Lazy Flake Inputs, with the `cache-flake-inputs` flake

.flake-lazy-inputs2

(see flake.nix)

flake.lock

{
  "nodes": {
    "cfi": {
      "inputs": {
        "cache-flake-inputs-global-override": [
          "cfi-override"
        ],
        "flu": "flu",
        "nixpkgs": "nixpkgs"
      },
      "locked": {
        "lastModified": 1657740012,
        "narHash": "sha256-l6aTMD1zkxRxYwka+eqQZ66YOmJ3xGUAtRTptzQEpFw=",
        "owner": "rrbutani",
        "repo": "cache-flake-inputs",
        "rev": "1213940c8196ecf569ccc80bc75b1028583c5678",
        "type": "github"
      },
      "original": {
        "owner": "rrbutani",
        "repo": "cache-flake-inputs",
        "type": "github"
      }
    },
    "cfi-override": {
      "locked": {
        "lastModified": 1657739253,
        "narHash": "sha256-L9eyTL7njtPBUYmZRYFKCzQFDgua9U9oE7UwCzjZfl8=",
        "owner": "boolean-option",
        "repo": "true",
        "rev": "6ecb49143ca31b140a5273f1575746ba93c3f698",
        "type": "github"
      },
      "original": {
        "owner": "boolean-option",
        "repo": "true",
        "type": "github"
      }
    },
    "flu": {
      "locked": {
        "lastModified": 1656928814,
        "narHash": "sha256-RIFfgBuKz6Hp89yRr7+NR5tzIAbn52h8vT6vXkYjZoM=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "7e2a3b3dfd9af950a856d66b0a7d01e3c18aa249",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "flu_2": {
      "locked": {
        "lastModified": 1656928814,
        "narHash": "sha256-RIFfgBuKz6Hp89yRr7+NR5tzIAbn52h8vT6vXkYjZoM=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "7e2a3b3dfd9af950a856d66b0a7d01e3c18aa249",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "llvm": {
      "flake": false,
      "locked": {
        "lastModified": 1655916384,
        "narHash": "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE=",
        "owner": "llvm",
        "repo": "llvm-project",
        "rev": "f28c006a5895fc0e329fe15fead81e37457cb1d1",
        "type": "github"
      },
      "original": {
        "owner": "llvm",
        "ref": "llvmorg-14.0.6",
        "repo": "llvm-project",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1653936696,
        "narHash": "sha256-M6bJShji9AIDZ7Kh7CPwPBPb/T7RiVev2PAcOi4fxDQ=",
        "owner": "nixOS",
        "repo": "nixpkgs",
        "rev": "ce6aa13369b667ac2542593170993504932eb836",
        "type": "github"
      },
      "original": {
        "owner": "nixOS",
        "ref": "22.05",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "nixpkgs_2": {
      "locked": {
        "lastModified": 1653936696,
        "narHash": "sha256-M6bJShji9AIDZ7Kh7CPwPBPb/T7RiVev2PAcOi4fxDQ=",
        "owner": "nixOS",
        "repo": "nixpkgs",
        "rev": "ce6aa13369b667ac2542593170993504932eb836",
        "type": "github"
      },
      "original": {
        "owner": "nixOS",
        "ref": "22.05",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "cfi": "cfi",
        "cfi-override": "cfi-override",
        "flu": "flu_2",
        "llvm": "llvm",
        "nixpkgs": "nixpkgs_2"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  nixConfig = {
    # A remote cache that you provide.
    extra-substituters = [
      "https://rrbutani.cachix.org"
    ];
    extra-trusted-public-keys = [
      "rrbutani.cachix.org-1:FUpcK9RyZjjdOm8qherJl9+wfTGf6ptANvH6LZF63Ro="
    ];
  };

  inputs = {
    # A large unwieldy flake input (an example).
    llvm = {
      url = github:llvm/llvm-project?ref=llvmorg-14.0.6;
      flake = false;
    };

    flu.url = github:numtide/flake-utils;
    nixpkgs.url = github:nixOS/nixpkgs/22.05;

    cfi-override.url = github:boolean-option/true;
    cfi = {
      url = github:rrbutani/cache-flake-inputs;

      # You can set `cfi-override.url` (above) to `github:boolean-option/false`
      # to disable `cacheInputs`.
      #
      # This is useful for changing the behavior of your _transitive_ flake dependencies.
      inputs.cache-flake-inputs-global-override.follows = "cfi-override";
    };
  };

  outputs = inputs@{ flu, cfi, nixpkgs, ... }: with flu.lib; eachDefaultSystem (sys: let
    knownCachedNarHashes = {
      # Keys should match the names of your flake inputs.
      "llvm" = {
        # Hashes should match what's in your `flake.lock`.
        "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE=" = true;
      };
    };

    np = import nixpkgs { system = sys; };
    inputs' = cfi.lib.${sys}.cacheInputs {
      inherit inputs knownCachedNarHashes;
      lockFile = ./flake.lock;

      # This tells `cfi` to assume that we can indeed use substituters to get the
      # things `knownCachedNarHashes` says we can.
      #
      # If this is set to `false`, `cfi` will effectively do nothing; derivations
      # in `inputs'` will be equivalent to the ones in `inputs` and flake inputs
      # will be fetched eagerly during evaluation.
      useSubstituters = true;

      # We already have a `nixpkgs` instance so pass it along.
      #
      # This is optional; if omitted, `cfi` will use it's own `nixpkgs` flake
      # input — which you can override with `inputs.cfi.inputs.nixpkgs.follows`).
      nixpkgs = np;
    };

    pkgC = src: np.stdenvNoCC.mkDerivation {
      name = "example";
      src = src;
      unpackPhase = "true";
      installPhase = ''
        cat $src/libunwind/CMakeLists.txt | grep PACKAGE_VERSION | cut -d' ' -f4- | cut -d')' -f1 | head -1 > $out
      '';
    };

    # Both `pkgA` and `pkgB` are in the substituter listed at the top of this flake.

    # Building `pkgA` will require fetching LLVM.
    pkgA = pkgC inputs.llvm;

    # Building `pkgB` will not.
    pkgB = pkgC inputs'.llvm;
  in {
    packages = {
      default = pkgB;
      inherit pkgA pkgB;
    };
  });
}