use cfg attribute instead of macro

compiler/rustc_codegen_ssa/src/base.rs

@@ -14,6 +14,8 @@
 use rustc_attr as attr;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::profiling::{get_resident_set_size, print_time_passes_entry};
+
+#[cfg(parallel_compiler)]
 use rustc_data_structures::sync::{par_iter, ParallelIterator};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
@@ -622,8 +624,8 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
     // This likely is a temporary measure. Once we don't have to support the
     // non-parallel compiler anymore, we can compile CGUs end-to-end in
     // parallel and get rid of the complicated scheduling logic.
+    #[cfg(parallel_compiler)]
     let pre_compile_cgus = |cgu_reuse: &[CguReuse]| {
-        if cfg!(parallel_compiler) {
         tcx.sess.time("compile_first_CGU_batch", || {
             // Try to find one CGU to compile per thread.
             let cgus: Vec<_> = cgu_reuse
@@ -645,11 +647,11 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
 
             (pre_compiled_cgus, start_time.elapsed())
         })
-        } else {
-            (FxHashMap::default(), Duration::new(0, 0))
-        }
     };
 
+    #[cfg(not(parallel_compiler))]
+    let pre_compile_cgus = |_: &[CguReuse]| (FxHashMap::default(), Duration::new(0, 0));
+
     let mut cgu_reuse = Vec::new();
     let mut pre_compiled_cgus: Option<FxHashMap<usize, _>> = None;
     let mut total_codegen_time = Duration::new(0, 0);