Add isSelfCompleted to RawSyntax

This should prevent completed node from not being reused due to the lookahead range

Author: StevenWong12

Sources/SwiftSyntax/IncrementalParseTransition.swift

@@ -323,9 +323,20 @@ public struct IncrementalParseLookup {
         nextLeafNodeLength = nextToken.raw.totalLength - nextToken.trailingTriviaLength
       }
     }
+
+    var underlyingRaw = node.raw
+    if let codeBlockItem = underlyingRaw.as(RawCodeBlockItemSyntax.self) {
+      underlyingRaw = codeBlockItem.item
+    }
+
+    let nodeAffectLength =
+      underlyingRaw.isSelfCompleted
+      ? (node.raw.totalLength + nextLeafNodeLength).utf8Length
+      : max(mergeLookaheadRange(at: node.position.utf8Offset, length: node.raw.totalLength.utf8Length), (node.raw.totalLength + nextLeafNodeLength).utf8Length)
+
     let nodeAffectRange = ByteSourceRange(
       offset: node.position.utf8Offset,
-      length: max(mergeLookaheadRange(at: node.position.utf8Offset, length: node.raw.totalLength.utf8Length), (node.raw.totalLength + nextLeafNodeLength).utf8Length)
+      length: nodeAffectLength
     )
 
     for edit in edits.edits {

Sources/SwiftSyntax/Raw/RawSyntax.swift

@@ -917,6 +917,24 @@ extension RawSyntax {
   }
 }
 
+extension RawSyntax {
+  public var isSelfCompleted: Bool {
+    if let children = self.layoutView?.children,
+      let lastNonNilChildIdx = children.lastIndex(where: {
+        $0 != nil
+      }),
+      lastNonNilChildIdx != children.endIndex
+    {
+
+      let nextPotentialChildIndex = children.index(after: lastNonNilChildIdx)
+      return nextPotentialChildIndex == children.index(before: children.endIndex)
+        || children.index(after: nextPotentialChildIndex) == children.index(before: children.endIndex)
+    }
+
+    return true
+  }
+}
+
 #if DEBUG
 /// See `SyntaxMemoryLayout`.
 var RawSyntaxDataMemoryLayouts: [String: SyntaxMemoryLayout.Value] = [

Tests/SwiftParserTest/IncrementalParsingTests.swift

@@ -62,4 +62,18 @@ public class IncrementalParsingTests: XCTestCase {
       ]
     )
   }
+
+  public func testDeclFollowedByLabeledStmt() {
+    assertIncrementalParse(
+      """
+      class foo {}
+      trailingClosure: ⏩️switch x {
+        default: break
+      }⏸️{}⏪️
+      """,
+      reusedNodes: [
+        ReusedNodeSpec("class foo {}", kind: .codeBlockItem)
+      ]
+    )
+  }
 }