Change fewerbraces to always use a colon, even before lambdas

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

@@ -179,7 +179,7 @@ object JavaScanners {
               nextChar()
 
             case ':' =>
-              token = COLON
+              token = COLONop
               nextChar()
 
             case '@' =>

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

@@ -75,13 +75,8 @@ object Scanners {
     def isNestedStart = token == LBRACE || token == INDENT
     def isNestedEnd = token == RBRACE || token == OUTDENT
 
-    /** Is token a COLON, after having converted COLONEOL to COLON?
-     *  The conversion means that indentation is not significant after `:`
-     *  anymore. So, warning: this is a side-effecting operation.
-     */
-    def isColon() =
-      if token == COLONEOL then token = COLON
-      token == COLON
+    def isColon =
+      token == COLONop || token == COLONfollow || token == COLONeol
 
     /** Is current token first one after a newline? */
     def isAfterLineEnd: Boolean = lineOffset >= 0
@@ -189,7 +184,10 @@ object Scanners {
     val indentSyntax =
       ((if (Config.defaultIndent) !noindentSyntax else ctx.settings.indent.value)
        || rewriteNoIndent)
-      && !isInstanceOf[LookaheadScanner]
+      && { this match
+        case self: LookaheadScanner => self.allowIndent
+        case _ => true
+      }
 
     if (rewrite) {
       val s = ctx.settings
@@ -206,12 +204,22 @@ 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)
+        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
 
@@ -386,10 +394,11 @@ object Scanners {
      */
     def nextToken(): Unit =
       val lastToken = token
+      val lastName = name
       adjustSepRegions(lastToken)
       getNextToken(lastToken)
       if isAfterLineEnd then handleNewLine(lastToken)
-      postProcessToken()
+      postProcessToken(lastToken, lastName)
       printState()
 
     final def printState() =
@@ -420,7 +429,7 @@ object Scanners {
       && {
         // Is current lexeme  assumed to start an expression?
         // This is the case if the lexime is one of the tokens that
-        // starts an expression or it is a COLONEOL. Furthermore, if
+        // starts an expression or it is a COLONeol. Furthermore, if
         // the previous token is in backticks, the lexeme may not be a binary operator.
         // I.e. in
         //
@@ -431,7 +440,7 @@ object Scanners {
         // in backticks and is a binary operator. Hence, `x` is not classified as a
         // leading infix operator.
         def assumeStartsExpr(lexeme: TokenData) =
-          (canStartExprTokens.contains(lexeme.token) || lexeme.token == COLONEOL)
+          (canStartExprTokens.contains(lexeme.token) || lexeme.token == COLONeol)
           && (!lexeme.isOperator || nme.raw.isUnary(lexeme.name))
         val lookahead = LookaheadScanner()
         lookahead.allowLeadingInfixOperators = false
@@ -607,12 +616,11 @@ object Scanners {
             currentRegion match
               case r: Indented =>
                 insert(OUTDENT, offset)
-                if next.token != COLON then
-                  handleNewIndentWidth(r.enclosing, ir =>
-                    errorButContinue(
-                      i"""The start of this line does not match any of the previous indentation widths.
-                          |Indentation width of current line : $nextWidth
-                          |This falls between previous widths: ${ir.width} and $lastWidth"""))
+                handleNewIndentWidth(r.enclosing, ir =>
+                  errorButContinue(
+                    i"""The start of this line does not match any of the previous indentation widths.
+                       |Indentation width of current line : $nextWidth
+                       |This falls between previous widths: ${ir.width} and $lastWidth"""))
               case r =>
                 if skipping then
                   if r.enclosing.isClosedByUndentAt(nextWidth) then
@@ -629,7 +637,7 @@ object Scanners {
             currentRegion.knownWidth = nextWidth
         else if (lastWidth != nextWidth)
           errorButContinue(spaceTabMismatchMsg(lastWidth, nextWidth))
-      if token != OUTDENT || next.token == COLON then
+      if token != OUTDENT then
         handleNewIndentWidth(currentRegion, _.otherIndentWidths += nextWidth)
     end handleNewLine
 
@@ -638,19 +646,24 @@ object Scanners {
          |Previous indent : $lastWidth
          |Latest indent   : $nextWidth"""
 
-    def observeColonEOL(): Unit =
-      if token == COLON then
+    def observeColonEOL(inTemplate: Boolean): Unit =
+      val enabled =
+        if token == COLONop && inTemplate then
+          report.deprecationWarning(em"`:` after symbolic operator is deprecated; use backticks around operator instead", sourcePos(offset))
+          true
+        else token == COLONfollow && (inTemplate || fewerBracesEnabled)
+      if enabled then
         lookAhead()
         val atEOL = isAfterLineEnd || token == EOF
         reset()
-        if atEOL then token = COLONEOL
+        if atEOL then token = COLONeol
 
     def observeIndented(): Unit =
       if indentSyntax && isNewLine then
         val nextWidth = indentWidth(next.offset)
         val lastWidth = currentRegion.indentWidth
         if lastWidth < nextWidth then
-          currentRegion = Indented(nextWidth, COLONEOL, currentRegion)
+          currentRegion = Indented(nextWidth, COLONeol, currentRegion)
           offset = next.offset
           token = INDENT
     end observeIndented
@@ -683,10 +696,10 @@ object Scanners {
       case _ =>
 
     /** - Join CASE + CLASS => CASECLASS, CASE + OBJECT => CASEOBJECT
-     *         SEMI + ELSE => ELSE, COLON + <EOL> => COLONEOL
+     *         SEMI + ELSE => ELSE, COLON following id/)/] => COLONfollow
      *  - Insert missing OUTDENTs at EOF
      */
-    def postProcessToken(): Unit = {
+    def postProcessToken(lastToken: Token, lastName: SimpleName): Unit = {
       def fuse(tok: Int) = {
         token = tok
         offset = prev.offset
@@ -721,8 +734,10 @@ object Scanners {
                 reset()
         case END =>
           if !isEndMarker then token = IDENTIFIER
-        case COLON =>
-          if fewerBracesEnabled then observeColonEOL()
+        case COLONop =>
+          if lastToken == IDENTIFIER && lastName != null && isIdentifierStart(lastName.head)
+              || colonEOLPredecessors.contains(lastToken)
+          then token = COLONfollow
         case RBRACE | RPAREN | RBRACKET =>
           closeIndented()
         case EOF =>
@@ -1067,7 +1082,7 @@ object Scanners {
         reset()
       next
 
-    class LookaheadScanner() extends Scanner(source, offset) {
+    class LookaheadScanner(val allowIndent: Boolean = false) extends Scanner(source, offset) {
       override def languageImportContext = Scanner.this.languageImportContext
     }
 
@@ -1179,7 +1194,7 @@ object Scanners {
       isSoftModifier && inModifierPosition()
 
     def isSoftModifierInParamModifierPosition: Boolean =
-      isSoftModifier && lookahead.token != COLON
+      isSoftModifier && !lookahead.isColon
 
     def isErased: Boolean = isIdent(nme.erased) && erasedEnabled
 
@@ -1518,7 +1533,9 @@ object Scanners {
       case NEWLINE => ";"
       case NEWLINES => ";;"
       case COMMA => ","
-      case _ => showToken(token)
+      case COLONfollow | COLONeol => "':'"
+      case _ =>
+        if debugTokenStream then showTokenDetailed(token) else showToken(token)
     }
 
     /* Resume normal scanning after XML */

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

@@ -16,13 +16,6 @@ abstract class TokensCommon {
 
   def tokenRange(lo: Int, hi: Int): TokenSet = BitSet(lo to hi: _*)
 
-  def showTokenDetailed(token: Int): String = debugString(token)
-
-  def showToken(token: Int): String = {
-    val str = tokenString(token)
-    if (isKeyword(token)) s"'$str'" else str
-  }
-
   val tokenString, debugString: Array[String] = new Array[String](maxToken + 1)
 
   def enter(token: Int, str: String, debugStr: String = ""): Unit = {
@@ -107,7 +100,7 @@ abstract class TokensCommon {
 
   /** special keywords */
   //inline val USCORE = 73;           enter(USCORE, "_")
-  inline val COLON = 74;            enter(COLON, ":")
+  inline val COLONop = 74;          enter(COLONop, ":")  // a stand-alone `:`, see also COLONfollow
   inline val EQUALS = 75;           enter(EQUALS, "=")
   //inline val LARROW = 76;           enter(LARROW, "<-")
   //inline val ARROW = 77;            enter(ARROW, "=>")
@@ -204,8 +197,11 @@ object Tokens extends TokensCommon {
 
   inline val QUOTE = 87;            enter(QUOTE, "'")
 
-  inline val COLONEOL = 88;         enter(COLONEOL, ":", ": at eol")
-  inline val SELFARROW = 89;        enter(SELFARROW, "=>") // reclassified ARROW following self-type
+  inline val COLONfollow = 88;      enter(COLONfollow, ":")
+    // A `:` following an alphanumeric identifier or one of the tokens in colonEOLPredecessors
+  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
 
   /** XML mode */
   inline val XMLSTART = 99;         enter(XMLSTART, "$XMLSTART$<") // TODO: deprecate
@@ -233,7 +229,7 @@ object Tokens extends TokensCommon {
   final val canStartExprTokens2: TokenSet = canStartExprTokens3 | BitSet(DO)
 
   final val canStartTypeTokens: TokenSet = literalTokens | identifierTokens | BitSet(
-    THIS, SUPER, USCORE, LPAREN, AT)
+    THIS, SUPER, USCORE, LPAREN, LBRACE, AT)
 
   final val templateIntroTokens: TokenSet = BitSet(CLASS, TRAIT, OBJECT, ENUM, CASECLASS, CASEOBJECT)
 
@@ -276,7 +272,7 @@ object Tokens extends TokensCommon {
   final val closingRegionTokens = BitSet(RBRACE, RPAREN, RBRACKET, CASE) | statCtdTokens
 
   final val canStartIndentTokens: BitSet =
-    statCtdTokens | BitSet(COLONEOL, WITH, EQUALS, 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.
@@ -287,7 +283,16 @@ object Tokens extends TokensCommon {
 
   final val endMarkerTokens = identifierTokens | BitSet(IF, WHILE, FOR, MATCH, TRY, NEW, THROW, GIVEN, VAL, THIS)
 
+  final val colonEOLPredecessors = BitSet(RPAREN, RBRACKET, BACKQUOTED_IDENT, THIS, SUPER, NEW)
+
   final val closingParens = BitSet(RPAREN, RBRACKET, RBRACE)
 
   final val softModifierNames = Set(nme.inline, nme.opaque, nme.open, nme.transparent, nme.infix)
+
+  def showTokenDetailed(token: Int): String = debugString(token)
+
+  def showToken(token: Int): String = {
+    val str = tokenString(token)
+    if isKeyword(token) || token == COLONfollow || token == COLONeol then s"'$str'" else str
+  }
 }

docs/_docs/internals/syntax.md

@@ -3,6 +3,20 @@ layout: doc-page
 title: "Scala 3 Syntax Summary"
 ---
 
+<!--
+
+This page has a companion page at _docs/reference/syntax.md.
+
+!! Make sure to edit both pages in sync. !!
+
+reference/syntax.md shows the official Scala 3 syntax, without deprecated or experimental features.
+
+internals/syntax.md shows the Scala 3 syntax as supported by the parser, including
+deprecated and experimental features. It also gives some indications how
+productions map to AST nodes.
+
+-->
+
 The following description of Scala tokens uses literal characters `‘c’` when
 referring to the ASCII fragment `\u0000` – `\u007F`.
 
@@ -88,22 +102,24 @@ nl               ::=  “new line character”
 semi             ::=  ‘;’ |  nl {nl}
 ```
 
-
 ## Optional Braces
 
 The lexical analyzer also inserts `indent` and `outdent` tokens that represent regions of indented code [at certain points](../reference/other-new-features/indentation.md)
 
 In the context-free productions below we use the notation `<<< ts >>>`
 to indicate a token sequence `ts` that is either enclosed in a pair of braces `{ ts }` or that constitutes an indented region `indent ts outdent`. Analogously, the
 notation `:<<< ts >>>` indicates a token sequence `ts` that is either enclosed in a pair of braces `{ ts }` or that constitutes an indented region `indent ts outdent` that follows
-a `:` at the end of a line.
+a `colon` token.
 
+A `colon` token reads as the standard colon "`:`" but is generated instead of it where `colon` is legal according to the context free syntax, but only if the previous token
+is an alphanumeric identifier, a backticked identifier, or one of the tokens `this`, `super`, `new`, "`)`", and "`]`".
 
 ```
+colon         ::=  ':'    -- with side conditions explained above
  <<< ts >>>   ::=  ‘{’ ts ‘}’
                 |  indent ts outdent
 :<<< ts >>>   ::=  [nl] ‘{’ ts ‘}’
-                |  `:` indent ts outdent
+                |  colon indent ts outdent
 ```
 
 ## Keywords
@@ -124,7 +140,7 @@ type      val       var       while     with      yield
 ### Soft keywords
 
 ```
-as  derives  end  extension  infix  inline  opaque  open  transparent  using  |  *  +  -
+as  derives  end  extension  infix  inline  opaque  open  throws transparent  using  |  *  +  -
 ```
 
 See the [separate section on soft keywords](../reference/soft-modifier.md) for additional
@@ -197,7 +213,7 @@ FunArgTypes       ::=  FunArgType { ‘,’ FunArgType }
 ParamType         ::=  [‘=>’] ParamValueType
 ParamValueType    ::=  Type [‘*’]                                               PostfixOp(t, "*")
 TypeArgs          ::=  ‘[’ Types ‘]’                                            ts
-Refinement        ::=  ‘{’ [RefineDcl] {semi [RefineDcl]} ‘}’                   ds
+Refinement        ::=  :<<< [RefineDcl] {semi [RefineDcl]} >>>                  ds
 TypeBounds        ::=  [‘>:’ Type] [‘<:’ Type]                                  TypeBoundsTree(lo, hi)
 TypeParamBounds   ::=  TypeBounds {‘:’ Type}                                    ContextBounds(typeBounds, tps)
 Types             ::=  Type {‘,’ Type}
@@ -231,13 +247,13 @@ Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
 Ascription        ::=  ‘:’ InfixType                                            Typed(expr, tp)
                     |  ‘:’ Annotation {Annotation}                              Typed(expr, Annotated(EmptyTree, annot)*)
 Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
-PostfixExpr       ::=  InfixExpr [id]                                           PostfixOp(expr, op)
+PostfixExpr       ::=  InfixExpr [id]                                           PostfixOp(expr, op) -- only if language.postfixOperators is enabled
 InfixExpr         ::=  PrefixExpr
                     |  InfixExpr id [nl] InfixExpr                              InfixOp(expr, op, expr)
-                    |  InfixExpr id ‘:’ IndentedExpr
+                    |  InfixExpr id ColonArgument
                     |  InfixExpr MatchClause
 MatchClause       ::=  ‘match’ <<< CaseClauses >>>                              Match(expr, cases)
-PrefixExpr        ::=  [PrefixOperator] SimpleExpr                             PrefixOp(expr, op)
+PrefixExpr        ::=  [PrefixOperator] SimpleExpr                              PrefixOp(expr, op)
 PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
 SimpleExpr        ::=  SimpleRef
                     |  Literal
@@ -253,11 +269,13 @@ SimpleExpr        ::=  SimpleRef
                     |  SimpleExpr ‘.’ MatchClause
                     |  SimpleExpr TypeArgs                                      TypeApply(expr, args)
                     |  SimpleExpr ArgumentExprs                                 Apply(expr, args)
-                    |  SimpleExpr ‘:’ IndentedExpr                              -- under language.experimental.fewerBraces
-                    |  SimpleExpr FunParams (‘=>’ | ‘?=>’) IndentedExpr         -- under language.experimental.fewerBraces
+                    |  SimpleExpr ColonArgument                                 -- under language.experimental.fewerBraces
                     |  SimpleExpr ‘_’                                           PostfixOp(expr, _) (to be dropped)
-                    |  XmlExpr													                        -- to be dropped
-IndentedExpr      ::=  indent CaseClauses | Block outdent
+                    |  XmlExpr							-- to be dropped
+ColonArgument     ::=  colon [LambdaStart]
+                       indent (CaseClauses | Block) outdent
+LambdaStart       ::=  FunParams (‘=>’ | ‘?=>’)
+                    |  HkTypeParamClause ‘=>’
 Quoted            ::=  ‘'’ ‘{’ Block ‘}’
                     |  ‘'’ ‘[’ Type ‘]’
 ExprSplice        ::= spliceId                                                  -- if inside quoted block
@@ -300,7 +318,7 @@ TypeCaseClause    ::=  ‘case’ (InfixType | ‘_’) ‘=>’ Type [semi]
 
 Pattern           ::=  Pattern1 { ‘|’ Pattern1 }                                Alternative(pats)
 Pattern1          ::=  Pattern2 [‘:’ RefinedType]                               Bind(name, Typed(Ident(wildcard), tpe))
-Pattern2          ::=  [id ‘@’] InfixPattern                                    Bind(name, pat)
+Pattern2          ::=  [id ‘@’] InfixPattern [‘*’]                              Bind(name, pat)
 InfixPattern      ::=  SimplePattern { id [nl] SimplePattern }                  InfixOp(pat, op, pat)
 SimplePattern     ::=  PatVar                                                   Ident(wildcard)
                     |  Literal                                                  Bind(name, Ident(wildcard))