MonoItem collector: Cleanup start fn root collection.

src/librustc_mir/monomorphize/collector.rs

@@ -341,6 +341,8 @@ fn collect_roots<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         };
 
         tcx.hir.krate().visit_all_item_likes(&mut visitor);
+
+        visitor.push_extra_entry_roots();
     }
 
     // We can only translate items that are instantiable - items all of
@@ -998,8 +1000,6 @@ impl<'b, 'a, 'v> RootCollector<'b, 'a, 'v> {
 
             let instance = Instance::mono(self.tcx, def_id);
             self.output.push(create_fn_mono_item(instance));
-
-            self.push_extra_entry_roots(def_id);
         }
     }
 
@@ -1008,20 +1008,22 @@ impl<'b, 'a, 'v> RootCollector<'b, 'a, 'v> {
     /// monomorphized copy of the start lang item based on
     /// the return type of `main`. This is not needed when
     /// the user writes their own `start` manually.
-    fn push_extra_entry_roots(&mut self, def_id: DefId) {
-        if self.entry_fn != Some(def_id) {
-            return;
-        }
-
+    fn push_extra_entry_roots(&mut self) {
         if self.tcx.sess.entry_type.get() != Some(config::EntryMain) {
-            return;
+            return
         }
 
+        let main_def_id = if let Some(def_id) = self.entry_fn {
+            def_id
+        } else {
+            return
+        };
+
         let start_def_id = match self.tcx.lang_items().require(StartFnLangItem) {
             Ok(s) => s,
             Err(err) => self.tcx.sess.fatal(&err),
         };
-        let main_ret_ty = self.tcx.fn_sig(def_id).output();
+        let main_ret_ty = self.tcx.fn_sig(main_def_id).output();
 
         // Given that `main()` has no arguments,
         // then its return type cannot have