Fix parsing attributes at the start of an accessor

Previously, it would consume an attribute without adding it to the tree if an identifier is not found. Now, it looks ahead to confirm there is an identifier before consuming.

Added two test cases to verify parseAccessorIntroducer

Fixes: #677

Author: liamrosenfeld

Sources/SwiftParser/Declarations.swift

@@ -1519,33 +1519,37 @@ extension Parser {
   struct AccessorIntroducer {
     var attributes: RawAttributeListSyntax?
     var modifier: RawDeclModifierSyntax?
-    var introducer: (AccessorKind, RawTokenSyntax)?
+    var kind: AccessorKind
+    var token: RawTokenSyntax
   }
 
-  mutating func parseAccessorIntroducer() -> AccessorIntroducer {
+  mutating func parseAccessorIntroducer() -> AccessorIntroducer? {
+    // Check there is an identifier before consuming
+    var look = self.lookahead()
+    let _ = look.consumeAttributeList()
+    let hasModifier = look.consume(ifAny: [], contextualKeywords: ["mutating", "nonmutating", "__consuming"]) != nil
+    guard let (kind, handle) = look.at(anyIn: AccessorKind.self) else {
+      return nil
+    }
+    
     let attrs = self.parseAttributeList()
 
     // Parse the contextual keywords for 'mutating' and 'nonmutating' before
     // get and set.
     let modifier: RawDeclModifierSyntax?
-    if let name = self.consume(ifAny: [], contextualKeywords: ["mutating", "nonmutating", "__consuming"]) {
+    if hasModifier {
       modifier = RawDeclModifierSyntax(
-        name: name,
+        name: self.consumeAnyToken(),
         detail: nil,
         arena: self.arena
       )
     } else {
       modifier = nil
     }
 
-    guard let (kind, handle) = self.at(anyIn: AccessorKind.self) else {
-      return AccessorIntroducer(
-        attributes: attrs, modifier: modifier, introducer: nil)
-    }
-
     let introducer = self.eat(handle)
     return AccessorIntroducer(
-      attributes: attrs, modifier: modifier, introducer: (kind, introducer))
+      attributes: attrs, modifier: modifier, kind: kind, token: introducer)
   }
 
   @_spi(RawSyntax)
@@ -1612,8 +1616,7 @@ extension Parser {
     do {
       var loopProgress = LoopProgressCondition()
       while !self.at(any: [.eof, .rightBrace]) && loopProgress.evaluate(currentToken) {
-        let introducer = self.parseAccessorIntroducer()
-        guard let (kind, kindToken) = introducer.introducer else {
+        guard let introducer = self.parseAccessorIntroducer() else {
           // There can only be an implicit getter if no other accessors were
           // seen before this one.
           guard elements.isEmpty else {
@@ -1649,7 +1652,7 @@ extension Parser {
         //
         //     set-name    ::= '(' identifier ')'
         let parameter: RawAccessorParameterSyntax?
-        if [ AccessorKind.set, .willSet, .didSet ].contains(kind), let lparen = self.consume(if: .leftParen) {
+        if [ AccessorKind.set, .willSet, .didSet ].contains(introducer.kind), let lparen = self.consume(if: .leftParen) {
           let (unexpectedBeforeName, name) = self.expectIdentifier()
           let (unexpectedBeforeRParen, rparen) = self.expect(.rightParen)
           parameter = RawAccessorParameterSyntax(
@@ -1681,7 +1684,7 @@ extension Parser {
         elements.append(RawAccessorDeclSyntax(
           attributes: introducer.attributes,
           modifier: introducer.modifier,
-          accessorKind: kindToken,
+          accessorKind: introducer.token,
           parameter: parameter,
           asyncKeyword: asyncKeyword,
           throwsKeyword: throwsKeyword,

Tests/SwiftParserTest/Declarations.swift

@@ -188,6 +188,28 @@ final class DeclarationTests: XCTestCase {
       async let theVeryLastPhotoWeWant = fetch("4.jpg")
       """
     )
+    
+    AssertParse(
+      """
+      var foo: Int {
+        @available(swift 5.0)
+        func myFun() -> Int {
+          return 42
+        }
+        return myFun()
+      }
+      """
+    )
+    
+    AssertParse(
+      """
+      var foo: Int {
+        mutating set {
+          test += 1
+        }
+      }
+      """
+    )
   }
 
   func testTypealias() {