From 3fec80bc0298cbf72d37579d526a72b4a97895dc Mon Sep 17 00:00:00 2001
From: Ben Barham <ben_barham@apple.com>
Date: Tue, 1 Jul 2025 18:06:27 -0700
Subject: [PATCH] [Frontend] Destroy compiling compiler instance before read

https://github.com/llvm/llvm-project/pull/134887 added a clone for the
compiler instance in `compileModuleAndReadASTImpl`, which would then be
destroyed *after* the corresponding read in the importing instance.

Swift has a `SwiftNameLookupExtension` module extension which updates
(effectively) global state - populating the lookup table for a
module on read and removing it when the module is destroyed.

With newly cloned instance, we would then see:
  - Module compiled with cloned instance
  - Module read with importing instance
  - Lookup table for that module added
  - Cloned instance destroyed
  - Module from that cloned instance destroyed
  - Lookup table for that module name removed

Depending on the original semantics is incredibly fragile, but for now
it's good enough to ensure that the read in the importing instance is
after the cloned instanced is destroyed. Ideally we'd only ever add to
the lookup tables in the original importing instance, never its clones.
---
 clang/lib/Frontend/CompilerInstance.cpp | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 4fedc66ce0334..6205236753be2 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -1643,16 +1643,18 @@ static bool compileModuleAndReadASTImpl(CompilerInstance &ImportingInstance,
                                         SourceLocation ModuleNameLoc,
                                         Module *Module,
                                         StringRef ModuleFileName) {
-  auto Instance = ImportingInstance.cloneForModuleCompile(ModuleNameLoc, Module,
-                                                          ModuleFileName);
-
-  if (!ImportingInstance.compileModule(ModuleNameLoc,
-                                       Module->getTopLevelModuleName(),
-                                       ModuleFileName, *Instance)) {
-    ImportingInstance.getDiagnostics().Report(ModuleNameLoc,
-                                              diag::err_module_not_built)
-        << Module->Name << SourceRange(ImportLoc, ModuleNameLoc);
-    return false;
+  {
+    auto Instance = ImportingInstance.cloneForModuleCompile(ModuleNameLoc, Module,
+                                                            ModuleFileName);
+
+    if (!ImportingInstance.compileModule(ModuleNameLoc,
+                                         Module->getTopLevelModuleName(),
+                                         ModuleFileName, *Instance)) {
+      ImportingInstance.getDiagnostics().Report(ModuleNameLoc,
+                                                diag::err_module_not_built)
+          << Module->Name << SourceRange(ImportLoc, ModuleNameLoc);
+      return false;
+    }
   }
 
   return readASTAfterCompileModule(ImportingInstance, ImportLoc, ModuleNameLoc,