the following would also be legal:
-```scala
-xs.foldLeft(0): (x, y) =>
-  x + y
-```
-
-The grammar changes for this variant are as follows.
-
-```
-SimpleExpr       ::=  ...
-                   |  SimpleExpr ColonArgument
-InfixExpr        ::=  ...
-                   |  InfixExpr id ColonArgument
-ColonArgument    ::=  colon [LambdaStart]
-                      indent (CaseClauses | Block) outdent
-LambdaStart      ::=  FunParams (‘=>’ | ‘?=>’)
-                   |  HkTypeParamClause ‘=>’
-```

docs/_docs/reference/syntax.md

@@ -249,6 +249,7 @@ Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
 PostfixExpr       ::=  InfixExpr [id]                                          -- only if language.postfixOperators is enabled
 InfixExpr         ::=  PrefixExpr
                     |  InfixExpr id [nl] InfixExpr
+                    |  InfixExpr id ColonArgument
                     |  InfixExpr MatchClause
 MatchClause       ::=  ‘match’ <<< CaseClauses >>>
 PrefixExpr        ::=  [PrefixOperator] SimpleExpr
@@ -267,6 +268,11 @@ SimpleExpr        ::=  SimpleRef
                     |  SimpleExpr ‘.’ MatchClause
                     |  SimpleExpr TypeArgs
                     |  SimpleExpr ArgumentExprs
+                    |  SimpleExpr ColonArgument
+ColonArgument     ::=  colon [LambdaStart]
+                       indent (CaseClauses | Block) outdent
+LambdaStart       ::=  FunParams (‘=>’ | ‘?=>’)
+                    |  HkTypeParamClause ‘=>’
 Quoted            ::=  ‘'’ ‘{’ Block ‘}’
                     |  ‘'’ ‘[’ Type ‘]’
 ExprSplice        ::= spliceId                                                  -- if inside quoted block

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

@@ -51,6 +51,7 @@ object language:
     /** Experimental support for using indentation for arguments
      */
     @compileTimeOnly("`fewerBraces` can only be used at compile time in import statements")
+    @deprecated("`fewerBraces` is now standard, no language import is needed", since = "3.3")
     object fewerBraces
 
     /** Experimental support for typechecked exception capabilities
@@ -192,7 +193,6 @@ object language:
   @compileTimeOnly("`3.2` can only be used at compile time in import statements")
   object `3.2`
 
-/* This can be added when we go to 3.3
   /** Set source version to 3.3-migration.
     *
     * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
@@ -206,5 +206,5 @@ object language:
     */
   @compileTimeOnly("`3.3` can only be used at compile time in import statements")
   object `3.3`
-*/
+
 end language

tests/neg-custom-args/no-experimental/experimental-nested-imports-2.scala

@@ -1,31 +1,27 @@
 import annotation.experimental
 
 class Class1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
   def g = 1
 
 object Object1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
   def g = 1
 
 def fun1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
   def g = 1
 
 val value1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition

tests/neg-custom-args/no-experimental/experimental-nested-imports-3.scala

@@ -1,25 +1,21 @@
 import annotation.experimental
 
 class Class1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
 
 object Object1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
 
 def fun1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
 
 val value1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition

tests/neg-custom-args/no-experimental/experimental-nested-imports.scala

@@ -1,28 +1,24 @@
 import annotation.experimental
 
 class Class1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
 
 object Object1:
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
 
 def fun1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
   @experimental def f = 1
 
 val value1 =
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition

tests/neg-custom-args/no-experimental/experimental-package-imports.scala

@@ -1,7 +1,6 @@
 import annotation.experimental
 
 package foo {
-  import language.experimental.fewerBraces // error
   import language.experimental.namedTypeArguments // error
   import language.experimental.genericNumberLiterals // error
   import language.experimental.erasedDefinitions // ok: only check at erased definition
@@ -13,7 +12,6 @@ package foo {
 
 package foo2 {
   // ok: all definitions are top-level @experimental
-  import language.experimental.fewerBraces
   import language.experimental.namedTypeArguments
   import language.experimental.genericNumberLiterals
   import language.experimental.erasedDefinitions

tests/neg/closure-args.scala

@@ -1,4 +1,4 @@
-import language.experimental.fewerBraces
+import language.`3.3`
 
 val x = List(1).map: (x: => Int) => // error
   ???

tests/neg/i10943.scala

@@ -1,4 +1,4 @@
-import language.experimental.fewerBraces
+import language.`3.3`
 
 object T:
   class A