added .replaceChild to FixIt.Change to allow replacing a nullable child node with a proper node

type erased keypath, parent node and child node
type safe operation is performed inside the `replaceRange` closure
modified `PeerMacroTests` to adopt `.replaceChild`

Author: AppAppWorks

Sources/SwiftCompilerPluginMessageHandling/Diagnostics.swift

@@ -130,6 +130,10 @@ extension PluginMessage.Diagnostic {
               to: .afterTrailingTrivia
             )
             text = newTrivia.description
+          case .replaceChild(_, let parent, let child, let replacingRange):
+            let localRange = replacingRange()
+            range = sourceManager.range(of: parent, localRange: localRange)
+            text = child.description
           #if RESILIENT_LIBRARIES
           @unknown default:
             fatalError()

Sources/SwiftCompilerPluginMessageHandling/PluginMacroExpansionContext.swift

@@ -186,6 +186,23 @@ class SourceManager {
     )
   }
 
+  func range(
+    of node: Syntax,
+    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/FixIt.swift

@@ -36,6 +36,33 @@ public struct FixIt: Sendable {
     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(
+      _ keyPath: AnyKeyPath & Sendable,
+      parent: Syntax,
+      child: Syntax,
+      replacingRange: @Sendable () -> Range<AbsolutePosition>
+    )
+
+    public init<P: SyntaxProtocol, C: SyntaxProtocol>(
+      parent: P,
+      replacingChildAt keyPath: WritableKeyPath<P, C?> & Sendable,
+      with newChild: C
+    ) {
+      let replacingRange: @Sendable () -> 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
+          }
+        }
+      }
+      self = .replaceChild(keyPath, parent: Syntax(parent), child: Syntax(newChild), replacingRange: replacingRange)
+    }
   }
 
   /// A description of what this Fix-It performs.
@@ -89,6 +116,11 @@ private extension FixIt.Change {
         range: token.endPositionBeforeTrailingTrivia..<token.endPosition,
         replacement: newTrivia.description
       )
+    case .replaceChild(_, _, let child, let replacingRange):
+      return SourceEdit(
+        range: replacingRange(),
+        replacement: child.description
+      )
     }
   }
 }

Sources/SwiftSyntaxMacrosGenericTestSupport/Assertions.swift

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

Tests/SwiftSyntaxMacroExpansionTest/PeerMacroTests.swift

@@ -49,8 +49,6 @@ 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(
@@ -62,9 +60,10 @@ final class PeerMacroTests: XCTestCase {
                 "add 'async'"
               ),
               changes: [
-                FixIt.Change.replace(
-                  oldNode: Syntax(funcDecl.signature),
-                  newNode: Syntax(newSignature)
+                FixIt.Change(
+                  parent: funcDecl,
+                  replacingChildAt: \.signature.effectSpecifiers,
+                  with: newEffects
                 )
               ]
             )