libcxxClang doesn't seem to work as a toolchain

[iphydf]

Describe the bug
Using clang with libc++ fails to find <cxxabi.h>.

To Reproduce
Use the following in WORKSPACE:

nixpkgs_cc_configure(
    attribute_path = "llvmPackages_16.libcxxClang",
    repository = "@nixpkgs",
)

Try to compile something that includes cxxabi.h (e.g. googletest) and observe:

external/com_google_googletest/googletest/include/gtest/internal/gtest-type-util.h:49:10: fatal error: 'cxxabi.h' file not found
#include <cxxabi.h>
         ^~~~~~~~~~
1 error generated.

Expected behavior
Compilation should succeed.

Environment

• OS name + version: NixOS 23.05
• Version of the code: 0.10.0 (loaded by rules_haskell).

Additional context
These files definitely exist:

/nix/store/6bi80vsz2aa0r5j9y16pcz3gkmx80qfj-libcxxabi-16.0.1-dev/include/cxxabi.h
/nix/store/6bi80vsz2aa0r5j9y16pcz3gkmx80qfj-libcxxabi-16.0.1-dev/include/c++/v1/cxxabi.h

[avdv]

Is this the same problem as NixOS/nixpkgs#150655 ?

Ie. does passing --cxxopt=-x --cxxopt=c++ help?

[iphydf]

No, all other C++ standard library headers can be found. Also, -x c++ was already passed by the toolchain. I can see:

/nix/store/y02xngjcbhfgliijh2wnpksc3dqhva56-clang-wrapper-16.0.1/bin/cc -U_FORTIFY_SOURCE -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -Wunused-but-set-parameter -Wno-free-nonheap-object -fcolor-diagnostics -fno-omit-frame-pointer -x c++ '-std=c++0x' -MD -MF [...]

[avdv]

I think this is a consequence of nixpkgs having split the toolchain into different packages and libcxxabi not directly being part of the toolchain by default.

A nix-shell provides an environment where these packages are combined again, using the NIX_CFLAGS_COMPILE / NIX_LDFLAGS variables which are respected by the clang wrapper.

Although this is also not enough, when trying to run nix-shell -p llvmPackages_16.libcxxClang --run "clang++ -E - <<<'#include <cxxabi.h>' > /dev/null" it bails out with

In file included from <stdin>:1:
/nix/store/8qwwfr56i3sy5b22a0njp4k3anrqb8c2-libcxxabi-16.0.6-dev/include/cxxabi.h:20:10: fatal error: '__cxxabi_config.h' file not found
#include <__cxxabi_config.h>
         ^~~~~~~~~~~~~~~~~~~
1 error generated.

You could make this work by using wrapCCWith, like this:

rules_nixpkgs/toolchains/cc/cc.nix

Lines 27 to 47 in 2c76769

	# Work around https://github.com/NixOS/nixpkgs/issues/42059.
	# See also https://github.com/NixOS/nixpkgs/pull/41589.
	pkgs.wrapCCWith rec {
	cc = stdenv.cc.cc;
	bintools = stdenv.cc.bintools.override { inherit postLinkSignHook; };
	extraBuildCommands = with pkgs.darwin.apple_sdk.frameworks; ''
	echo "-Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
	echo "-Wno-elaborated-enum-base" >> $out/nix-support/cc-cflags
	echo "-isystem ${pkgs.llvmPackages.libcxx.dev}/include/c++/v1" >> $out/nix-support/cc-cflags
	echo "-isystem ${pkgs.llvmPackages.clang-unwrapped.lib}/lib/clang/${cc.version}/include" >> $out/nix-support/cc-cflags
	echo "-F${CoreFoundation}/Library/Frameworks" >> $out/nix-support/cc-cflags
	echo "-F${CoreServices}/Library/Frameworks" >> $out/nix-support/cc-cflags
	echo "-F${Security}/Library/Frameworks" >> $out/nix-support/cc-cflags
	echo "-F${Foundation}/Library/Frameworks" >> $out/nix-support/cc-cflags
	echo "-L${pkgs.llvmPackages.libcxx}/lib" >> $out/nix-support/cc-cflags
	echo "-L${pkgs.llvmPackages.libcxxabi}/lib" >> $out/nix-support/cc-cflags
	echo "-L${pkgs.libiconv}/lib" >> $out/nix-support/cc-cflags
	echo "-L${pkgs.darwin.libobjc}/lib" >> $out/nix-support/cc-cflags
	echo "-resource-dir=${pkgs.stdenv.cc}/resource-root" >> $out/nix-support/cc-cflags
	'';
	};

Adding in any library and include directories needed.

I am not sure how we could provide a better experience here, we probably would have to run the setupHooks somehow in order to realize the env variables so that the resulting paths would be reflected in the toolchain.