Add diagnostics for wrong modifiers for operator declarations & update test cases

Author: StevenWong12

Sources/SwiftParser/Declarations.swift

@@ -1888,7 +1888,7 @@ extension Parser {
     _ attrs: DeclAttributes,
     _ handle: RecoveryConsumptionHandle
   ) -> RawOperatorDeclSyntax {
-    let (unexpectedBeforeOperatorKeyword, operatorKeyword) = self.eat(handle)
+    var (unexpectedBeforeOperatorKeyword, operatorKeyword) = self.eat(handle)
     let unexpectedBeforeName: RawUnexpectedNodesSyntax?
     let name: RawTokenSyntax
     switch self.canRecoverTo(anyIn: OperatorLike.self) {
@@ -1965,14 +1965,29 @@ extension Parser {
       unexpectedAtEnd = nil
     }
 
+    var unexpectedBetweenAttributesAndModifiers: RawUnexpectedNodesSyntax?
     var modifiers: RawModifierListSyntax? = attrs.modifiers
+
+    let expectedModifiers = [Keyword.prefix.defaultText, Keyword.postfix.defaultText, Keyword.infix.defaultText]
+
+    if let firstModifier = modifiers?.elements.first {
+      if !expectedModifiers.contains(firstModifier.name.tokenText) {
+        unexpectedBetweenAttributesAndModifiers = RawUnexpectedNodesSyntax([modifiers], arena: self.arena)
+        modifiers = nil
+      } else {
+        unexpectedBeforeOperatorKeyword = RawUnexpectedNodesSyntax([unexpectedBeforeOperatorKeyword, RawUnexpectedNodesSyntax(Array(modifiers!.elements.dropFirst()), arena: self.arena)], arena: self.arena)
+        modifiers = RawModifierListSyntax(elements: [firstModifier], arena: self.arena)
+      }
+    }
+
     if modifiers == nil, unexpectedBeforeOperatorKeyword == nil {
       let missedModifier = RawDeclModifierSyntax(name: self.missingToken(.prefix), detail: nil, arena: self.arena)
       modifiers = RawModifierListSyntax(elements: [missedModifier], arena: self.arena)
     }
 
     return RawOperatorDeclSyntax(
       attributes: attrs.attributes,
+      unexpectedBetweenAttributesAndModifiers,
       modifiers: modifiers,
       unexpectedBeforeOperatorKeyword,
       operatorKeyword: operatorKeyword,

Tests/SwiftParserTest/translated/OperatorDeclTests.swift

@@ -480,6 +480,40 @@ final class OperatorDeclTests: XCTestCase {
     )
   }
 
+  func testOperatorDecl124() {
+    assertParse(
+      """
+      prefix 1️⃣mutating operator ?? : UndeclaredPrecedenceGroup
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "unexpected code 'mutating' in operator declaration")
+      ]
+    )
+  }
+
+  func testOperatorDecl125() {
+    assertParse(
+      """
+      prefix 1️⃣postfix operator ** : PrecedenceGroup
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "unexpected code 'postfix' in operator declaration")
+      ]
+    )
+  }
+
+  func testOperatorDecl126() {
+    assertParse(
+      """
+      1️⃣mutating 2️⃣operator ?? : PrecedenceGroup
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "unexpected code 'mutating' before operator declaration"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "operator must be declared as 'prefix', 'postfix', or 'infix'"),
+      ]
+    )
+  }
+
   func testIdentifierAsOperatorName() {
     assertParse(
       "postfix operator 1️⃣aa",