Take this flake's inputs, for example.

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    rust-overlay.url = "github:oxalica/rust-overlay";
    systems = {
      url = "github:nix-systems/default";
      flake = false;
    };
  };

This would be the content of the flake.lock for the above inputs,

{
  "nodes": {
    "nixpkgs": {
      // ...
    },
    "nixpkgs_2": {
      // ...
    },
    "rust-overlay": {
      "inputs": {
        "nixpkgs": "nixpkgs_2"
      },
      // ...
    },
    "systems": {
      // ...
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs",
        "rust-overlay": "rust-overlay",
        "systems": "systems"
      }
    }
  },
  "root": "root",
  "version": 7
}

You can see two instances of nixpkgs. This means our flake's closure size is rather large, and we don't really want to download nixpkgs twice.

This can be fixed by defining rust-overlay.inputs.nixpkgs.follows to the name of our nixpkgs.

  inputs = {
    # ...
    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    # ...
  };

That change would result in the following in the lock file:

  {
    "nodes": {
      "nixpkgs": {
        // ...
      },
-     "nixpkgs_2": {
-       // ...
-     },
      "rust-overlay": {
        "inputs": {
-         "nixpkgs": "nixpkgs_2"
+         "nixpkgs": [
+           "nixpkgs"
+         ]
        },
        // ...
      },
      // ...
    },
    "root": "root",
    "version": 7
  }

This tool, allfollow, will effectively do the same as a post-process to your flake.lock.

Large flakes that aggregate other packages need to add inputs.*.inputs.*.follows = "*" for each input of every top-level input, perhaps even recursively.

Take a look at the [Hyprland's flake.nix]. Every single repository in the HyprWM organization with a flake.nix is riddled with the words inputs and follows, and it is a pain to maintain this web of dependencies manually.

As of writing, for my flake Hyprnix, I was successfully able to remove 47+ tediously-maintained lines from the flake.nix. the original flake.lock is 933 lines long, the only 150 after using allfollow.