[clang] [clang][Modules] Diagnose mismatching pcm dependencies in explicit buiilds (PR #137068)

[Cyndy Ishida via cfe-commits]

================
@@ -3311,11 +3312,22 @@ ASTReader::ReadControlBlock(ModuleFile &F,
                                                    SignatureBytes.end());
         Blob = Blob.substr(ASTFileSignature::size);
 
+        // Use BaseDirectoryAsWritten to ensure we use the same path in the
+        // ModuleCache as when writing.
+        StoredFile = ReadPathBlob(BaseDirectoryAsWritten, Record, Idx, Blob);
         if (ImportedFile.empty()) {
-          // Use BaseDirectoryAsWritten to ensure we use the same path in the
-          // ModuleCache as when writing.
-          ImportedFile =
-              ReadPathBlob(BaseDirectoryAsWritten, Record, Idx, Blob);
+          ImportedFile = StoredFile;
+        } else {
+          auto ImportedFileRef =
+              PP.getFileManager().getOptionalFileRef(ImportedFile);
+          auto StoredFileRef =
+              PP.getFileManager().getOptionalFileRef(StoredFile);
+          if ((ImportedFileRef && StoredFileRef) &&
+              (*ImportedFileRef != *StoredFileRef)) {
+            Diag(diag::warn_lazy_pcm_mismatch) << ImportedFile << StoredFile;
+            Diag(diag::note_module_file_imported_by)
----------------
cyndyishida wrote:

e.g. 

```
$ /Users/cishida/Builds/llvm-build/bin/clang -cc1 -internal-isystem /Users/cishida/Builds/llvm-build/lib/clang/21/include -nostdsysteminc -x c -fmodules -fno-implicit-modules -isysroot /Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/Sysroot -I/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/BuildDir -I/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/Sysroot/usr/include -fmodule-map-file=/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/BuildDir/A/module.modulemap -fmodule-map-file=/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/Sysroot/usr/include/B/module.modulemap -fmodule-file=A=/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/A-2.pcm -fmodule-file=B=/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/B-1.pcm  /Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c  -Wlazy-pcm-mismatch
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:38:2: warning: loaded module file '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/A-2.pcm' conflicts with imported file '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/A-1.pcm' [-Wlazy-pcm-mismatch]
   38 | #include <B/B.h>
      |  ^
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:38:2: note: imported by module 'B' in '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/B-1.pcm'
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:38:2: fatal error: module file '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/A-2.pcm' is out of date and needs to be rebuilt: module file has a different size or mtime than expected
   38 | #include <B/B.h>
      |  ^
/Users/cishida/Projects/llvm/monorepo/llvm-project/clang/test/Modules/invalid-module-dep.c:38:2: note: imported by module 'B' in '/Users/cishida/Builds/llvm-build/tools/clang/test/Modules/Output/invalid-module-dep.c.tmp/B-1.pcm'
1 warning and 1 error generated.

```

https://github.com/llvm/llvm-project/pull/137068