llvmPackages_15.libcxxabi: fix cycles that arise when stdenv is the LLVM stdenv

pkgs/development/compilers/llvm/15/default.nix

@@ -1,4 +1,4 @@
-{ lowPrio, newScope, pkgs, lib, stdenv, cmake
+{ lowPrio, newScope, pkgs, lib, stdenv, stdenvNoCC, cmake
 , gccForLibs, preLibcCrossHeaders
 , libxml2, python3, isl, fetchFromGitHub, overrideCC, wrapCCWith, wrapBintoolsWith
 , buildLlvmTools # tools, but from the previous stage, for cross
@@ -253,6 +253,19 @@ let
       # what stdenv we use here, as long as CMake is happy.
       cxx-headers = callPackage ./libcxx {
         inherit llvm_meta;
+        # Note that if we use the regular stdenv here we'll get cycle errors
+        # when attempting to use this compiler in the stdenv.
+        #
+        # The final stdenv pulls `cxx-headers` from the package set where
+        # hostPlatform *is* the target platform which means that `stdenv` at
+        # that point attempts to use this toolchain.
+        #
+        # So, we use `stdenv_` (the stdenv containing `clang` from this package
+        # set, defined below) to sidestep this issue.
+        #
+        # Because we only use `cxx-headers` in `libcxxabi` (which depends on the
+        # clang stdenv _anyways_), this is okay.
+        stdenv = stdenv_;
         headersOnly = true;
       };
 

pkgs/development/compilers/llvm/15/libcxx/default.nix

@@ -80,7 +80,13 @@ stdenv.mkDerivation rec {
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
-    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF";
+    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF"
+    # If we're only building the headers we don't actually *need* a functioning
+    # C/C++ compiler:
+    ++ lib.optionals (headersOnly) [
+      "-DCMAKE_C_COMPILER_WORKS=ON"
+      "-DCMAKE_CXX_COMPILER_WORKS=ON"
+    ];
 
   buildFlags = lib.optional headersOnly "generate-cxx-headers";
   installTargets = lib.optional headersOnly "install-cxx-headers";