From e0c2ab07ede8b7e2f1a3857385ab355b0040a27d Mon Sep 17 00:00:00 2001
From: Saleem Abdulrasool <compnerd@compnerd.org>
Date: Fri, 22 May 2026 11:18:47 -0700
Subject: [PATCH] SourceKitLSP: fallback to Swift source for SDK symbol resolve

When resolving a workspace symbol whose location points at an SDK
.swiftmodule/.swiftinterface, SourceKit-LSP currently asks IndexStoreDB for
main files containing that module file and uses one of those files to provide
build settings for opening the generated interface.

That relationship is not guaranteed to exist for SDK module files. The module
file can appear as the indexed symbol location, while
mainFiles(containing:) still returns no source file. This causes
workspaceSymbol/resolve to fail before calling sourcekitd, with errors like:

  No source file found that imports Swift.String; cannot open generated interface

If no main file is found for the module file, fall back to a buildable Swift
source file from the workspace. The file is only used as the source of compiler
arguments for generating the interface, so any buildable Swift source in the
workspace is sufficient.

This fixes resolving stdlib/SDK workspace symbols such as Swift.String on
Windows SDK layouts where the stdlib location is a .swiftmodule under the SDK.
---
 Sources/SourceKitLSP/SourceKitLSPServer.swift | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/Sources/SourceKitLSP/SourceKitLSPServer.swift b/Sources/SourceKitLSP/SourceKitLSPServer.swift
index 59bf0b7fb..e547e5d6d 100644
--- a/Sources/SourceKitLSP/SourceKitLSPServer.swift
+++ b/Sources/SourceKitLSP/SourceKitLSPServer.swift
@@ -1733,10 +1733,21 @@ extension SourceKitLSPServer {
       }()
     )
     for workspace in workspaces {
-      let mainFile = await workspace.buildServerManager
+      var candidateMainFiles = await workspace.buildServerManager
         .mainFiles(containing: moduleFileURI)
         .sorted(by: { $0.arbitrarySchemeURL.absoluteString < $1.arbitrarySchemeURL.absoluteString })
-        .first
+      if candidateMainFiles.isEmpty {
+        // SDK module files don't necessarily have a recorded main-file relationship in the index. We only need a Swift
+        // source file from the workspace to provide compiler arguments for generating the interface.
+        candidateMainFiles =
+          await orLog("Getting Swift source files for workspaceSymbol/resolve generated interface") {
+            try await workspace.buildServerManager.projectSourceFiles()
+              .keys
+              .filter { $0.fileURL?.pathExtension == "swift" }
+              .sorted(by: { $0.arbitrarySchemeURL.absoluteString < $1.arbitrarySchemeURL.absoluteString })
+          } ?? []
+      }
+      let mainFile = candidateMainFiles.first
       guard let mainFile else {
         continue
       }