Mini Compile Commands

Mini compile commands instruments the compiler wrappers in nixpkgs to generate compile_commands.json files. Using a version of nixpkgs after this PR, it can be used in a shell.nix as follows:

with import <nixpkgs> {};
let mcc-env = (callPackage <this_repo> {}).wrap stdenv;
in (mkShell.override {stdenv = mcc-env;}) {
   buildInputs = [ cmake gtest ];
}

When the compiler is invoked, it will send a message to mini_compile_commands_server.py:

example.mp4

Mini Compile Commands and flakes

If your project is flake based, add mini-compile-commands = { url = github:danielbarter/mini_compile_commands; flake = false;}; to your inputs and mini-compile-commands as an argument to your outputs. Then the above development shell output would be specified as

devShell.x86_64-linux =
  with import nixpkgs { system = "x86_64-linux"; };
  let mcc-env = (callPackage mini-compile-commands {}).wrap stdenv;
  in (mkShell.override {stdenv = mcc-env;}) {
    buildInputs = [ cmake gtest ];
  };

Examples

Mini compile commands can be used to generate a compile_commands.json for the linux kernel:

nix-shell -E 'with import <nixpkgs> {}; let mcc-env = (callPackage <this_repo> {}).wrap stdenv; in linux.override { stdenv = mcc-env; }'

As demonstrated in the above video, create two shells. In one, run mini_compile_commands_server.py compile_commands.json and in the other, run genericBuild.

For certain packages (like those included in python3Packages), there is currently no easy way to override the standard environment. To use mini compile commands for these packages, we can override the standard environment for all nixpkgs as follows:

nix-build -E 'with import <nixpkgs> { config.replaceStdenv = ({ pkgs }: (pkgs.callPackage <this_repo> {}).wrap pkgs.stdenv);}; python3Packages.pybind11'

Warning: This requires a huge amount of rebuilding.

Testing

There are tests for gcc and clang in tests/gcc and tests/clang respectively. In either of these directories, running nix-shell will generate a compile_commands.json. To test if things are working, open test.cc (make sure your editor can locate clangd) and try and jump into the iostream header.

Build input hook

It is possible to use mini compile commands to generate a compile_commands.json while building a derivation. This involves using a wrapped standard standard environment and adding a hook to the buildInputs:

with import <nixpkgs> {};
let mcc-env = (callPackage <this_repo> {}).wrap stdenv;
    mcc-hook = (callPackage <this_repo> {}).hook;
in (hello.override { stdenv = mcc-env; }).overrideAttrs
  (finalAttrs: previousAttrs: {
    buildInputs = (previousAttrs.buildInputs or []) ++ [ mcc-hook ];
  })

Running nix-build on this derivation will generate a compile_commands.json and move it into $out. As explained in the compile commands specification, each entry contains a directory attribute which is the absolute path of the directory from which the compiler invocation occurred. To use this generated json with clangd, the source code needs to be located in the same place as during the nix build (typically /build). Since this is inconvenient, we recommend using mini compile commands interactively.