[clang] [llvm] [clang] Use the VFS in `ModuleDependencyCollector` (PR #160944)

[Jan Svoboda via llvm-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/160944

This PR starts using the correct VFS in `ModuleDependencyCollector` instead of using the real FS directly. This matches compiler's behavior for other input files.

>From 9311be23d860f7bd7c0bb561c750ff709359d290 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Fri, 26 Sep 2025 13:34:44 -0700
Subject: [PATCH] [clang] Use the VFS in `ModuleDependencyCollector`

---
 clang/lib/Frontend/ModuleDependencyCollector.cpp | 9 +++++----
 llvm/include/llvm/Support/FileCollector.h        | 3 +++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/clang/lib/Frontend/ModuleDependencyCollector.cpp b/clang/lib/Frontend/ModuleDependencyCollector.cpp
index 3b363f948a3a8..dad39df59ad45 100644
--- a/clang/lib/Frontend/ModuleDependencyCollector.cpp
+++ b/clang/lib/Frontend/ModuleDependencyCollector.cpp
@@ -91,10 +91,10 @@ void ModuleDependencyCollector::attachToPreprocessor(Preprocessor &PP) {
       std::make_unique<ModuleDependencyMMCallbacks>(*this));
 }
 
-static bool isCaseSensitivePath(StringRef Path) {
+static bool isCaseSensitivePath(llvm::vfs::FileSystem &VFS, StringRef Path) {
   SmallString<256> TmpDest = Path, UpperDest, RealDest;
   // Remove component traversals, links, etc.
-  if (llvm::sys::fs::real_path(Path, TmpDest))
+  if (VFS.getRealPath(Path, TmpDest))
     return true; // Current default value in vfs.yaml
   Path = TmpDest;
 
@@ -104,7 +104,7 @@ static bool isCaseSensitivePath(StringRef Path) {
   // already expects when sensitivity isn't setup.
   for (auto &C : Path)
     UpperDest.push_back(toUppercase(C));
-  if (!llvm::sys::fs::real_path(UpperDest, RealDest) && Path == RealDest)
+  if (!VFS.getRealPath(UpperDest, RealDest) && Path == RealDest)
     return false;
   return true;
 }
@@ -121,7 +121,8 @@ void ModuleDependencyCollector::writeFileMap() {
 
   // Explicitly set case sensitivity for the YAML writer. For that, find out
   // the sensitivity at the path where the headers all collected to.
-  VFSWriter.setCaseSensitivity(isCaseSensitivePath(VFSDir));
+  VFSWriter.setCaseSensitivity(
+      isCaseSensitivePath(Canonicalizer.getFileSystem(), VFSDir));
 
   // Do not rely on real path names when executing the crash reproducer scripts
   // since we only want to actually use the files we have on the VFS cache.
diff --git a/llvm/include/llvm/Support/FileCollector.h b/llvm/include/llvm/Support/FileCollector.h
index 9cc6776b948ba..9fa11ba362241 100644
--- a/llvm/include/llvm/Support/FileCollector.h
+++ b/llvm/include/llvm/Support/FileCollector.h
@@ -81,6 +81,9 @@ class LLVM_ABI FileCollector : public FileCollectorBase {
     /// Canonicalize a pair of virtual and real paths.
     LLVM_ABI PathStorage canonicalize(StringRef SrcPath);
 
+    /// Return the underlying file system.
+    vfs::FileSystem &getFileSystem() const { return *VFS; };
+
     explicit PathCanonicalizer(IntrusiveRefCntPtr<vfs::FileSystem> VFS)
         : VFS(std::move(VFS)) {}