Simplify checking if a token can be an argument label

# Conflicts:
#	Sources/SwiftParser/Types.swift

Author: ahoppen

Sources/SwiftParser/Expressions.swift

@@ -2092,7 +2092,7 @@ extension Parser {
       let unexpectedBeforeLabel: RawUnexpectedNodesSyntax?
       let label: RawTokenSyntax?
       let colon: RawTokenSyntax?
-      if currentToken.canBeArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon {
+      if self.atArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon {
         (unexpectedBeforeLabel, label) = parseArgumentLabel()
         colon = consumeAnyToken()
       } else {
@@ -2177,7 +2177,7 @@ extension Parser.Lookahead {
     // Fast path: the next two tokens must be a label and a colon.
     // But 'default:' is ambiguous with switch cases and we disallow it
     // (unless escaped) even outside of switches.
-    if !self.currentToken.canBeArgumentLabel()
+    if !self.atArgumentLabel()
       || self.at(.keyword(.default))
       || self.peek().rawTokenKind != .colon
     {

Sources/SwiftParser/Names.swift

@@ -24,7 +24,7 @@ extension Parser {
   }
 
   mutating func parseArgumentLabel() -> (RawUnexpectedNodesSyntax?, RawTokenSyntax) {
-    guard self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) else {
+    guard self.atArgumentLabel(allowDollarIdentifier: true) else {
       return (nil, missingToken(.identifier))
     }
     if let dollarIdent = self.consume(if: .dollarIdentifier) {
@@ -98,7 +98,7 @@ extension Parser {
     // A close parenthesis, if empty lists are allowed.
     let nextIsRParen = flags.contains(.zeroArgCompoundNames) && next.rawTokenKind == .rightParen
     // An argument label.
-    let nextIsArgLabel = next.canBeArgumentLabel() || next.rawTokenKind == .colon
+    let nextIsArgLabel = next.isArgumentLabel() || next.rawTokenKind == .colon
 
     guard nextIsRParen || nextIsArgLabel else {
       return nil
@@ -117,7 +117,7 @@ extension Parser {
       var loopProgress = LoopProgressCondition()
       while !self.at(.endOfFile, .rightParen) && loopProgress.evaluate(currentToken) {
         // Check to see if there is an argument label.
-        precondition(self.currentToken.canBeArgumentLabel() && self.peek().rawTokenKind == .colon)
+        precondition(self.atArgumentLabel() && self.peek().rawTokenKind == .colon)
         let name = self.consumeAnyToken()
         let (unexpectedBeforeColon, colon) = self.expect(.colon)
         elements.append(
@@ -242,7 +242,7 @@ extension Parser.Lookahead {
     var loopProgress = LoopProgressCondition()
     while !lookahead.at(.endOfFile, .rightParen) && loopProgress.evaluate(lookahead.currentToken) {
       // Check to see if there is an argument label.
-      guard lookahead.currentToken.canBeArgumentLabel() && lookahead.peek().rawTokenKind == .colon else {
+      guard lookahead.atArgumentLabel() && lookahead.peek().rawTokenKind == .colon else {
         return false
       }
 
@@ -261,18 +261,16 @@ extension Parser.Lookahead {
 }
 
 extension Lexer.Lexeme {
-  func canBeArgumentLabel(allowDollarIdentifier: Bool = false) -> Bool {
-    // `inout` is reserved as an argument label for historical reasons.
-    if TypeSpecifier(lexeme: self) == .inout {
-      return false
-    }
-
-    switch self.rawTokenKind {
+  func isArgumentLabel(allowDollarIdentifier: Bool = false) -> Bool {
+    switch self {
     case .identifier, .wildcard:
       // Identifiers, escaped identifiers, and '_' can be argument labels.
       return true
     case .dollarIdentifier:
       return allowDollarIdentifier
+    case .keyword(.inout):
+      // `inout` is reserved as an argument label for historical reasons.
+      return false
     default:
       // All other keywords can be argument labels.
       return self.isLexerClassifiedKeyword

Sources/SwiftParser/Parameters.swift

@@ -64,15 +64,15 @@ extension Parser {
   fileprivate mutating func parseParameterNames() -> ParameterNames {
     let unexpectedBeforeFirstName: RawUnexpectedNodesSyntax?
     let firstName: RawTokenSyntax?
-    if self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) {
+    if self.atArgumentLabel(allowDollarIdentifier: true) {
       (unexpectedBeforeFirstName, firstName) = self.parseArgumentLabel()
     } else {
       (unexpectedBeforeFirstName, firstName) = (nil, nil)
     }
 
     let unexpectedBeforeSecondName: RawUnexpectedNodesSyntax?
     let secondName: RawTokenSyntax?
-    if self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) {
+    if self.atArgumentLabel(allowDollarIdentifier: true) {
       (unexpectedBeforeSecondName, secondName) = self.parseArgumentLabel()
     } else {
       (unexpectedBeforeSecondName, secondName) = (nil, nil)
@@ -280,7 +280,7 @@ extension Parser {
     // to be an argument label, don't parse any parameters.
     let shouldSkipParameterParsing =
       lparen.isMissing
-      && (!currentToken.canBeArgumentLabel(allowDollarIdentifier: true) || currentToken.isLexerClassifiedKeyword)
+      && (!self.atArgumentLabel(allowDollarIdentifier: true) || currentToken.isLexerClassifiedKeyword)
     if !shouldSkipParameterParsing {
       var keepGoing = true
       var loopProgress = LoopProgressCondition()

Sources/SwiftParser/Parser.swift

@@ -90,7 +90,12 @@ public struct Parser {
   var arena: ParsingSyntaxArena
   /// A view of the sequence of lexemes in the input.
   var lexemes: Lexer.LexemeSequence
-  /// The current token. If there was no input, this token will have a kind of `.endOfFile`.
+  /// The current token that should be consumed next.
+  ///
+  /// If the end of the source file is reached, this is `.endOfFile`.
+  ///
+  /// - Important: You should almost never need to access this token directly
+  ///   in the parser. Instead, prefer using the `at` methods.
   var currentToken: Lexer.Lexeme
 
   /// The current nesting level, i.e. the number of tokens that

Sources/SwiftParser/Patterns.swift

@@ -386,7 +386,7 @@ extension Parser.Lookahead {
     }
 
     // To have a parameter name here, we need a name.
-    guard self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) else {
+    guard self.atArgumentLabel(allowDollarIdentifier: true) else {
       return false
     }
 
@@ -397,7 +397,7 @@ extension Parser.Lookahead {
     }
 
     // If the next token can be an argument label, we might have a name.
-    if nextTok.canBeArgumentLabel(allowDollarIdentifier: true) {
+    if nextTok.isArgumentLabel(allowDollarIdentifier: true) {
       // If the first name wasn't a contextual keyword, we're done.
       if !self.at(.keyword(.isolated))
         && !self.at(.keyword(.some))
@@ -420,7 +420,7 @@ extension Parser.Lookahead {
           return true  // isolated :
         }
         self.consumeAnyToken()
-        return self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon
+        return self.atArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon
       }
     }
 

Sources/SwiftParser/TokenConsumer.swift

@@ -278,4 +278,9 @@ extension TokenConsumer {
       return false
     }
   }
+
+  /// Whether the current token can be a function argument label.
+  func atArgumentLabel(allowDollarIdentifier: Bool = false) -> Bool {
+    return self.currentToken.isArgumentLabel(allowDollarIdentifier: allowDollarIdentifier)
+  }
 }

Sources/SwiftParser/TopLevel.swift

@@ -26,7 +26,7 @@ extension Parser {
   /// as unexpected nodes that have the `isMaximumNestingLevelOverflow` bit set.
   /// Check this in places that are likely to cause deep recursion and if this returns non-nil, abort parsing.
   mutating func remainingTokensIfMaximumNestingLevelReached() -> RawUnexpectedNodesSyntax? {
-    if nestingLevel > self.maximumNestingLevel && self.currentToken.rawTokenKind != .endOfFile {
+    if nestingLevel > self.maximumNestingLevel && !self.at(.endOfFile) {
       let remainingTokens = self.consumeRemainingTokens()
       return RawUnexpectedNodesSyntax(elements: remainingTokens, isMaximumNestingLevelOverflow: true, arena: self.arena)
     } else {

Sources/SwiftParser/Types.swift

@@ -514,7 +514,7 @@ extension Parser {
             second = nil
             unexpectedBeforeColon = nil
             colon = parsedColon
-          } else if self.currentToken.canBeArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon {
+          } else if self.atArgumentLabel(allowDollarIdentifier: true) && self.peek().rawTokenKind == .colon {
             (unexpectedBeforeSecond, second) = self.parseArgumentLabel()
             (unexpectedBeforeColon, colon) = self.expect(.colon)
           } else {
@@ -808,7 +808,7 @@ extension Parser.Lookahead {
       // by a type annotation.
       if self.startsParameterName(isClosure: false, allowMisplacedSpecifierRecovery: false) {
         self.consumeAnyToken()
-        if self.currentToken.canBeArgumentLabel() {
+        if self.atArgumentLabel() {
           self.consumeAnyToken()
           guard self.at(.colon) else {
             return false
@@ -1070,16 +1070,6 @@ extension Parser {
 }
 
 extension Lexer.Lexeme {
-  var isAnyOperator: Bool {
-    return self.rawTokenKind == .binaryOperator
-      || self.rawTokenKind == .postfixOperator
-      || self.rawTokenKind == .prefixOperator
-  }
-
-  var isEllipsis: Bool {
-    return self.isAnyOperator && self.tokenText == "..."
-  }
-
   var isGenericTypeDisambiguatingToken: Bool {
     switch self.rawTokenKind {
     case .rightParen,