use ty::TraitRef::identity where possible

Co-authored-by: Tyler Mandry <tmandry@gmail.com>

Author: nikomatsakis

src/librustc/traits/object_safety.rs

@@ -23,7 +23,6 @@ use hir::def_id::DefId;
 use lint;
 use traits;
 use ty::{self, Ty, TyCtxt, TypeFoldable};
-use ty::subst::Substs;
 use ty::util::ExplicitSelf;
 use std::borrow::Cow;
 use syntax::ast;
@@ -173,10 +172,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         trait_def_id: DefId,
         supertraits_only: bool) -> bool
     {
-        let trait_ref = ty::Binder::dummy(ty::TraitRef {
-            def_id: trait_def_id,
-            substs: Substs::identity_for_item(self, trait_def_id)
-        });
+        let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id));
         let predicates = if supertraits_only {
             self.super_predicates_of(trait_def_id)
         } else {
@@ -391,10 +387,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
                     // Compute supertraits of current trait lazily.
                     if supertraits.is_none() {
-                        let trait_ref = ty::Binder::bind(ty::TraitRef {
-                            def_id: trait_def_id,
-                            substs: Substs::identity_for_item(self, trait_def_id)
-                        });
+                        let trait_ref = ty::Binder::bind(
+                            ty::TraitRef::identity(self, trait_def_id),
+                        );
                         supertraits = Some(traits::supertraits(self, trait_ref).collect());
                     }
 

src/librustc/ty/sty.rs

@@ -614,6 +614,15 @@ impl<'tcx> TraitRef<'tcx> {
         TraitRef { def_id: def_id, substs: substs }
     }
 
+    /// Returns a TraitRef of the form `P0: Foo<P1..Pn>` where `Pi`
+    /// are the parameters defined on trait.
+    pub fn identity<'a, 'gcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>, def_id: DefId) -> TraitRef<'tcx> {
+        TraitRef {
+            def_id,
+            substs: Substs::identity_for_item(tcx, def_id),
+        }
+    }
+
     pub fn self_ty(&self) -> Ty<'tcx> {
         self.substs.type_at(0)
     }

src/librustc_traits/lowering.rs

@@ -15,7 +15,6 @@ use rustc::hir::{self, ImplPolarity};
 use rustc::traits::{Clause, Clauses, DomainGoal, Goal, PolyDomainGoal, ProgramClause,
                     WhereClause, FromEnv, WellFormed};
 use rustc::ty::query::Providers;
-use rustc::ty::subst::Substs;
 use rustc::ty::{self, Slice, TyCtxt};
 use rustc_data_structures::fx::FxHashSet;
 use std::mem;
@@ -225,10 +224,7 @@ fn program_clauses_for_trait<'a, 'tcx>(
 
     // `Self: Trait<P1..Pn>`
     let trait_pred = ty::TraitPredicate {
-        trait_ref: ty::TraitRef {
-            def_id,
-            substs: Substs::identity_for_item(tcx, def_id),
-        },
+        trait_ref: ty::TraitRef::identity(tcx, def_id),
     };
 
     // `Implemented(Self: Trait<P1..Pn>)`

src/librustc_typeck/collect.rs

@@ -274,10 +274,9 @@ fn type_param_predicates<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 ItemTrait(_, _, ref generics, ..) => {
                     // Implied `Self: Trait` and supertrait bounds.
                     if param_id == item_node_id {
-                        result.predicates.push(ty::TraitRef {
-                            def_id: item_def_id,
-                            substs: Substs::identity_for_item(tcx, item_def_id)
-                        }.to_predicate());
+                        result.predicates.push(
+                            ty::TraitRef::identity(tcx, item_def_id).to_predicate()
+                        );
                     }
                     generics
                 }
@@ -1359,10 +1358,7 @@ pub fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 ItemUnion(_, ref generics) => generics,
 
                 ItemTrait(_, _, ref generics, .., ref items) => {
-                    is_trait = Some((ty::TraitRef {
-                        def_id,
-                        substs: Substs::identity_for_item(tcx, def_id)
-                    }, items));
+                    is_trait = Some((ty::TraitRef::identity(tcx, def_id), items));
                     generics
                 }
                 ItemExistential(ref exist_ty) => {