Make fewerBraces a standard feature (#16297)

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

The old 

    import language.experimental.fewerBraces

is deprecated but it still works. This is so that one can continue to
mix fewerBraces in for projects compiling with earlier versions. We can
turn the language import into a no-op or drop it completely when 3.3 is
in the rear mirror.

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