readme: Fix ./script/gen

I've accidentally broken the script during the recent test refactoring,
since now test.nix doesn't evaluate to an attrset.

readme.nix

@@ -0,0 +1,40 @@
+# This script is used to test & generate `README.md`.
+
+let
+  sources = import ./nix/sources.nix;
+
+  pkgs = import ./nix {
+    system = builtins.currentSystem;
+    nixpkgs = "nixpkgs";
+  };
+
+  naersk = pkgs.callPackage ./default.nix {
+    inherit (pkgs.rustPackages) cargo rustc;
+  };
+
+  docparse = naersk.buildPackage {
+    root = ./docparse;
+
+    src = builtins.filterSource (
+      p: t:
+        let
+          p' = pkgs.lib.removePrefix (toString ./docparse + "/") p;
+        in
+        p' == "Cargo.lock" || p' == "Cargo.toml" || p' == "src" || p' == "src/main.rs"
+    ) ./docparse;
+  };
+
+in rec {
+  body = pkgs.runCommand "readme-body" {
+    buildInputs = [ docparse ];
+  } ''
+    cat ${./README.tpl.md} > $out
+    docparse ${./config.nix} >> gen
+    sed -e '/GEN_CONFIGURATION/{r gen' -e 'd}' -i $out
+  '';
+
+  test = pkgs.runCommand "readme-test" {} ''
+    diff ${./README.md} ${body}
+    touch $out
+  '';
+}

script/gen

@@ -7,6 +7,6 @@ set -euo pipefail
 
 echo "Updating README"
 
-cat $(nix-build ./test.nix -A readme) > README.md
+cat $(nix-build ./readme.nix -A body) > README.md
 
 echo done

test/fast/readme/default.nix

@@ -1,25 +1,2 @@
-{ naersk, pkgs, ... }:
-let
-docparse = naersk.buildPackage {
-  root = ../../../docparse;
-
-  src = builtins.filterSource (
-    p: t:
-      let
-        p' = pkgs.lib.removePrefix (toString ../../../docparse + "/") p;
-      in
-      p' == "Cargo.lock" || p' == "Cargo.toml" || p' == "src" || p' == "src/main.rs"
-  ) ../../../docparse;
-};
-
-readme = pkgs.runCommand "readme-gen" {} ''
-  cat ${../../../README.tpl.md} > $out
-  ${docparse}/bin/docparse ${../../../config.nix} >> gen
-  sed -e '/GEN_CONFIGURATION/{r gen' -e 'd}' -i $out
-'';
-
-in
-pkgs.runCommand "readme-test" {} ''
-  diff ${../../../README.md} ${readme}
-  touch $out
-''
+{ ... }:
+(import ../../../readme.nix).test