[clang][Darwin] Prefer the toolchain-provided libc++.dylib if there is one

When libc++ is bootstrapped with clang, the resulting clang uses the
just-built libcxx headers from <install>/bin/../include/c++/v1. However,
before this patch, clang would still use the system-provided libc++.dylib
(usually in the Apple SDK) because it would fail to add the corresponding
linker flag to find the just-built libc++. After this patch, Clang will
also add `-L <install>/bin/../lib` to the linker search paths, which will
allow the just-built libc++.dylib to be found.

Fixes #77653
rdar://107060541

Author: ldionne

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -426,6 +426,24 @@ void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
   Args.AddAllArgs(CmdArgs, options::OPT_sub__library);
   Args.AddAllArgs(CmdArgs, options::OPT_sub__umbrella);
 
+  // Pass -L <toolchain-install>/bin/../lib/ to linker to prioritize the
+  // toolchain's libc++.dylib over the sysroot-provided one. This matches
+  // what we do for determining which libc++ headers to use.
+  //
+  // If the toolchain does not provide libc++.dylib, we will fall back to the
+  // normal system search paths for finding libc++.dylib (on Darwin, that would
+  // be in the SDK so we would find Apple's system libc++ instead of using
+  // LLVM's).
+  {
+    llvm::SmallString<128> InstallBinPath =
+        llvm::StringRef(D.Dir); // <install>/bin
+    llvm::SmallString<128> InstallLibPath = InstallBinPath;
+    llvm::sys::path::append(InstallLibPath, "..",
+                            "lib"); // <install>/bin/../lib
+    CmdArgs.push_back("-L");
+    CmdArgs.push_back(C.getArgs().MakeArgString(InstallLibPath));
+  }
+
   // Give --sysroot= preference, over the Apple specific behavior to also use
   // --isysroot as the syslibroot.
   // We check `OPT__sysroot_EQ` directly instead of `getSysRoot` to make sure we

clang/test/Driver/darwin-header-search-libcxx.cpp

@@ -92,7 +92,7 @@
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
-// Make sure that using -nostdinc, -nostdinc++ or -nostdlib will drop both the toolchain
+// Make sure that using -nostdinc, -nostdinc++ or -nostdlibinc will drop both the toolchain
 // C++ include path and the sysroot one.
 //
 // RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -116,7 +116,7 @@
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     -stdlib=platform \
 // RUN:     -nostdinc++ \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINCXX %s
 // CHECK-LIBCXX-NOSTDINCXX: "-cc1"

clang/test/Driver/darwin-link-libcxx-from-toolchain.cpp

@@ -0,0 +1,39 @@
+// UNSUPPORTED: system-windows
+
+// Tests to check that we pass -L <install>/bin/../lib/ to the linker to prioritize the toolchain's
+// libc++.dylib over the system's libc++.dylib on Darwin. This matches the behavior we have for
+// header search paths, where we prioritize toolchain headers and then fall back to the sysroot ones.
+
+// Check that we pass the right -L to the linker even when -stdlib=libc++ is not passed.
+//
+// RUN: %clang -### %s 2>&1                                                     \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx    \
+// RUN:               --check-prefix=CHECK-1 %s
+//
+// CHECK-1: "/usr/bin/ld"
+// CHECK-1: "-L" "[[TOOLCHAIN]]/usr/bin/../lib"
+
+// Check that we pass the right -L to the linker when -stdlib=libc++ is passed, both in the
+// case where there is libc++.dylib in the toolchain and when there isn't.
+//
+// RUN: %clang -### %s 2>&1                                                     \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -stdlib=libc++                                                      \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx    \
+// RUN:               --check-prefix=CHECK-2 %s
+//
+// CHECK-2: "/usr/bin/ld"
+// CHECK-2: "-L" "[[TOOLCHAIN]]/usr/bin/../lib"
+//
+// RUN: %clang -### %s 2>&1                                             \
+// RUN:     --target=x86_64-apple-darwin                                \
+// RUN:     -stdlib=libc++                                              \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin   \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain      \
+// RUN:               --check-prefix=CHECK-3 %s
+//
+// CHECK-3: "/usr/bin/ld"
+// CHECK-3: "-L" "[[TOOLCHAIN]]/usr/bin/../lib"