[5.9] Fix two memory corruption bugs in SwiftSyntax

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxNodesFile.swift

@@ -221,7 +221,7 @@ func syntaxNode(emitKind: String) -> SourceFileSyntax {
             AccessorDeclSyntax(
               """
               set(value) {
-                self = \(raw: node.name)(data.replacingChild(at: \(raw: index), with: value\(raw: child.isOptional ? "?" : "").raw, arena: SyntaxArena()))
+                self = \(raw: node.name)(data.replacingChild(at: \(raw: index), with: value\(raw: child.isOptional ? "?" : "").data, arena: SyntaxArena()))
               }
               """
             )

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxRewriterFile.swift

@@ -342,8 +342,9 @@ let syntaxRewriterFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
       /// a parent if `node` had one.
       public func rewrite(_ node: Syntax) -> Syntax {
         let rewritten = self.visit(node)
-        let arena = SyntaxArena()
-        return Syntax(node.data.replacingSelf(rewritten.raw, arena: arena))
+        return withExtendedLifetime(rewritten) {
+          return Syntax(node.data.replacingSelf(rewritten.raw, arena: rewritten.raw.arena))
+        }
       }
       """
     )

Sources/SwiftSyntax/SyntaxData.swift

@@ -311,8 +311,10 @@ struct SyntaxData {
       let newParent = Syntax(parentData)
       return SyntaxData(absoluteRaw.replacingSelf(newRaw, newRootId: parentData.nodeId.rootId), parent: newParent)
     } else {
-      // Otherwise, we're already the root, so return the new root data.
-      return .forRoot(newRaw)
+      return withExtendedLifetime(arena) {
+        // Otherwise, we're already the root, so return the new root data.
+        return .forRoot(newRaw)
+      }
     }
   }
 
@@ -332,6 +334,15 @@ struct SyntaxData {
     return replacingSelf(newRaw, arena: arena)
   }
 
+  /// Identical to `replacingChild(at: Int, with: RawSyntax?, arena: SyntaxArena)`
+  /// that ensures that the arena of`newChild` doesn’t get de-allocated before
+  /// `newChild` has been addded to the result.
+  func replacingChild(at index: Int, with newChild: SyntaxData?, arena: SyntaxArena) -> SyntaxData {
+    return withExtendedLifetime(newChild) {
+      return replacingChild(at: index, with: newChild?.raw, arena: arena)
+    }
+  }
+
   func withLeadingTrivia(_ leadingTrivia: Trivia, arena: SyntaxArena) -> SyntaxData {
     if let raw = raw.withLeadingTrivia(leadingTrivia, arena: arena) {
       return replacingSelf(raw, arena: arena)

Sources/SwiftSyntax/generated/SyntaxRewriter.swift

@@ -6996,7 +6996,8 @@ open class SyntaxRewriter {
   /// a parent if `node` had one.
   public func rewrite(_ node: Syntax) -> Syntax {
     let rewritten = self.visit(node)
-    let arena = SyntaxArena()
-    return Syntax(node.data.replacingSelf(rewritten.raw, arena: arena))
+    return withExtendedLifetime(rewritten) {
+      return Syntax(node.data.replacingSelf(rewritten.raw, arena: rewritten.raw.arena))
+    }
   }
 }