Swift normally looks for the Clang resource dir in a subdir/symlink of its own
resource dir. We provide a symlink to the Swift build-time Clang as a default
there, but we also here patch two checks to try locate it via NIX_CC.

The first (ClangImporter.cpp) happens when Swift code imports C modules. The
second (ToolChains.cpp) happens when Swift is used to link the final product.

--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -73,6 +73,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Memory.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/YAMLParser.h"
 #include <algorithm>
@@ -786,6 +787,17 @@ importer::addCommonInvocationArguments(
 
   const std::string &overrideResourceDir = importerOpts.OverrideResourceDir;
   if (overrideResourceDir.empty()) {
+    // Prefer the Clang resource directory from NIX_CC, to allow swapping in a
+    // different stdenv.
+    // TODO: Figure out how to provide a user override for this. Probably a
+    // niche use case, though, and for now a user can unset NIX_CC to work
+    // around it if necessary.
+    if (auto nixCC = llvm::sys::Process::GetEnv("NIX_CC")) {
+      llvm::SmallString<128> resourceDir(nixCC.getValue());
+      llvm::sys::path::append(resourceDir, "resource-root");
+      invocationArgStrs.push_back("-resource-dir");
+      invocationArgStrs.push_back(std::string(resourceDir.str()));
+    } else {
     llvm::SmallString<128> resourceDir(searchPathOpts.RuntimeResourcePath);
 
     // Adjust the path to refer to our copy of the Clang resource directory
@@ -801,6 +813,7 @@ importer::addCommonInvocationArguments(
     // Set the Clang resource directory to the path we computed.
     invocationArgStrs.push_back("-resource-dir");
     invocationArgStrs.push_back(std::string(resourceDir.str()));
+    } // nixCC
   } else {
     invocationArgStrs.push_back("-resource-dir");
     invocationArgStrs.push_back(overrideResourceDir);
--- a/lib/Driver/ToolChains.cpp
+++ b/lib/Driver/ToolChains.cpp
@@ -1393,10 +1393,20 @@ void ToolChain::getClangLibraryPath(const ArgList &Args,
                                     SmallString<128> &LibPath) const {
   const llvm::Triple &T = getTriple();
 
+  // Nix: We provide a `clang` symlink in the default Swift resource root, but
+  // prefer detecting the Clang resource root via NIX_CC, to allow swapping in
+  // a different stdenv. However, always honor a user-provided `-resource-dir`.
+  auto nixCC = llvm::sys::Process::GetEnv("NIX_CC");
+  if (nixCC && !Args.hasArgNoClaim(options::OPT_resource_dir)) {
+    LibPath.assign(nixCC.getValue());
+    llvm::sys::path::append(LibPath, "resource-root");
+  } else {
   getResourceDirPath(LibPath, Args, /*Shared=*/true);
   // Remove platform name.
   llvm::sys::path::remove_filename(LibPath);
-  llvm::sys::path::append(LibPath, "clang", "lib",
+  llvm::sys::path::append(LibPath, "clang");
+  } // nixCC
+  llvm::sys::path::append(LibPath, "lib",
                           T.isOSDarwin() ? "darwin"
                                          : getPlatformNameForTriple(T));
 }