Remove the unsupported anyScalar case (#650)

We decided not to support the `anyScalar` character class, which would match a single Unicode scalar regardless of matching mode. However, its representation was still included in the various character class types in the regex engine, leading to unreachable code and unclear requirements when changing or adding new code. This change removes that representation where possible. The `DSLTree.Atom.CharacterClass` enum is left unchanged, since it is marked `@_spi(RegexBuilder) public`. Any use of that enum case is handled with a `fatalError("Unsupported")`, and it isn't produced on any code path.

Sources/_StringProcessing/ByteCodeGen.swift

@@ -702,9 +702,6 @@ fileprivate extension Compiler.ByteCodeGen {
  case .characterClass(let cc):
  // Custom character class that consumes a single grapheme
  let model = cc.asRuntimeModel(options)
- guard model.consumesSingleGrapheme else {
- return false
- }
  builder.buildQuantify(
  model: model,
  kind,

Sources/_StringProcessing/Engine/MEBuiltins.swift

@@ -193,8 +193,6 @@ extension String {
  switch (isScalarSemantics, cc) {
  case (_, .anyGrapheme):
  next = index(after: currentPosition)
- case (_, .anyScalar):
- next = unicodeScalars.index(after: currentPosition)
  case (true, _):
  next = unicodeScalars.index(after: currentPosition)
  case (false, _):
@@ -204,12 +202,6 @@ extension String {
  switch cc {
  case .any, .anyGrapheme:
  matched = true
- case .anyScalar:
- if isScalarSemantics {
- matched = true
- } else {
- matched = isOnGraphemeClusterBoundary(next)
- }
  case .digit:
  if isScalarSemantics {
  matched = scalar.properties.numericType != nil && asciiCheck

Sources/_StringProcessing/PrintAsPattern.swift

@@ -760,8 +760,6 @@ extension DSLTree.Atom.CharacterClass {
  switch self {
  case .anyGrapheme:
  return ".anyGraphemeCluster"
- case .anyUnicodeScalar:
- return ".anyUnicodeScalar"
  case .digit:
  return ".digit"
  case .notDigit:
@@ -786,6 +784,8 @@ extension DSLTree.Atom.CharacterClass {
  return ".whitespace"
  case .notWhitespace:
  return ".whitespace.inverted"
+ case .anyUnicodeScalar:
+ fatalError("Unsupported")
  }
  }
 }

Sources/_StringProcessing/Regex/ASTConversion.swift

@@ -183,7 +183,6 @@ extension AST.Atom.EscapedBuiltin {
  case .wordCharacter: return .word
  case .notWordCharacter: return .notWord
  case .graphemeCluster: return .anyGrapheme
- case .trueAnychar: return .anyUnicodeScalar
  default: return nil
  }
  }

Sources/_StringProcessing/Regex/DSLTree.swift

@@ -260,7 +260,6 @@ extension DSLTree.Atom.CharacterClass {
  public var inverted: DSLTree.Atom.CharacterClass? {
  switch self {
  case .anyGrapheme: return nil
- case .anyUnicodeScalar: return nil
  case .digit: return .notDigit
  case .notDigit: return .digit
  case .word: return .notWord
@@ -273,6 +272,8 @@ extension DSLTree.Atom.CharacterClass {
  case .notVerticalWhitespace: return .verticalWhitespace
  case .whitespace: return .notWhitespace
  case .notWhitespace: return .whitespace
+ case .anyUnicodeScalar:
+ fatalError("Unsupported")
  }
  }
 }

Sources/_StringProcessing/Unicode/ASCII.swift

@@ -134,8 +134,7 @@ extension String {
 
  // TODO: bitvectors
  switch cc {
- case .any, .anyGrapheme, .anyScalar:
- // TODO: should any scalar not consume CR-LF in scalar semantic mode?
+ case .any, .anyGrapheme:
  return (next, true)
 
  case .digit:

Sources/_StringProcessing/_CharacterClassModel.swift

@@ -45,8 +45,6 @@ struct _CharacterClassModel: Hashable {
  case any = 0
  /// Any grapheme cluster
  case anyGrapheme
- /// Any Unicode scalar
- case anyScalar
  /// Character.isDigit
  case digit
  /// Horizontal whitespace: `[:blank:]`, i.e
@@ -90,15 +88,6 @@ struct _CharacterClassModel: Hashable {
  }
 }
 
-extension _CharacterClassModel {
- var consumesSingleGrapheme: Bool {
- switch self.cc {
- case .anyScalar: return false
- default: return true
- }
- }
-}
-
 extension _CharacterClassModel.Representation {
  /// Returns true if this CharacterClass should be matched by strict ascii under the given options
  func isStrictAscii(options: MatchingOptions) -> Bool {
@@ -119,7 +108,6 @@ extension _CharacterClassModel.Representation: CustomStringConvertible {
  switch self {
  case .any: return "<any>"
  case .anyGrapheme: return "<any grapheme>"
- case .anyScalar: return "<any scalar>"
  case .digit: return "<digit>"
  case .horizontalWhitespace: return "<horizontal whitespace>"
  case .newlineSequence: return "<newline sequence>"
@@ -185,7 +173,7 @@ extension DSLTree.Atom.CharacterClass {
  case .anyGrapheme:
  cc = .anyGrapheme
  case .anyUnicodeScalar:
- cc = .anyScalar
+ fatalError("Unsupported")
  }
  return _CharacterClassModel(cc: cc, options: options, isInverted: inverted)
  }