introduced .replaceChild to FixIt.Change to allow replacing an optional child node with a proper node

- introduced `ReplacingChildData` as the type-erased payload for `.replaceChild`

Author: AppAppWorks

Sources/SwiftCompilerPluginMessageHandling/Diagnostics.swift

@@ -130,12 +130,16 @@ extension PluginMessage.Diagnostic {
               to: .afterTrailingTrivia
             )
             text = newTrivia.description
+          case .replaceChild(let replaceChildData):
+            let localRange = replaceChildData.replacementRange
+            range = sourceManager.range(of: replaceChildData.parent, localRange: localRange)
+            text = replaceChildData.newChild.description
           #if RESILIENT_LIBRARIES
           @unknown default:
             fatalError()
           #endif
           }
-          guard let range = range else {
+          guard let range else {
             return nil
           }
           return .init(

Sources/SwiftCompilerPluginMessageHandling/PluginMacroExpansionContext.swift

@@ -186,6 +186,23 @@ class SourceManager {
     )
   }
 
+  func range(
+    of node: some SyntaxProtocol,
+    localRange: Range<AbsolutePosition>
+  ) -> SourceRange? {
+    guard let base = self.knownSourceSyntax[node.root.id] else {
+      return nil
+    }
+
+    let positionOffset = base.location.offset
+
+    return SourceRange(
+      fileName: base.location.fileName,
+      startUTF8Offset: localRange.lowerBound.advanced(by: positionOffset).utf8Offset,
+      endUTF8Offset: localRange.upperBound.advanced(by: positionOffset).utf8Offset
+    )
+  }
+
   /// Get location of `node` in the known root nodes.
   /// The root node of `node` must be one of the returned value from `add(_:)`.
   func location(

Sources/SwiftDiagnostics/Convenience.swift

@@ -51,4 +51,20 @@ extension FixIt {
       ]
     )
   }
+
+  public static func replaceChild<Parent: SyntaxProtocol, Child: SyntaxProtocol>(
+    message: FixItMessage,
+    parent: Parent,
+    replacingChildAt keyPath: WritableKeyPath<Parent, Child?> & Sendable,
+    with newChild: Child
+  ) -> Self {
+    FixIt(
+      message: message,
+      changes: [
+        .replaceChild(
+          data: FixIt.Change.ReplacingOptionalChildData(parent: parent, newChild: newChild, keyPath: keyPath)
+        )
+      ]
+    )
+  }
 }

Sources/SwiftDiagnostics/FixIt.swift

@@ -27,15 +27,45 @@ public protocol FixItMessage: Sendable {
   var fixItID: MessageID { get }
 }
 
+public protocol ReplacingChildData: Sendable {
+  associatedtype Parent: SyntaxProtocol
+  associatedtype Child: SyntaxProtocol
+
+  var replacementRange: Range<AbsolutePosition> { get }
+  var parent: Parent { get }
+  var newChild: Child { get }
+}
+
 /// A Fix-It that can be applied to resolve a diagnostic.
 public struct FixIt: Sendable {
   public enum Change: Sendable {
+    struct ReplacingOptionalChildData<Parent: SyntaxProtocol, Child: SyntaxProtocol>: ReplacingChildData {
+      let parent: Parent
+      let newChild: Child
+      let keyPath: WritableKeyPath<Parent, Child?> & Sendable
+
+      var replacementRange: Range<AbsolutePosition> {
+        if let oldChild = parent[keyPath: keyPath] {
+          return oldChild.range
+        } else {
+          let newParent = parent.with(keyPath, newChild)
+          if let previousToken = newParent[keyPath: keyPath]?.previousToken(viewMode: .all) {
+            return previousToken.endPosition..<previousToken.endPosition
+          } else {
+            return parent.position..<parent.position
+          }
+        }
+      }
+    }
+
     /// Replace `oldNode` by `newNode`.
     case replace(oldNode: Syntax, newNode: Syntax)
     /// Replace the leading trivia on the given token
     case replaceLeadingTrivia(token: TokenSyntax, newTrivia: Trivia)
     /// Replace the trailing trivia on the given token
     case replaceTrailingTrivia(token: TokenSyntax, newTrivia: Trivia)
+    /// Replace a child of a parent node
+    case replaceChild(data: any ReplacingChildData)
   }
 
   /// A description of what this Fix-It performs.
@@ -89,6 +119,12 @@ private extension FixIt.Change {
         range: token.endPositionBeforeTrailingTrivia..<token.endPosition,
         replacement: newTrivia.description
       )
+
+    case .replaceChild(let replacingChildData):
+      return SourceEdit(
+        range: replacingChildData.replacementRange,
+        replacement: replacingChildData.newChild.description
+      )
     }
   }
 }

Sources/SwiftSyntaxMacrosGenericTestSupport/Assertions.swift

@@ -622,6 +622,15 @@ fileprivate extension FixIt.Change {
         range: start..<end,
         replacement: newTrivia.description
       )
+
+    case .replaceChild(let replacingChildData):
+      let range = replacingChildData.replacementRange
+      let start = expansionContext.position(of: range.lowerBound, anchoredAt: replacingChildData.parent)
+      let end = expansionContext.position(of: range.upperBound, anchoredAt: replacingChildData.parent)
+      return SourceEdit(
+        range: start..<end,
+        replacement: replacingChildData.newChild.description
+      )
     }
   }
 }

Tests/SwiftSyntaxMacroExpansionTest/PeerMacroTests.swift

@@ -49,24 +49,17 @@ final class PeerMacroTests: XCTestCase {
           newEffects = FunctionEffectSpecifiersSyntax(asyncSpecifier: .keyword(.async))
         }
 
-        let newSignature = funcDecl.signature.with(\.effectSpecifiers, newEffects)
-
         let diag = Diagnostic(
           node: Syntax(funcDecl.funcKeyword),
           message: SwiftSyntaxMacros.MacroExpansionErrorMessage(
             "can only add a completion-handler variant to an 'async' function"
           ),
           fixIts: [
-            FixIt(
-              message: SwiftSyntaxMacros.MacroExpansionFixItMessage(
-                "add 'async'"
-              ),
-              changes: [
-                FixIt.Change.replace(
-                  oldNode: Syntax(funcDecl.signature),
-                  newNode: Syntax(newSignature)
-                )
-              ]
+            .replaceChild(
+              message: SwiftSyntaxMacros.MacroExpansionFixItMessage("add 'async'"),
+              parent: funcDecl,
+              replacingChildAt: \.signature.effectSpecifiers,
+              with: newEffects
             )
           ]
         )