nix flake show: Skip IFDs instead of throwing

[ulucs]

Motivation

As mentioned in #4265 (comment), nix flake show fails if it encounters an IFD while it's trying to show the derivations inside the flake. This means that nix flake show is effectively broken if you are using IFD's in almost any place in your flake.

Context

Following --all-systems, nix flake show now shows an "omitted" message when it encounters an IFD. To this end, I gave IFD errors an explicit type that made this possible.

Example output with this patch:

git+file:///Users/ulucs/projects/flake-filter
├───checks
│   ├───aarch64-darwin
│   │   └───flake-filter omitted due to use of import from derivation
│   ├───aarch64-linux
│   │   └───flake-filter omitted (use '--all-systems' to show)
│   ├───x86_64-darwin
│   │   └───flake-filter omitted (use '--all-systems' to show)
│   └───x86_64-linux
│       └───flake-filter omitted (use '--all-systems' to show)
├───devShells
│   ├───aarch64-darwin
│   │   ├───default: development environment 'nix-shell'
│   │   └───rust: development environment 'nix-shell'
│   ├───aarch64-linux
│   │   ├───default omitted (use '--all-systems' to show)
│   │   └───rust omitted (use '--all-systems' to show)
│   ├───x86_64-darwin
│   │   ├───default omitted (use '--all-systems' to show)
│   │   └───rust omitted (use '--all-systems' to show)
│   └───x86_64-linux
│       ├───default omitted (use '--all-systems' to show)
│       └───rust omitted (use '--all-systems' to show)
└───packages
    ├───aarch64-darwin
    │   ├───default omitted due to use of import from derivation
    │   └───rust omitted due to use of import from derivation
    ├───aarch64-linux
    │   ├───default omitted (use '--all-systems' to show)
    │   └───rust omitted (use '--all-systems' to show)
    ├───x86_64-darwin
    │   ├───default omitted (use '--all-systems' to show)
    │   └───rust omitted (use '--all-systems' to show)
    └───x86_64-linux
        ├───default omitted (use '--all-systems' to show)
        └───rust omitted (use '--all-systems' to show)

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[tomberek]

From meeting:

Not sure about adding a dedicated flag that is so similar to an existing flag + option. Let's start with only the descriptive warning that the output is omitted.

[nixos-discourse]

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

https://discourse.nixos.org/t/nix-2-29-0-released/64609/1