nix: output docs in

package.nix

@@ -58,6 +58,8 @@ in {
     inherit (self) strictDeps __structuredAttrs;
     inherit (self) doCheck doInstallCheck;
 
+    outputs = [ "out" "doc" ];
+
     src = lib.fileset.toSource {
       root = ./.;
       fileset = lib.fileset.unions [
@@ -71,6 +73,12 @@ in {
       lockFile = ./Cargo.lock;
     };
 
+    postInstall = ''
+      cargo doc --document-private-items
+      mkdir -p "$doc"
+      cp -r ./target/doc/* "$doc/"
+    '';
+
     nativeBuildInputs = rustHooks.asList ++ [
       cargo
     ];