[AccessorMacro] disallow accessor macro apply to variable declaration

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -633,6 +633,7 @@ private enum MacroApplicationError: DiagnosticMessage, Error {
   case accessorMacroOnVariableWithMultipleBindings
   case peerMacroOnVariableWithMultipleBindings
   case malformedAccessor
+  case accessorMacroOnLetVariable
 
   var diagnosticID: MessageID {
     return MessageID(domain: diagnosticDomain, id: "\(self)")
@@ -650,6 +651,8 @@ private enum MacroApplicationError: DiagnosticMessage, Error {
       return """
         macro returned a malformed accessor. Accessors should start with an introducer like 'get' or 'set'.
         """
+    case .accessorMacroOnLetVariable:
+      return "accessor macro is not allowed to be applied to a 'let' variable"
     }
   }
 }
@@ -923,6 +926,11 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
       return DeclSyntax(node)
     }
 
+    guard node.bindingSpecifier.tokenKind != .keyword(.let) else {
+      contextGenerator(Syntax(node)).addDiagnostics(from: MacroApplicationError.accessorMacroOnLetVariable, node: node)
+      return DeclSyntax(node)
+    }
+
     guard node.bindings.count == 1, let binding = node.bindings.first else {
       contextGenerator(Syntax(node)).addDiagnostics(
         from: MacroApplicationError.accessorMacroOnVariableWithMultipleBindings,

Tests/SwiftSyntaxMacroExpansionTest/AccessorMacroTests.swift

@@ -411,4 +411,21 @@ final class AccessorMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testAccessorOnLetVariableDeclaration() {
+    assertMacroExpansion(
+      """
+      @constantOne
+      let x: Int = 1
+      """,
+      expandedSource: """
+        let x: Int = 1
+        """,
+      diagnostics: [
+        DiagnosticSpec(message: "accessor macro is not allowed to be applied to a 'let' variable", line: 1, column: 1)
+      ],
+      macros: ["constantOne": ConstantOneGetter.self],
+      indentationWidth: indentationWidth
+    )
+  }
 }