NFC: Introduce peek(isAtAnyIn:)

Author: hamishknight

Sources/SwiftParser/Attributes.swift

@@ -223,7 +223,7 @@ extension Parser {
       )
     }
 
-    switch DeclarationAttributeWithSpecialSyntax(lexeme: self.peek()) {
+    switch peek(isAtAnyIn: DeclarationAttributeWithSpecialSyntax.self) {
     case .available, ._spi_available:
       return parseAttribute(argumentMode: .required) { parser in
         return .availability(parser.parseAvailabilityArgumentSpecList())

Sources/SwiftParser/Statements.swift

@@ -943,15 +943,13 @@ extension Parser.Lookahead {
       return false
     }
 
-    let next = peek()
-
     // If 'then' is followed by a binary or postfix operator, prefer to parse as
     // an expr.
-    if BinaryOperatorLike(lexeme: next) != nil || PostfixOperatorLike(lexeme: next) != nil {
+    if peek(isAtAnyIn: BinaryOperatorLike.self) != nil || peek(isAtAnyIn: PostfixOperatorLike.self) != nil {
       return false
     }
 
-    switch PrepareForKeywordMatch(next) {
+    switch PrepareForKeywordMatch(peek()) {
     case TokenSpec(.is), TokenSpec(.as):
       // Treat 'is' and 'as' like the binary operator case, and parse as an
       // expr.
@@ -965,7 +963,7 @@ extension Parser.Lookahead {
       // These are handled based on whether there is trivia between the 'then'
       // and the token. If so, it's a 'then' statement. Otherwise it should
       // be treated as an expression, e.g `then(...)`, `then[...]`, `then.foo`.
-      return !self.currentToken.trailingTriviaText.isEmpty || !next.leadingTriviaText.isEmpty
+      return !self.currentToken.trailingTriviaText.isEmpty || !peek().leadingTriviaText.isEmpty
     default:
       break
     }

Sources/SwiftParser/TokenConsumer.swift

@@ -148,6 +148,17 @@ extension TokenConsumer {
     return nil
   }
 
+  /// Checks whether the parser's next token is any token in `SpecSet`.
+  /// If this is the case, return the corresponding `SpecSet` case.
+  @inline(__always)
+  func peek<SpecSet: TokenSpecSet>(isAtAnyIn specSet: SpecSet.Type) -> SpecSet? {
+    guard let matchedKind = SpecSet(lexeme: self.peek()) else {
+      return nil
+    }
+    precondition(matchedKind.spec ~= self.peek())
+    return matchedKind
+  }
+
   /// Whether the current token’s text starts with the given prefix.
   @inline(__always)
   mutating func at(prefix: SyntaxText) -> Bool {

Sources/SwiftParser/Types.swift

@@ -798,7 +798,7 @@ extension Parser.Lookahead {
         return true
       }
 
-      if EffectSpecifier(lexeme: self.peek()) != nil {
+      if peek(isAtAnyIn: EffectSpecifier.self) != nil {
         var backtrack = self.lookahead()
         backtrack.consumeAnyToken()
         backtrack.consumeAnyToken()
@@ -909,9 +909,7 @@ extension Parser {
   }
 
   mutating func parseTypeAttribute() -> RawAttributeListSyntax.Element {
-    let typeAttr = TypeAttribute(lexeme: self.peek())
-
-    switch typeAttr {
+    switch peek(isAtAnyIn: TypeAttribute.self) {
     case ._local, ._noMetadata, .async, .escaping, .noDerivative, .noescape, .Sendable, .unchecked, .autoclosure:
       // Known type attribute that doesn't take any arguments
       return parseAttributeWithoutArguments()