Clang attempts to guess file names based on the OS and distro (yes!),
but unfortunately, that doesn't work for us.

This patch makes it easy to insert libc's $libdir so that Clang passes the
correct absolute file name of crt1.o etc. to 'ld'.  It also disables all
the distro-specific stuff and removes the hard-coded FHS directory names
to make sure Clang also works on foreign distros.

diff --git a/lib/Driver/Distro.cpp b/lib/Driver/Distro.cpp
index 396d0bee..90e06613 100644
--- a/lib/Driver/Distro.cpp
+++ b/lib/Driver/Distro.cpp
@@ -19,6 +19,10 @@ using namespace clang::driver;
 using namespace clang;
 
 static Distro::DistroType DetectDistro(llvm::vfs::FileSystem &VFS) {
+  // The compiler should always behave the same, even when used via Guix on a
+  // foreign distro.
+  return Distro::UnknownDistro;
+
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
       VFS.getBufferForFile("/etc/lsb-release");
   if (File) {
diff --git a/lib/Driver/ToolChains/Cuda.cpp b/lib/Driver/ToolChains/Cuda.cpp
index 57b8d434..57339f1a 100644
--- a/lib/Driver/ToolChains/Cuda.cpp
+++ b/lib/Driver/ToolChains/Cuda.cpp
@@ -68,6 +68,9 @@ CudaInstallationDetector::CudaInstallationDetector(
     const Driver &D, const llvm::Triple &HostTriple,
     const llvm::opt::ArgList &Args)
     : D(D) {
+  // Don't look for CUDA in /usr.
+  return;
+
   struct Candidate {
     std::string Path;
     bool StrictChecking;
diff --git a/lib/Driver/ToolChains/Linux.cpp b/lib/Driver/ToolChains/Linux.cpp
index 65ab9b2d..5ed28e0a 100644
--- a/lib/Driver/ToolChains/Linux.cpp
+++ b/lib/Driver/ToolChains/Linux.cpp
@@ -233,7 +233,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
                          GCCInstallation.getTriple().str() + "/bin")
                        .str());
-
+  // Comment out the distro-specific tweaks so that they don't bite when
+  // using Guix on a foreign distro.
+#if 0
   Distro Distro(D.getVFS());
 
   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
@@ -304,6 +306,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   if (IsAndroid || Distro.IsOpenSUSE())
     ExtraOpts.push_back("--enable-new-dtags");
+#endif
 
   // The selection of paths to try here is designed to match the patterns which
   // the GCC driver itself uses, as this is part of the GCC-compatible driver.
@@ -366,6 +369,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     }
   }
 
+#if 0
   // Similar to the logic for GCC above, if we currently running Clang inside
   // of the requested system root, add its parent library paths to
   // those searched.
@@ -411,6 +415,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   // Try walking via the GCC triple path in case of biarch or multiarch GCC
   // installations with strange symlinks.
   if (GCCInstallation.isValid()) {
+    // The following code would end up adding things like
+    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
     addPathIfExists(D,
                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
                         "/../../" + OSLibDir,
@@ -446,9 +452,14 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   // directory ('Dir' below) or the ResourceDir.
   if (StringRef(D.Dir).startswith(SysRoot))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
+#endif
+
+  // Add libc's lib/ directory to the search path, so that crt1.o, crti.o,
+  // and friends can be found.
+  addPathIfExists(D, "@GLIBC_LIBDIR@", Paths);
 
-  addPathIfExists(D, SysRoot + "/lib", Paths);
-  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+  // Add GCC's lib/ directory so libstdc++.so can be found.
+  addPathIfExists(D, GCCInstallation.getParentLibPath(), Paths);
 }
 
 ToolChain::CXXStdlibType Linux::GetDefaultCXXStdlibType() const {
ry' preserves transformations by default.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-01-19 17:45:39 +0100'>2021-01-19</span></td><td><a href='/guix/commit/tests/transformations.scm?id=9ab817b2a4601b4a6755983590ed7d93ebdc8d09'>transformations: Add '--with-latest'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-27 17:23:40 +0100'>2020-12-27</span></td><td><a href='/guix/commit/tests/transformations.scm?id=e38d90d497e19e00263fa28961c688a433154386'>transformations: Add '--with-patch'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-21 17:47:35 +0100'>2020-12-21</span></td><td><a href='/guix/commit/tests/transformations.scm?id=527551287011888c2ba13fe92e636300ce625430'>tests: Check the effect of '--without-tests' on implicit inputs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-10-31 23:16:43 +0100'>2020-10-31</span></td><td><a href='/guix/commit/tests/transformations.scm?id=f68b3ba12dc7532dddde7dc63afb81a8492c661e'>guix build: Move transformation options to (guix transformations).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>