Add a --num-threads to the prime-caches CLI command

And make it parallel by default (and remove the `--parallel` flag) to mirror the IDE cache priming.

Author: ChayimFriedman2

crates/rust-analyzer/src/cli/flags.rs

@@ -150,8 +150,8 @@ xflags::xflags! {
             optional --disable-proc-macros
             /// Run the proc-macro-srv binary at the specified path.
             optional --proc-macro-srv path: PathBuf
-            /// Run cache priming in parallel.
-            optional --parallel
+            /// The number of threads to use. Defaults to the number of physical cores.
+            optional --num-threads num_threads: usize
         }
 
         cmd ssr {
@@ -299,7 +299,7 @@ pub struct PrimeCaches {
     pub disable_build_scripts: bool,
     pub disable_proc_macros: bool,
     pub proc_macro_srv: Option<PathBuf>,
-    pub parallel: bool,
+    pub num_threads: Option<usize>,
 }
 
 #[derive(Debug)]

crates/rust-analyzer/src/cli/prime_caches.rs

@@ -52,7 +52,7 @@ impl flags::PrimeCaches {
             elapsed.memory.allocated.megabytes() as u64
         );
 
-        let threads = if self.parallel { num_cpus::get() } else { 1 };
+        let threads = self.num_threads.unwrap_or_else(num_cpus::get_physical);
         ide_db::prime_caches::parallel_prime_caches(&db, threads, &|_| ());
 
         let elapsed = stop_watch.elapsed();