set default threads to 8

Author: SparrowLii

compiler/rustc_session/src/options.rs

@@ -1800,7 +1800,7 @@ written to standard error output)"),
     /// in the future. Note that -Zthreads=0 is the way to get
     /// the num_cpus behavior.
     #[rustc_lint_opt_deny_field_access("use `Session::threads` instead of this field")]
-    threads: usize = (2, parse_threads, [UNTRACKED],
+    threads: usize = (8, parse_threads, [UNTRACKED],
         "use a thread pool with N threads"),
     time_llvm_passes: bool = (false, parse_bool, [UNTRACKED],
         "measure time of each LLVM pass (default: no)"),