Detect circular macro expansion

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -63,6 +63,7 @@ enum MacroExpansionError: Error, CustomStringConvertible {
   case noFreestandingMacroRoles(Macro.Type)
   case moreThanOneBodyMacro
   case preambleWithoutBody
+  case recursiveExpansion(any Macro.Type)
 
   var description: String {
     switch self {
@@ -92,6 +93,9 @@ enum MacroExpansionError: Error, CustomStringConvertible {
 
     case .preambleWithoutBody:
       return "preamble macro cannot be applied to a function with no body"
+
+    case .recursiveExpansion(let type):
+      return "recursive expansion of macro '\(type)'"
     }
   }
 }

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -667,6 +667,12 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
   /// added to top-level 'CodeBlockItemList'.
   var extensions: [CodeBlockItemSyntax] = []
 
+  /// Stores the types of the freestanding macros that are currently expanding.
+  ///
+  /// As macros are expanded by DFS, `expandingFreestandingMacros` always represent the expansion path starting from
+  /// the root macro node to the last macro node currently expanding.
+  var expandingFreestandingMacros: [any Macro.Type] = []
+
   init(
     macroSystem: MacroSystem,
     contextGenerator: @escaping (Syntax) -> Context,
@@ -683,7 +689,7 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
   }
 
   override func visitAny(_ node: Syntax) -> Syntax? {
-    if skipVisitAnyHandling.contains(node) {
+    guard !skipVisitAnyHandling.contains(node) else {
       return nil
     }
 
@@ -692,8 +698,10 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
     // position are handled by 'visit(_:CodeBlockItemListSyntax)'.
     // Only expression expansions inside other syntax nodes is handled here.
     switch expandExpr(node: node) {
-    case .success(let expanded):
-      return Syntax(visit(expanded))
+    case .success(let expansion):
+      return expansion.withExpandedNode { expandedNode in
+        Syntax(visit(expandedNode))
+      }
     case .failure:
       return Syntax(node)
     case .notAMacro:
@@ -794,9 +802,11 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
     func addResult(_ node: CodeBlockItemSyntax) {
       // Expand freestanding macro.
       switch expandCodeBlockItem(node: node) {
-      case .success(let expanded):
-        for item in expanded {
-          addResult(item)
+      case .success(let expansion):
+        expansion.withExpandedNode { expandedNode in
+          for item in expandedNode {
+            addResult(item)
+          }
         }
         return
       case .failure:
@@ -839,9 +849,11 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
     func addResult(_ node: MemberBlockItemSyntax) {
       // Expand freestanding macro.
       switch expandMemberDecl(node: node) {
-      case .success(let expanded):
-        for item in expanded {
-          addResult(item)
+      case .success(let expansion):
+        expansion.withExpandedNode { expandedNode in
+          for item in expandedNode {
+            addResult(item)
+          }
         }
         return
       case .failure:
@@ -1205,9 +1217,36 @@ extension MacroApplication {
 // MARK: Freestanding macro expansion
 
 extension MacroApplication {
+  /// Encapsulates an expanded node, the type of the macro from which the node was expanded, and the macro application,
+  /// such that recursive macro expansion can be consistently detected.
+  struct MacroExpansion<ResultType> {
+    private let expandedNode: ResultType
+    private let macro: any Macro.Type
+    private unowned let macroApplication: MacroApplication
+
+    fileprivate init(expandedNode: ResultType, macro: any Macro.Type, macroApplication: MacroApplication) {
+      self.expandedNode = expandedNode
+      self.macro = macro
+      self.macroApplication = macroApplication
+    }
+
+    /// Invokes the given closure with the node resulting from a macro expansion.
+    ///
+    /// This method inserts a pair of push and pop operations immediately around the invocation of `body` to maintain
+    /// an exact stack of expanding freestanding macros to detect recursive macro expansion. Callers should perform any
+    /// further macro expansion on `expanded` only within the scope of `body`.
+    func withExpandedNode<T>(_ body: (_ expandedNode: ResultType) throws -> T) rethrows -> T {
+      macroApplication.expandingFreestandingMacros.append(macro)
+      defer {
+        macroApplication.expandingFreestandingMacros.removeLast()
+      }
+      return try body(expandedNode)
+    }
+  }
+
   enum MacroExpansionResult<ResultType> {
     /// Expansion of the macro succeeded.
-    case success(ResultType)
+    case success(expansion: MacroExpansion<ResultType>)
 
     /// Macro system found the macro to expand but running the expansion threw
     /// an error and thus no expansion result exists.
@@ -1217,18 +1256,37 @@ extension MacroApplication {
     case notAMacro
   }
 
+  /// Expands the given freestanding macro node into a syntax node by invoking the given closure.
+  ///
+  /// Any error thrown by `expandMacro` and circular expansion error will be added to diagnostics.
+  ///
+  /// - Parameters:
+  ///   - node: The freestanding macro node to be expanded.
+  ///   - expandMacro: The closure that expands the given macro type and macro node into a syntax node.
+  ///
+  /// - Returns:
+  /// Returns `.notAMacro` if `node` is `nil` or `node.macroName` isn't registered with any macro type.
+  /// Returns `.failure` if `expandMacro` throws an error or returns `nil`, or recursive expansion is detected.
+  /// Returns `.success` otherwise.
   private func expandFreestandingMacro<ExpandedMacroType: SyntaxProtocol>(
     _ node: (any FreestandingMacroExpansionSyntax)?,
-    expandMacro: (_ macro: Macro.Type, _ node: any FreestandingMacroExpansionSyntax) throws -> ExpandedMacroType?
+    expandMacro: (_ macro: any Macro.Type, _ node: any FreestandingMacroExpansionSyntax) throws -> ExpandedMacroType?
   ) -> MacroExpansionResult<ExpandedMacroType> {
     guard let node,
       let macro = macroSystem.lookup(node.macroName.text)?.type
     else {
       return .notAMacro
     }
+
     do {
+      guard !expandingFreestandingMacros.contains(where: { $0 == macro }) else {
+        // We may think of any ongoing macro expansion as a tree in which macro types being expanded are nodes.
+        // Any macro type being expanded more than once will create a cycle which the compiler as of now doesn't allow.
+        throw MacroExpansionError.recursiveExpansion(macro)
+      }
+
       if let expanded = try expandMacro(macro, node) {
-        return .success(expanded)
+        return .success(expansion: MacroExpansion(expandedNode: expanded, macro: macro, macroApplication: self))
       } else {
         return .failure
       }

Tests/SwiftSyntaxMacroExpansionTest/ExpressionMacroTests.swift

@@ -37,6 +37,46 @@ fileprivate struct StringifyMacro: ExpressionMacro {
   }
 }
 
+private struct InfiniteRecursionMacro: ExpressionMacro {
+  static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> ExprSyntax {
+    if let i = node.arguments.first?.expression.as(IntegerLiteralExprSyntax.self)?.representedLiteralValue {
+      return "\(raw: i) + #infiniteRecursion(i: \(raw: i + 1))"
+    } else {
+      return "#nested1"
+    }
+  }
+}
+
+private struct Nested1RecursionMacro: ExpressionMacro {
+  static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> ExprSyntax {
+    "(#nested2, #nested3, #infiniteRecursion(i: 1), #infiniteRecursion)"
+  }
+}
+
+private struct Nested2RecursionMacro: ExpressionMacro {
+  static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> ExprSyntax {
+    "(#nested3, #nested3)"
+  }
+}
+
+private struct Nested3RecursionMacro: ExpressionMacro {
+  static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> ExprSyntax {
+    "0"
+  }
+}
+
 final class ExpressionMacroTests: XCTestCase {
   private let indentationWidth: Trivia = .spaces(2)
 
@@ -292,4 +332,31 @@ final class ExpressionMacroTests: XCTestCase {
       macros: ["test": DiagnoseFirstArgument.self]
     )
   }
+
+  func testDetectCircularExpansion() {
+    assertMacroExpansion(
+      "#nested1",
+      expandedSource: "((0, 0), 0, 1 + #infiniteRecursion(i: 2), #nested1)",
+      diagnostics: [
+        DiagnosticSpec(
+          message:
+            "recursive expansion of macro 'InfiniteRecursionMacro'",
+          line: 1,
+          column: 5
+        ),
+        DiagnosticSpec(
+          message:
+            "recursive expansion of macro 'Nested1RecursionMacro'",
+          line: 1,
+          column: 1
+        ),
+      ],
+      macros: [
+        "nested1": Nested1RecursionMacro.self,
+        "nested2": Nested2RecursionMacro.self,
+        "nested3": Nested3RecursionMacro.self,
+        "infiniteRecursion": InfiniteRecursionMacro.self,
+      ]
+    )
+  }
 }