default.nix: init

Move recipes from flake.nix:s `packages` to a `callPackage`-able scope.nix.
This way out-of-tree projects can reuse the `buildRustPackage`
definitions (e.g. the in-sync `cargoLock` hashes) without depending on
the unsupported flakes feature.

scope.nix

@@ -0,0 +1,41 @@
+{ lib, newScope }:
+
+lib.makeScope newScope (self: {
+  forgetScope = lib.flip lib.removeAttrs [
+    "override"
+    "overrideDerivation"
+    "overrideScope"
+    "newScope"
+    "packages"
+    "forgetScope"
+  ];
+  buildEnvVars = self.forgetScope (
+    self.callPackage (
+      { xdg-desktop-portal, virtiofsd }:
+      {
+        BIN_XDG_PERMISSION_STORE = "${xdg-desktop-portal}/libexec/xdg-permission-store";
+        BIN_XDG_DOCUMENT_PORTAL = "${xdg-desktop-portal}/libexec/xdg-document-portal";
+        BIN_VIRTIOFSD = "${virtiofsd}/bin/virtiofsd";
+      }
+    ) { }
+  );
+  rustPackage =
+    {
+      crate,
+      cargoToml ? builtins.fromTOML (builtins.readFile ./${crate}/Cargo.toml),
+      rustPlatform,
+      buildEnvVars,
+    }:
+    rustPlatform.buildRustPackage {
+      inherit (cargoToml.package) name version;
+      src = ./.;
+      cargoLock.lockFile = ./Cargo.lock;
+      cargoLock.outputHashes = {
+        "busd-0.5.0" = "sha256-IZZ2MeEmUbzRrH6SUz0pnecMH4f8Mh54WdhI4q44YfI=";
+      };
+      buildAndTestSubdir = crate;
+      env = buildEnvVars;
+    };
+    sidebus-agent = self.callPackage self.rustPackage { crate = "sidebus-agent"; };
+    sidebus-broker = self.callPackage self.rustPackage { crate = "sidebus-broker"; };
+})