perf: Share Cargo build cache across git worktrees (#60)

Use a common target directory for all worktrees to avoid recompiling the backend in every worktree directory.

.cargo/config.toml

@@ -1,7 +1,5 @@
-[env]
 # Set macOS deployment target to avoid linker warnings
 # Use Rust project defaults: 10.12 for x86_64, 11.0 for aarch64
-
 [target.x86_64-apple-darwin.env]
 MACOSX_DEPLOYMENT_TARGET = "10.12"
 

package.json

@@ -3,17 +3,20 @@
   "version": "0.0.37",
   "private": true,
   "scripts": {
-    "dev": "export FRONTEND_PORT=$(node scripts/setup-dev-environment.js frontend) && export BACKEND_PORT=$(node scripts/setup-dev-environment.js backend) && concurrently \"cargo watch -w backend -x 'run --manifest-path backend/Cargo.toml'\" \"npm run frontend:dev\"",
+    "dev": "export FRONTEND_PORT=$(node scripts/setup-dev-environment.js frontend) && export BACKEND_PORT=$(node scripts/setup-dev-environment.js backend) && concurrently \"npm run backend:dev:watch\" \"npm run frontend:dev\"",
-    "build": "npm run frontend:build && cargo build --release --manifest-path backend/Cargo.toml && cargo build --release --bin mcp_task_server --manifest-path backend/Cargo.toml",
+    "build": "npm run frontend:build && npm run backend:build",
-    "build:single": "npm run frontend:build && cargo build --release --manifest-path backend/Cargo.toml",
+    "build:single": "npm run frontend:build && npm run backend:build:single",
     "build:npm": "./build-npm-package.sh",
     "test:npm": "./test-npm-package.sh",
     "frontend:dev": "cd frontend && npm run dev -- --port ${FRONTEND_PORT:-3000} --open",
     "frontend:build": "cd frontend && npm run build",
-    "backend:dev": "BACKEND_PORT=$(node scripts/setup-dev-environment.js backend) cargo watch -w backend -x 'run --manifest-path backend/Cargo.toml'",
+    "cargo": "node scripts/cargo.js",
-    "backend:build": "cargo build --release --manifest-path backend/Cargo.toml",
+    "backend:dev": "BACKEND_PORT=$(node scripts/setup-dev-environment.js backend) npm run backend:dev:watch",
-    "backend:run": "cargo run --manifest-path backend/Cargo.toml",
+    "backend:dev:watch": "npm run cargo -- watch -w backend -x 'run --manifest-path backend/Cargo.toml'",
-    "backend:test": "cargo test --lib",
+    "backend:build": "npm run cargo -- build --release --manifest-path backend/Cargo.toml && npm run cargo -- build --release --bin mcp_task_server --manifest-path backend/Cargo.toml",
+    "backend:build:single": "npm run cargo -- build --release --manifest-path backend/Cargo.toml",
+    "backend:run"  : "npm run cargo -- run --manifest-path backend/Cargo.toml",
+    "backend:test" : "npm run cargo -- test --lib",
     "generate-types": "cd backend && cargo run --bin generate_types",
     "prepare-db": "node scripts/prepare-db.js",
     "dev:clear-ports": "node scripts/setup-dev-environment.js clear"

scripts/cargo.js

@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+/**
+ * Drop-in replacement for the `cargo` binary.
+ * If CARGO_TARGET_DIR is already set we leave it alone.
+ * Otherwise we compute a path that is the same for every git-worktree:
+ *   <main-clone-root>/target
+ *
+ * Usage   :  node scripts/cargo.js <cargo-subcommand> […]
+ * NPM     :  "backend:build": "node scripts/cargo.js build --release …"
+ */
+const { spawnSync, execSync } = require('node:child_process');
+const path = require('node:path');
+
+function sharedTarget() {
+  try {
+    // Works in any work-tree folder
+    const common = execSync('git rev-parse --git-common-dir', { encoding: 'utf8' }).trim();
+    return path.resolve(common, '..', 'target');
+  } catch {
+    return undefined;                 // not a git repo? -> let Cargo use ./target
+  }
+}
+
+const env = { ...process.env };
+if (!env.CARGO_TARGET_DIR) {
+  const dir = sharedTarget();
+  if (dir) env.CARGO_TARGET_DIR = dir;
+}
+
+const result = spawnSync('cargo', process.argv.slice(2), { stdio: 'inherit', env });
+process.exitCode = result.status;