make impl_subject more readable

Author: kylematsuda

compiler/rustc_middle/src/hir/mod.rs

@@ -105,12 +105,10 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn impl_subject(self, def_id: DefId) -> EarlyBinder<ImplSubject<'tcx>> {
-        EarlyBinder(
-            self.impl_trait_ref(def_id)
-                .map(|t| t.subst_identity())
-                .map(ImplSubject::Trait)
-                .unwrap_or_else(|| ImplSubject::Inherent(self.type_of(def_id).subst_identity())),
-        )
+        match self.impl_trait_ref(def_id) {
+            Some(t) => t.map_bound(ImplSubject::Trait),
+            None => self.type_of(def_id).map_bound(ImplSubject::Inherent),
+        }
     }
 }