nix: build nova-plugin via crane to cache deps separately from crate src

flake.lock

@@ -1,5 +1,20 @@
 {
   "nodes": {
+    "crane": {
+      "locked": {
+        "lastModified": 1777830388,
+        "narHash": "sha256-2uoQAqUk2H0ijQtGiWAyNeQYGYc6yfAcRRLlJAz4Gp8=",
+        "owner": "ipetkov",
+        "repo": "crane",
+        "rev": "d459c1350e96ce1a7e3859c513ef5e9869d67d6f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "ipetkov",
+        "repo": "crane",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1776169885,
@@ -38,6 +53,7 @@
     },
     "root": {
       "inputs": {
+        "crane": "crane",
         "nixpkgs": "nixpkgs",
         "quickshell": "quickshell",
         "treefmt-nix": "treefmt-nix"

flake.nix

@@ -13,6 +13,8 @@
       url = "github:numtide/treefmt-nix";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    crane.url = "github:ipetkov/crane";
   };
 
   outputs =
@@ -21,6 +23,7 @@
       nixpkgs,
       quickshell,
       treefmt-nix,
+      crane,
       ...
     }:
     let
@@ -57,6 +60,7 @@
           fn rec {
             rawPkgs = nixpkgs.legacyPackages.${system};
             pkgs = rawPkgs.extend qtbaseOverlay;
+            craneLib = crane.mkLib pkgs;
             treefmt-eval = treefmt-nix.lib.evalModule rawPkgs treefmt-config;
           }
         );
@@ -65,7 +69,7 @@
       formatter = forAllSystems ({ treefmt-eval, ... }: treefmt-eval.config.build.wrapper);
 
       packages = forAllSystems (
-        { pkgs, rawPkgs, ... }:
+        { pkgs, rawPkgs, craneLib, ... }:
         let
           # Rebuild quickshell against patched Qt via its overlay
           qs = (pkgs.extend quickshell.overlays.default).quickshell.override {
@@ -74,7 +78,7 @@
           };
           nova-stats = pkgs.callPackage ./nix/stats-daemon.nix { };
           nova-shaders = pkgs.callPackage ./nix/shaders.nix { };
-          nova-plugin = pkgs.callPackage ./nix/plugin.nix { };
+          nova-plugin = pkgs.callPackage ./nix/plugin.nix { inherit craneLib; };
         in
         rec {
           inherit nova-stats nova-shaders nova-plugin;
@@ -122,12 +126,13 @@
                 withX11 = false;
                 withI3 = false;
               };
+              craneLibRaw = crane.mkLib rawPkgs;
             in
             rawPkgs.callPackage ./nix/package.nix {
               quickshell = qsUnpatched;
               nova-stats = rawPkgs.callPackage ./nix/stats-daemon.nix { };
               nova-shaders = rawPkgs.callPackage ./nix/shaders.nix { };
-              nova-plugin = rawPkgs.callPackage ./nix/plugin.nix { };
+              nova-plugin = rawPkgs.callPackage ./nix/plugin.nix { craneLib = craneLibRaw; };
             };
         }
       );

nix/plugin.nix

@@ -1,6 +1,6 @@
 {
   lib,
-  rustPlatform,
+  craneLib,
   pkg-config,
   qt6,
   writeShellScript,
@@ -33,19 +33,21 @@ let
     fi
     exec ${qt6.qtbase}/bin/qmake6 "$@"
   '';
-in
-rustPlatform.buildRustPackage {
+
+  # Args shared between the deps-only build (cached on Cargo.lock changes only)
+  # and the final crate build. cxx-qt-build runs during the deps build too, so it
+  # needs the Qt env to find qmltyperegistrar/qmlcachegen.
+  commonArgs = {
     pname = "nova-plugin";
     version = "0.1.0";
-  src = lib.cleanSource ../plugin;
-  cargoLock.lockFile = ../plugin/Cargo.lock;
+    src = craneLib.cleanCargoSource ../plugin;
+    strictDeps = true;
 
     nativeBuildInputs = [
       pkg-config
       qt6.qtbase
       qt6.qtdeclarative
     ];
-
     buildInputs = [
       qt6.qtbase
       qt6.qtdeclarative
@@ -58,22 +60,35 @@ rustPlatform.buildRustPackage {
     preBuild = ''
       export QMAKE=${qmakeWrapper}
     '';
+  };
 
-  installPhase = ''
-    runHook preInstall
+  cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+in
+craneLib.buildPackage (
+  commonArgs
+  // {
+    inherit cargoArtifacts;
 
+    # Crane's default cargo build doesn't set CARGO_BUILD_TARGET, so artifacts land
+    # in `target/release/` and cxx-qt files in `target/cxxqt/...` (no triple subdir,
+    # unlike nixpkgs rustPlatform). Use `find` to stay agnostic to either layout.
+    installPhaseCommand = ''
       qml_dir="$out/lib/qt-6/qml/NovaStats"
       mkdir -p "$qml_dir"
 
-    install -m755 target/*/release/libnova_plugin.so "$qml_dir/libNovaStats.so"
-    install -m644 target/*/cxxqt/qml_modules/NovaStats/qmldir "$qml_dir/"
-    install -m644 target/*/cxxqt/qml_modules/NovaStats/plugin.qmltypes "$qml_dir/" 2>/dev/null || true
+      so=$(find target -name 'libnova_plugin.so' -path '*/release/*' | head -1)
+      install -m755 "$so" "$qml_dir/libNovaStats.so"
 
-    runHook postInstall
+      qmldir=$(find target -path '*/cxxqt/qml_modules/NovaStats/qmldir' | head -1)
+      install -m644 "$qmldir" "$qml_dir/"
+
+      qmltypes=$(find target -path '*/cxxqt/qml_modules/NovaStats/plugin.qmltypes' | head -1)
+      [ -n "$qmltypes" ] && install -m644 "$qmltypes" "$qml_dir/" || true
     '';
 
     meta = {
       description = "In-process system stats QML plugin for nova-shell";
       platforms = lib.platforms.linux;
     };
-}
+  }
+)

plugin/build.rs

@@ -8,7 +8,15 @@ fn main() {
     // plugin loader finds nothing exported. --export-dynamic re-exposes them.
     println!("cargo:rustc-link-arg=-Wl,--export-dynamic");
 
+    // Crane's deps-only build stubs out lib.rs and removes our bridge sources to
+    // build a dummy crate that only compiles dependencies. Skip cxx-qt codegen in
+    // that case - we just want cxx-qt-lib (the heavy dep) cached, not our own glue.
+    let bridge_files = ["src/system_stats.rs", "src/cpu_service.rs"];
+    if !bridge_files.iter().all(|p| std::path::Path::new(p).exists()) {
+        return;
+    }
+
     CxxQtBuilder::new_qml_module(QmlModule::new("NovaStats").plugin_type(PluginType::Dynamic))
-        .files(["src/system_stats.rs", "src/cpu_service.rs"])
+        .files(bridge_files)
         .build();
 }