Extract header nodes from DeclGroups

This also involves changing Child.Refactoring to support a new kind of change: “extracted”, in which a group of adjacent children are moved into a new child node.

Future commits will introduce clients that use the new *DeclHeaderSyntax nodes directly.

Author: beccadax

CodeGeneration/Sources/SyntaxSupport/Child.swift

@@ -371,10 +371,10 @@ extension Child {
     /// should be provided. The associated value is the *old*, now-deprecated name.
     case renamed(from: String)
 
-//    /// Several adjacent children were extracted into a separate node at some
-//    /// point in the past, so deprecated aliases that flatten the other node's 
-//    /// children into this node should be provided.
-//    case extracted
+    /// Several adjacent children were extracted into a separate node at some
+    /// point in the past, so deprecated aliases that flatten the other node's 
+    /// children into this node should be provided.
+    case extracted
   }
 }
 

CodeGeneration/Sources/SyntaxSupport/CompatibilityLayers.swift

@@ -33,6 +33,17 @@ public struct CompatibilityLayers /*: Sendable*/ {
     }
   }
 
+  private func extractedChildren(in layoutNode: LayoutNode) -> ArraySlice<Child> {
+    var children = layoutNode.children[...]
+    if children.first?.isUnexpectedNodes ?? false {
+      children.removeFirst()
+    }
+    if children.last?.isUnexpectedNodes ?? false {
+      children.removeLast()
+    }
+    return children
+  }
+
   /// Returns the child or children that would have existed in place of this
   /// child before this refactoring was applied.
   private mutating func replacementChildren(for newerChild: Child, by refactoring: Child.Refactoring) -> [Child] {
@@ -46,8 +57,16 @@ public struct CompatibilityLayers /*: Sendable*/ {
           )
         ]
 
-//        case .extracted:
-//          fatalError("unimplemented")
+      case .extracted:
+        let extractedNode = SYNTAX_NODE_MAP[newerChild.syntaxNodeKind]!
+        realizeLayers(for: extractedNode)
+
+        // Drop the leading and trailing unexpected nodes--these will be newly introduced.
+        let newerGrandchildren = extractedChildren(in: extractedNode.layoutNode!)
+
+        return newerGrandchildren.map { newerGrandchild in
+          Child(newerChildPath: [newerChild, newerGrandchild])
+        }
       }
     }