ThreadPool::enqueue(): Use move semantics

This avoids a superfluous copy of the work item.

src/libutil/include/nix/util/thread-pool.hh

@@ -36,7 +36,7 @@ public:
     /**
      * Enqueue a function to be executed by the thread pool.
      */
-    void enqueue(const work_t & t);
+    void enqueue(work_t t);
 
     /**
      * Execute work items until the queue is empty.

src/libutil/thread-pool.cc

@@ -41,12 +41,12 @@ void ThreadPool::shutdown()
         thr.join();
 }
 
-void ThreadPool::enqueue(const work_t & t)
+void ThreadPool::enqueue(work_t t)
 {
     auto state(state_.lock());
     if (quit)
         throw ThreadPoolShutDown("cannot enqueue a work item while the thread pool is shutting down");
-    state->pending.push(t);
+    state->pending.push(std::move(t));
     /* Note: process() also executes items, so count it as a worker. */
     if (state->pending.size() > state->workers.size() + 1 && state->workers.size() + 1 < maxThreads)
         state->workers.emplace_back(&ThreadPool::doWork, this, false);