patch-compile-commands

Make your compile_commands.json tell the whole truth when you build C/C++ projects inside a Nix development shell.

TLDR

• Nix compiler-wrappers silently inject
• -isystem /nix/store/…/include flags.
• The build succeeds, but those flags never reach clangd, ccls, cpp-tools
• … so your editor shows false “cannot find header” errors.
• This tool adds the same flags back into the compile database **after the
• build** so every LSP sees the correct include paths.

---

How it works

1. Every dev package in your buildInputs contributes its include path to the environment variable NIX_CFLAGS_COMPILE via Nix setup-hooks.

2. The Python script:

   • reads $NIX_CFLAGS_COMPILE
   • reads compile_commands.json
   • prepends the missing -isystem flags (idempotently)
   

   You can run it directly:

   $ patch-compile-commands          # ← fixes build/compile_commands.json
   $ PATCH_CC_DB=out/compile_commands.json patch-compile-commands

3. A ready-made shell hook (patch_cc.lib.shellHook) calls the tool automatically every time you regenerate the build inside nix develop.

---

Quick start (as a flake input)

{
  description = "example-cpp";

  inputs = {
    nixpkgs.url     = "github:nixos/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
    patch_cc.url    = "github:LarssonMartin1998/patch-compile-commands";
  };

  outputs = { self, nixpkgs, flake-utils, patch_cc }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
      in {
        packages.example = pkgs.stdenv.mkDerivation {
          pname = "example";
          version = "0.1.0";
          src = ./.;

          nativeBuildInputs = [
            pkgs.cmake
            patch_cc.packages.${system}.default
          ];

          # ← add *one* line and every interactive build shell gets patched
          shellHook = ''
            ${patch_cc.lib.shellHook}
          '';
        };

        packages.default = self.packages.${system}.example;
      });
}