Using Clang instead of GCC

You can use Clang instead of GCC as a compiler for any package by overriding stdenv, which contains the compilation toolchain, with:

stdenv = pkgs.clangStdenv;

or to get a specific version of clang:

stdenv = pkgs.llvmPackages_9.stdenv;

Depending on the case you may want to set this value in different location, and using different mechanism.

Note you may get errors like fatal error: ... file not found on standard library #include directives, because of this bug https://github.com/NixOS/nixpkgs/issues/150655

Globally, in a package repository tree

If you have a set of packages in a repository tree, you can set the stdenv value in the scope where the callPackage are called. Be carefull all the packages present in the scope will be built with Clang because the callPackage that resolves the package function inputs will use the pkgs.clangStdenv for all packages.

rec {
    stdenv = pkgs.clangStdenv;
    foo = callPackage ./foo { };
    bar = callPackage ./bar { };
}

or import nixpkgs with replaceStdenv.

import <nixpkgs> { config = { replaceStdenv = ({ pkgs }: pkgs.clangStdenv); }; }

For a specific package in a repository tree

If you a one specific package in your package repository that you want to build with Clang. You can either override stdenv in the callPackage or creating a package override.

Here only foo will be built with Clang, and only with Clang.

rec {
    foo = callPackage ./foo { stdenv = pkgs.clangStdenv; };
    bar = callPackage ./bar { };
}

But if you want both toolchains you can use:

rec {
    foo_gcc = callPackage ./foo { };
    foo_clang = callPackage ./foo { stdenv = pkgs.clangStdenv; };
    bar = callPackage ./bar { };
}

Using Nix CLI on existing packages

Directly inline with CLI just do:

nix-build -E "with import <nixpkgs> {}; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }"

Flakes:

nix build --impure --expr 'with builtins.getFlake "nixpkgs"; with legacyPackages.x86_64-linux; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }'

or, if you want a shell for development:

nix-shell -E "with import <nixpkgs> {}; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }"

Flakes:

nix shell --impure --expr 'with builtins.getFlake "nixpkgs"; with legacyPackages.x86_64-linux; pkgs.hello.override{ stdenv = pkgs.clangStdenv; }'

Using an external override definition

# in file ./hello_with_clan.nix
with import <nixpkgs> {};
hello.override {
    # use Clang instead of GCC
    stdenv = pkgs.clangStdenv;
}

nix-build ./hello_with_clan.nix

With nix-shell

To use clang in nix-shell instead of gcc:

# in file ./shell.nix
with import <nixpkgs> {};
clangStdenv.mkDerivation {
  name = "clang-nix-shell";
  buildInputs = [ /* add libraries here */ ];
}

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