Refactor local monomorphization logic to be easier to comprehend

src/librustc_mir/monomorphize/collector.rs

@@ -187,7 +187,6 @@
 use rustc::hir::{self, CodegenFnAttrFlags};
 use rustc::hir::itemlikevisit::ItemLikeVisitor;
 
-use rustc::hir::Node;
 use rustc::hir::def_id::DefId;
 use rustc::mir::interpret::{AllocId, ConstValue};
 use rustc::middle::lang_items::{ExchangeMallocFnLangItem, StartFnLangItem};
@@ -737,27 +736,27 @@ fn should_monomorphize_locally<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, instance:
         ty::InstanceDef::CloneShim(..) => return true
     };
 
-    return match tcx.hir.get_if_local(def_id) {
-        Some(Node::ForeignItem(..)) => {
-            false // foreign items are linked against, not codegened.
+    if tcx.is_foreign_item(def_id) {
+        // We can always link to foreign items
+        return false;
     }
-        Some(_) => true,
-        None => {
+
+    if def_id.is_local() {
+        // local items cannot be referred to locally without monomorphizing them locally
+        return true;
+    }
+
     if tcx.is_reachable_non_generic(def_id) ||
-                tcx.is_foreign_item(def_id) ||
-                is_available_upstream_generic(tcx, def_id, instance.substs)
-            {
+       is_available_upstream_generic(tcx, def_id, instance.substs) {
         // We can link to the item in question, no instance needed
         // in this crate
-                false
-            } else {
+        return false;
+    }
+
     if !tcx.is_mir_available(def_id) {
         bug!("Cannot create local mono-item for {:?}", def_id)
     }
-                true
-            }
-        }
-    };
+    return true;
 
     fn is_available_upstream_generic<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                                def_id: DefId,