feat: improve CPU usage

closes #9922

Turned out to be trivial after preliminary refactor.

The intended behavior is that we schedule cache priming once ws become
quiescent (that is, we fully load cargo project), and we continue to
rschedule it until it completes (priming might get cancelled by user
typing into a file).

Author: matklad

crates/rust-analyzer/src/main_loop.rs

@@ -296,12 +296,15 @@ impl GlobalState {
                             ));
                             fraction = Progress::fraction(report.n_done, report.n_total);
                         }
-                        PrimeCachesProgress::End { cancelled: _ } => {
+                        PrimeCachesProgress::End { cancelled } => {
                             state = Progress::End;
                             message = None;
                             fraction = 1.0;
 
                             self.prime_caches_queue.op_completed(());
+                            if cancelled {
+                                self.prime_caches_queue.request_op();
+                            }
                         }
                     };
 
@@ -424,11 +427,10 @@ impl GlobalState {
                 for flycheck in &self.flycheck {
                     flycheck.update();
                 }
+                self.prime_caches_queue.request_op();
             }
 
             if !was_quiescent || state_changed {
-                self.prime_caches_queue.request_op();
-
                 // Refresh semantic tokens if the client supports it.
                 if self.config.semantic_tokens_refresh() {
                     self.semantic_tokens_cache.lock().clear();