packaging: don't block main crate build on its docs

package.nix

@@ -67,7 +67,9 @@ in {
     inherit (self) strictDeps __structuredAttrs;
     inherit (self) doCheck doInstallCheck;
 
-    outputs = [ "out" "doc" ];
+    outputs = [ "out" ];
+    # "Fake" doc output, since it's actually built as a separate derivation.
+    passthru.doc = self.dynixCrateDocs;
 
     src = lib.fileset.toSource {
       root = ./.;
@@ -82,12 +84,6 @@ in {
       lockFile = ./Cargo.lock;
     };
 
-    postInstall = ''
-      cargo doc --document-private-items
-      mkdir -p "$doc"
-      cp -r ./target/doc/* "$doc/"
-    '';
-
     nativeBuildInputs = rustHooks.asList ++ [
       cargo
     ];
@@ -103,6 +99,37 @@ in {
     };
   };
 
+  # Run `cargo doc` as a separate derivation, so it can be run in parallel while the main
+  # `cargo build` of `dynixCommand`.
+  # This does effectively run `cargo check` twice, but that's by far the fastest part of the build,
+  # so it's fine imo.
+  dynixCrateDocs = stdenv.mkDerivation {
+    pname = "${self.pname}-crate-docs";
+    inherit (self) version;
+    inherit (self) strictDeps __structuredAttrs;
+    inherit (self.dynixCommand) src cargoDeps nativeBuildInputs;
+
+    phases = [ "unpackPhase" "patchPhase" "cargoDocPhase" "installPhase" ];
+
+    cargoDocPhase = ''
+      runHook preCargoDoc
+      cargo doc --document-private-items
+      runHook postCargoDoc
+    '';
+
+    installPhase = ''
+      runHook preInstall
+      mkdir -p "$out"
+      cp -r ./target/doc/* "$out"
+      runHook postInstall
+    '';
+
+    meta = {
+      description = "Crate Rustdoc for Dynix";
+      inherit (self.meta) license;
+    };
+  };
+
   dynixModules = stdenv.mkDerivation {
     pname = "${self.pname}-modules";
     inherit (self) version;