LLVM

The way LLVM is packaged in Nix is an example of the stdenv pattern described in the previous chapter. To integrate with the LLVM compiler infrastructure, there is a pkgs.llvmPackages.libcxxStdenv derivation — however this will not provide a pure llvm stdenv in which all dependencies come from the LLVM project and none from GCC.¹ A different approach would be something like this:

stdenv = pkgs.overrideCC
  (
    pkgs.llvmPackages.libcxxStdenv.override {
      targetPlatform.useLLVM = true;
      targetPlatform.linker = "lld";
    }
  )
  pkgs.llvmPackages.clangUseLLVM

That stdenv is what the stdenv option is given, and the devShell conan-flake computes from it can then be appended to an inputsFrom option for composition:

# file: examples/llvm-flake-parts/flake.nix
{
  outputs = inputs@{ nixpkgs, flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      systems = nixpkgs.lib.systems.flakeExposed;
      imports = [
        inputs.conan-flake.flakeModule
      ];
      perSystem = { pkgs, config, ... }:
      {
        conan = {
          profiles.default = {
            settings = {
              build_type = "Release";
              "compiler.cppstd" = "23";
            };
          };

          stdenv = pkgs.overrideCC
            (
              pkgs.llvmPackages.libcxxStdenv.override {
                targetPlatform.useLLVM = true;
                targetPlatform.linker = "lld";
              }
            )
            pkgs.llvmPackages.clangUseLLVM;
        };

        devShells.default = pkgs.mkShell {
          inputsFrom = [
            config.conan.outputs.devShell
          ];
        };
      };
    };
}

The above example is on the examples/llvm-flake-parts directory:

cd examples/llvm-flake-parts
direnv allow .

By default, conan-flake sets defaults.profiles.settings."compiler.libcxx" to "libstdc++11", which would result in the wrong choice for compiler.libcxx — with the LLVM stdenv above it is detected as libc++ instead:

conan profile show

To the conan.profiles.default.settings.build_type and conan.profiles.default.settings."compiler.cppstd" options correspond, respectivelly, the build_type and compiler.cppstd entries in the command output:

Host profile:
[settings]
arch=x86_64
build_type=Release
compiler=clang
compiler.cppstd=23
compiler.libcxx=libc++
compiler.version=21.1.8
os=Linux
[platform_tool_requires]
cmake/4.3.4
[conf]
tools.build:compiler_executables={'c': '/nix/store/va889lnfilh11sjb1rcnrdvp813jpg03-clang-wrapper-21.1.8/bin/clang', 'cpp': '/nix/store/va889lnfilh11sjb1rcnrdvp813jpg03-clang-wrapper-21.1.8/bin/clang++'}

Build profile:
[settings]
arch=x86_64
build_type=Release
compiler=clang
compiler.cppstd=23
compiler.libcxx=libc++
compiler.version=21.1.8
os=Linux
[platform_tool_requires]
cmake/4.3.4
[conf]
tools.build:compiler_executables={'c': '/nix/store/va889lnfilh11sjb1rcnrdvp813jpg03-clang-wrapper-21.1.8/bin/clang', 'cpp': '/nix/store/va889lnfilh11sjb1rcnrdvp813jpg03-clang-wrapper-21.1.8/bin/clang++'}

The package defined in the examples/llvm-flake-parts/conanfile.py recipe — example/0.0.1 — can be created in order to validate these settings:

conan create . --build=missing

Lines from its output correspond to entries from the Conan profile and, ultimately, to the conan-flake options:

hello-conan: Hello World Release!
  hello-conan: __x86_64__ defined
  hello-conan: __cplusplus202302
  hello-conan: __GNUC__4
  hello-conan: __GNUC_MINOR__2
  hello-conan: __clang_major__21
  hello-conan: __clang_minor__1
example/0.0.1 test_package

1. See this question, or this issue, for further details on how to create a LLVM-based stdenv for C++ development. ↩