Improve composability of mkShell

[basvandijk]

Motivation for this change

This improves the composability of mkShell in two ways:

• shellHook is now also composed. For example running the following expression with nix-shell:

let
  pkgs = import <nixpkgs> {};

  shell1 = pkgs.mkShell {
    shellHook = ''
      echo shell1
    '';
  };

  shell2 = pkgs.mkShell {
    shellHook = ''
      echo shell2
    '';
  };

  shell3 = pkgs.mkShell {
    inputsFrom = [ shell1 shell2 ];
    shellHook = ''
      echo shell3
    '';
  };
in shell3

will now result in:

shell2
shell1
shell3

Note that packages in the front of inputsFrom have precedence over packages in the back. The outermost mkShell has precedence over all.

• mergeInputs will now result in a more sensible order of $PATH. For example running following with nix-shell:

let
  pkgs = import <nixpkgs> {};

  shell1 = pkgs.mkShell {
    buildInputs = [ pkgs.htop ];
  };

  shell2 = pkgs.mkShell {
    buildInputs = [ pkgs.hello ];
  };

  shell3 = pkgs.mkShell {
    inputsFrom = [ shell1 shell2 ];
    buildInputs = [ pkgs.tree ];
  };

in shell3

results in the following $PATH:

$ echo $PATH
...
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
...

Previously the order was:

/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:

The new order makes more sense because it allows to override $PATH in the outermost mkShell and this order is consistent with the shellHook precedence.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[zimbatm]

good idea

[basvandijk]

Note I also backported this to release-19.03 in #63718.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-shell-tips-tricks-and-best-practices/22332/1