core/pkgs/build-support/mkshell/default.nix

{ lib, stdenv, buildEnv }:

# A special kind of derivation that is only meant to be consumed by the
# nix-shell.
{ name ? "nix-shell", # a list of packages to add to the shell environment
packages ? [ ], # propagate all the inputs from the given derivations
inputsFrom ? [ ], buildInputs ? [ ], nativeBuildInputs ? [ ]
, propagatedBuildInputs ? [ ], propagatedNativeBuildInputs ? [ ], ... }@attrs:
let
  mergeInputs = name:
    (attrs.${name} or [ ]) ++
    # 1. get all `{build,nativeBuild,...}Inputs` from the elements of `inputsFrom`
    # 2. since that is a list of lists, `flatten` that into a regular list
    # 3. filter out of the result everything that's in `inputsFrom` itself
    # this leaves actual dependencies of the derivations in `inputsFrom`, but never the derivations themselves
    (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs name inputsFrom)));

  rest = builtins.removeAttrs attrs [
    "name"
    "packages"
    "inputsFrom"
    "buildInputs"
    "nativeBuildInputs"
    "propagatedBuildInputs"
    "propagatedNativeBuildInputs"
    "shellHook"
  ];

in stdenv.mkDerivation ({
  inherit name;

  buildInputs = mergeInputs "buildInputs";
  nativeBuildInputs = packages ++ (mergeInputs "nativeBuildInputs");
  propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
  propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";

  shellHook = lib.concatStringsSep "\n"
    (lib.catAttrs "shellHook" (lib.reverseList inputsFrom ++ [ attrs ]));

  phases = [ "buildPhase" ];

  buildPhase = ''
    { echo "------------------------------------------------------------";
      echo " WARNING: the existence of this path is not guaranteed.";
      echo " It is an internal implementation detail for pkgs.mkShell.";
      echo "------------------------------------------------------------";
      echo;
      # Record all build inputs as runtime dependencies
      export;
    } >> "$out"
  '';

  preferLocalBuild = true;
} // rest)