{
  description = "fumi - Smol Mail client (Rust)";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";

    # RNS carrier sources (upstream RNS.md sec 13): the build sandbox has no
    # network, so every microReticulum FetchContent dependency is vendored as
    # its own flake input and handed over via RNS_<DEP>_SOURCE_DIR.
    # microReticulum is pinned at the commit bunshin verified against.
    microReticulum = {
      url = "github:attermann/microReticulum/40fa628809d57140180c1c833559ab96fec992c1";
      flake = false;
    };
    rns-arduinojson = {
      url = "github:bblanchon/ArduinoJson/ed69feadb95182dc53ac978975d310122060a767";
      flake = false;
    };
    rns-msgpack = {
      url = "github:hideakitai/MsgPack/1f552c31b940d6e9063ee17a4b3fa10c47b27169";
      flake = false;
    };
    rns-arxcontainer = {
      url = "github:hideakitai/ArxContainer/d6affcd0bc83219b863c20abf7c269214db8db2a";
      flake = false;
    };
    rns-arxtypetraits = {
      url = "github:hideakitai/ArxTypeTraits/702de9cc59c7e047cdc169ae3547718b289d2c02";
      flake = false;
    };
    rns-debuglog = {
      url = "github:hideakitai/DebugLog/b581f7dde6c276c5df684e2328f406d9754d2f46";
      flake = false;
    };
    rns-crypto = {
      url = "github:attermann/Crypto/984dc891330986c302a86c4e312d4f5abcc28359";
      flake = false;
    };
    rns-microstore = {
      url = "github:attermann/microStore/0f28567fe00ab8ab14624a34c2e9e0a000a44c46";
      flake = false;
    };
  };

  outputs = { self, nixpkgs, flake-utils, microReticulum, rns-arduinojson
            , rns-msgpack, rns-arxcontainer, rns-arxtypetraits, rns-debuglog
            , rns-crypto, rns-microstore }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
        inherit (pkgs) lib;

        # build.rs consumes these directly; with every dependency vendored,
        # the cmake configure step never fetches.
        rnsSourceEnv = {
          MICRORETICULUM_SOURCE_DIR = "${microReticulum}";
          RNS_ARDUINOJSON_SOURCE_DIR = "${rns-arduinojson}";
          RNS_MSGPACK_SOURCE_DIR = "${rns-msgpack}";
          RNS_ARXCONTAINER_SOURCE_DIR = "${rns-arxcontainer}";
          RNS_ARXTYPETRAITS_SOURCE_DIR = "${rns-arxtypetraits}";
          RNS_DEBUGLOG_SOURCE_DIR = "${rns-debuglog}";
          RNS_CRYPTO_SOURCE_DIR = "${rns-crypto}";
          RNS_MICROSTORE_SOURCE_DIR = "${rns-microstore}";
        };

        fumi = { rns ? false }: pkgs.rustPlatform.buildRustPackage {
          pname = "fumi";
          version = "0.1.0";
          src = ./.;
          cargoLock.lockFile = ./Cargo.lock;
          nativeBuildInputs = [ pkgs.pkg-config ]
            ++ lib.optionals rns [ pkgs.cmake pkgs.ninja pkgs.gcc ];
          buildFeatures = lib.optionals rns [ "rns" ];
          env = lib.optionalAttrs rns rnsSourceEnv;
          # ninja's setup hook claims buildPhase before the cargo hooks
          # can, which would run ninja against no build.ninja instead of
          # cargo; ninja here is only for build.rs to invoke through cmake.
          dontUseNinjaBuild = rns;
          dontUseNinjaCheck = rns;
          dontUseNinjaInstall = rns;
        };
      in
      {
        packages.default = fumi { };
        packages.rns = fumi { rns = true; };

        apps.default = flake-utils.lib.mkApp {
          drv = fumi { };
          name = "fumi";
        };

        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            cargo
            rustc
            rustfmt
            clippy
            pkg-config
            sqlite
            cmake
            ninja
            gcc
          ];
          env = rnsSourceEnv;
        };
      });
}