llvmPackages_git.libcxx: fix darwin build

pkgs/development/compilers/llvm/git/default.nix

@@ -247,11 +247,18 @@ let
                else stdenv;
     };
 
-    libcxxabi = callPackage ./libcxxabi {
-      inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+    libcxxabi = let
+      stdenv_ = if stdenv.hostPlatform.useLLVM or false
                then overrideCC stdenv buildLlvmTools.clangNoLibcxx
                else stdenv;
+      cxx-headers = callPackage ./libcxx {
+        inherit llvm_meta;
+        stdenv = stdenv_;
+        headersOnly = true;
+      };
+    in callPackage ./libcxxabi {
+      stdenv = stdenv_;
+      inherit llvm_meta cxx-headers;
     };
 
     libunwind = callPackage ./libunwind {

pkgs/development/compilers/llvm/git/libcxx/default.nix

@@ -1,15 +1,23 @@
 { lib, stdenv, llvm_meta, src, cmake, python3, fixDarwinDylibNames, version
+, libcxxabi
 , enableShared ? !stdenv.hostPlatform.isStatic
+
+# If headersOnly is true, the resulting package would only include the headers.
+# Use this to break the circular dependency between libcxx and libcxxabi.
+#
+# Some context:
+# https://reviews.llvm.org/rG1687f2bbe2e2aaa092f942d4a97d41fad43eedfb
+, headersOnly ? false
 }:
 
 stdenv.mkDerivation rec {
-  pname = "libcxx";
+  pname = if headersOnly then "cxx-headers" else "libcxx";
   inherit version;
 
   inherit src;
-  sourceRoot = "source/${pname}";
+  sourceRoot = "source/libcxx";
 
-  outputs = [ "out" "dev" ];
+  outputs = [ "out" ] ++ lib.optional (!headersOnly) "dev";
 
   patches = [
     ./gnu-install-dirs.patch
@@ -24,15 +32,29 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake python3 ]
     ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
-  cmakeFlags = [
-  ] ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
+  buildInputs = lib.optionals (!headersOnly) [ libcxxabi ];
+
+  cmakeFlags = [ "-DLIBCXX_CXX_ABI=libcxxabi" ]
+    ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
     ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
-    ++ lib.optional stdenv.hostPlatform.isWasm [
+    ++ lib.optionals stdenv.hostPlatform.isWasm [
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
     ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF";
 
+  buildFlags = lib.optional headersOnly "generate-cxx-headers";
+  installTargets = lib.optional headersOnly "install-cxx-headers";
+
+  # At this point, cxxabi headers would be installed in the dev output, which
+  # prevents moveToOutput from doing its job later in the build process.
+  postInstall = lib.optionalString (!headersOnly) ''
+    mv "$dev/include/c++/v1/"* "$out/include/c++/v1/"
+    pushd "$dev"
+    rmdir -p include/c++/v1
+    popd
+  '';
+
   passthru = {
     isLLVM = true;
   };

pkgs/development/compilers/llvm/git/libcxxabi/default.nix

@@ -1,6 +1,5 @@
-{ lib, stdenv, llvm_meta, cmake, python3, src, libunwind, version
+{ lib, stdenv, llvm_meta, cmake, python3, src, cxx-headers, libunwind, version
 , enableShared ? !stdenv.hostPlatform.isStatic
-, libcxx
 }:
 
 stdenv.mkDerivation rec {
@@ -26,7 +25,7 @@ stdenv.mkDerivation rec {
   buildInputs = lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm) libunwind;
 
   cmakeFlags = [
-    "-DLIBCXXABI_LIBCXX_INCLUDES=${libcxx.dev}/include/c++/v1"
+    "-DLIBCXXABI_LIBCXX_INCLUDES=${cxx-headers}/include/c++/v1"
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"