flake.lock

{
  "nodes": {
    "flake-utils": {
      "inputs": {
        "systems": "systems"
      },
      "locked": {
        "lastModified": 1710146030,
        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1713805509,
        "narHash": "sha256-YgSEan4CcrjivCNO5ZNzhg7/8ViLkZ4CB/GrGBVSudo=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "1e1dc66fe68972a76679644a5577828b6a7e8be4",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixpkgs-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": "nixpkgs"
      }
    },
    "systems": {
      "locked": {
        "lastModified": 1681028828,
        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
        "owner": "nix-systems",
        "repo": "default",
        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
        "type": "github"
      },
      "original": {
        "owner": "nix-systems",
        "repo": "default",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  inputs = {
    flake-utils.url = "github:numtide/flake-utils";
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
  };
  outputs = {
    self,
    flake-utils,
    nixpkgs,
  }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let pkgs = nixpkgs.legacyPackages.${system}; in
        {
          packages.hello =
            pkgs.callPackage ./hello.nix {
              trivialBuild = pkgs.emacs.pkgs.trivialBuild;
            };
        }
      );
}

hello.el

;;; hello.el --- Description -*- lexical-binding: t; -*-

(defun hello ()
  "Say hello."
  (interactive)
  (message "Hello, world!"))

(provide 'hello)
;;; hello.el ends here

hello.nix

{
  trivialBuild,
}:
trivialBuild rec {
  pname = "hello";
  version = "v1.0.0";
  src = ./.;
}

Use this with e.g. a flake.nix like:

{
  inputs = {
    flake-utils.url = "github:numtide/flake-utils";
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    hello.url = "git+https://gist.github.com/rgoulter/fc8863d085d98276d8d51b1f301e150a";
  };
  outputs = {
    self,
    flake-utils,
    nixpkgs,
    hello,
  }@inputs:
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          pkgs = nixpkgs.legacyPackages.${system};
          hello = inputs.hello.packages.${system}.hello;
        in
        {
          packages.emacs =
            pkgs.emacs.pkgs.emacsWithPackages (epkgs: [
              hello
            ]);
        }
      );
}

Then running the resulting emacs with emacs -Q, the following Elisp should echo 'hello world':

(require 'hello)
(hello)