Only check if a document has been modified while loading diagnostics during publishDiagnostics

I was seeing some of these errors in VS Code, which uses the pull diagnostics request even though I couldn’t see any concurrent modifications to the document. Move the check up to `publishDiagnostics`, where it belongs, and log the version of the old and new document version so we can investiage what’s causing the issue.

Author: ahoppen

Sources/SourceKitLSP/Swift/DiagnosticReportManager.swift

@@ -110,12 +110,6 @@ actor DiagnosticReportManager {
     let dict = try await self.sourcekitd.send(skreq, fileContents: snapshot.text)
 
     try Task.checkCancellation()
-    guard (try? documentManager.latestSnapshot(snapshot.uri).id) == snapshot.id else {
-      // Check that the document wasn't modified while we were getting diagnostics. This could happen because we are
-      // calling `fullDocumentDiagnosticReport` from `publishDiagnosticsIfNeeded` outside of `messageHandlingQueue`
-      // and thus a concurrent edit is possible while we are waiting for the sourcekitd request to return a result.
-      throw ResponseError.unknown("Document was modified while loading diagnostics")
-    }
 
     let diagnostics: [Diagnostic] =
       dict[keys.diagnostics]?.compactMap({ diag in

Sources/SourceKitLSP/Swift/SwiftLanguageServer.swift

@@ -417,6 +417,20 @@ extension SwiftLanguageServer {
           for: snapshot,
           buildSettings: buildSettings
         )
+        let latestSnapshotID = try? await documentManager.latestSnapshot(snapshot.uri).id
+        if latestSnapshotID != snapshot.id {
+          // Check that the document wasn't modified while we were getting diagnostics. This could happen because we are
+          // calling `publishDiagnosticsIfNeeded` outside of `messageHandlingQueue` and thus a concurrent edit is
+          // possible while we are waiting for the sourcekitd request to return a result.
+          logger.log(
+            """
+            Document was modified while loading diagnostics. \
+            Loaded diagnostics for \(snapshot.id.version, privacy: .public), \
+            latest snapshot is \((latestSnapshotID?.version).map(String.init) ?? "<nil>", privacy: .public)
+            """
+          )
+          throw CancellationError()
+        }
 
         await sourceKitServer.sendNotificationToClient(
           PublishDiagnosticsNotification(