Make PCH's respect any VFS specified.

clang/lib/Frontend/FrontendActions.cpp

@@ -348,7 +348,8 @@ void VerifyPCHAction::ExecuteAction() {
       DisableValidationForModuleKind::None,
       /*AllowASTWithCompilerErrors*/ false,
       /*AllowConfigurationMismatch*/ true,
-      /*ValidateSystemInputs*/ true));
+      /*ValidateSystemInputs*/ true,
+      CI.getHeaderSearchOpts().ValidateASTInputFilesContent));
 
   Reader->ReadAST(getCurrentFile(),
                   Preamble ? serialization::MK_Preamble

clang/lib/Serialization/ASTWriter.cpp

@@ -4781,6 +4781,24 @@ bool ASTWriter::PreparePathForOutput(SmallVectorImpl<char> &Path) {
     Changed = true;
   }
 
+  // If we are generating a normal PCH (EG. not a C++ module).
+  if (!WritingModule) {
+    // Use the vfs overlay if it exists to translate paths.
+    auto &FileSys =
+        Context->getSourceManager().getFileManager().getVirtualFileSystem();
+    if (auto *RFS = dyn_cast<llvm::vfs::RedirectingFileSystem>(&FileSys)) {
+      if (auto Result = RFS->lookupPath(StringRef(Path.data(), Path.size()))) {
+        if (std::optional<StringRef> Redirect = Result->getExternalRedirect()) {
+          const char *data = Redirect->data();
+          const size_t size = Redirect->size();
+          Path.resize(size);
+          Path.assign(data, data + size);
+          Changed = true;
+        }
+      }
+    }
+  }
+
   return Changed;
 }
 

clang/test/PCH/verify_no_timestamp.c

@@ -0,0 +1,8 @@
+// RUN: echo 'int SomeFunc() { return 42; }' > %t.h
+// RUN: %clang_cc1 -Werror -fno-pch-timestamp -fvalidate-ast-input-files-content -emit-pch -o "%t.pch" %t.h
+
+// Now change the source file, which should cause the verifier to fail with content mismatch.
+// RUN: echo 'int SomeFunc() { return 13; }' > %t.h
+// RUN: not %clang_cc1 -fno-pch-timestamp -fvalidate-ast-input-files-content -verify-pch %t.pch 2>&1 | FileCheck %s -DT=%t
+
+// CHECK: fatal error: file '[[T]].h' has been modified since the precompiled header '[[T]].pch' was built: content changed

clang/test/VFS/Inputs/vfsoverlay-directory-remap.yaml

@@ -0,0 +1,9 @@
+{
+  'version': 0,
+  'roots': [
+    { 'name': 'FROM_DIR',
+      'type': 'directory-remap',
+      'external-contents': 'TO_DIR'
+    }
+  ]
+}

clang/test/VFS/remap-to-fake.c

@@ -0,0 +1,27 @@
+// RUN: split-file %s %t
+// RUN: sed -e "s@FROM_DIR@%{/t:regex_replacement}/From@g" -e "s@TO_DIR@%{/t:regex_replacement}/Fake@g" %S/Inputs/vfsoverlay-directory-remap.yaml > %t/to-fake.yaml
+// RUN: sed -e "s@FROM_DIR@%{/t:regex_replacement}/Fake@g" -e "s@TO_DIR@%{/t:regex_replacement}/To@g" %S/Inputs/vfsoverlay-directory-remap.yaml > %t/from-fake.yaml
+
+// RUN: %clang_cc1 -Werror -fno-pch-timestamp -fvalidate-ast-input-files-content -ivfsoverlay %t/to-fake.yaml -emit-pch -o "%t.pch" %t/From/../From/B.h
+
+// Remove the `From` directory as we don't want to accidentally find that source if the PCH hasn't remapped using the VFS!
+// RUN: rm -rf %t/From
+
+// The PCH will be invalid because the `Fake` directory does not exist.
+// RUN: not %clang_cc1 -fno-pch-timestamp -fvalidate-ast-input-files-content -verify-pch %t.pch
+
+// But if we specify the correct VFS overlay it'll verify clean.
+// RUN: %clang_cc1 -fno-pch-timestamp -fvalidate-ast-input-files-content -verify-pch -ivfsoverlay %t/from-fake.yaml %t.pch
+
+// RUN: %clang_cc1 -fno-pch-timestamp -fvalidate-ast-input-files-content -Werror -I %t/To -ivfsoverlay %t/from-fake.yaml -include-pch "%t.pch" -emit-llvm -C %t/Test.c -o %t.o
+
+//--- From/B.h
+#pragma once
+int SomeFunc() { return 13; }
+//--- To/B.h
+#pragma once
+int SomeFunc() { return 13; }
+//--- Test.c
+#include "B.h"
+
+int UseSomeFunc() { return SomeFunc(); }