Check the invariant for `principal` inside the method

5 files changed, 11 insertions, 14 deletions

diff --git a/src/librustc/ty/error.rs b/src/librustc/ty/error.rs
index 3123f0fbe31..d886d5ed204 100644
--- a/src/librustc/ty/error.rs
+++ b/src/librustc/ty/error.rs
@@ -208,8 +208,7 @@ impl<'a, 'gcx, 'lcx, 'tcx> ty::TyS<'tcx> {
             ty::FnDef(..) => "fn item".into(),
             ty::FnPtr(_) => "fn pointer".into(),
             ty::Dynamic(ref inner, ..) => {
-                inner.principal().map_or_else(|| "trait".into(),
-                    |p| format!("trait {}", tcx.item_path_str(p.def_id())).into())
+                format!("trait {}", tcx.item_path_str(inner.principal().def_id())).into()
             }
             ty::Closure(..) => "closure".into(),
             ty::Generator(..) => "generator".into(),
diff --git a/src/librustc/ty/fast_reject.rs b/src/librustc/ty/fast_reject.rs
index 0f68e7aba4d..e6aaf8b1bb2 100644
--- a/src/librustc/ty/fast_reject.rs
+++ b/src/librustc/ty/fast_reject.rs
@@ -78,7 +78,7 @@ pub fn simplify_type<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
         ty::Array(..) | ty::Slice(_) => Some(ArraySimplifiedType),
         ty::RawPtr(_) => Some(PtrSimplifiedType),
         ty::Dynamic(ref trait_info, ..) => {
-            trait_info.principal().map(|p| TraitSimplifiedType(p.def_id()))
+            Some(TraitSimplifiedType(trait_info.principal().def_id()))
         }
         ty::Ref(_, ty, _) => {
             // since we introduce auto-refs during method lookup, we
diff --git a/src/librustc/ty/item_path.rs b/src/librustc/ty/item_path.rs
index ab081324036..c4a25971da3 100644
--- a/src/librustc/ty/item_path.rs
+++ b/src/librustc/ty/item_path.rs
@@ -436,7 +436,7 @@ pub fn characteristic_def_id_of_type(ty: Ty<'_>) -> Option<DefId> {
     match ty.sty {
         ty::Adt(adt_def, _) => Some(adt_def.did),
 
-        ty::Dynamic(data, ..) => data.principal().map(|p| p.def_id()),
+        ty::Dynamic(data, ..) => Some(data.principal().def_id()),
 
         ty::Array(subty, _) |
         ty::Slice(subty) => characteristic_def_id_of_type(subty),
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 145c122e75d..cc6e6b2861e 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -559,10 +559,10 @@ impl<'a, 'gcx, 'tcx> Binder<ExistentialPredicate<'tcx>> {
 impl<'tcx> serialize::UseSpecializedDecodable for &'tcx List<ExistentialPredicate<'tcx>> {}
 
 impl<'tcx> List<ExistentialPredicate<'tcx>> {
-    pub fn principal(&self) -> Option<ExistentialTraitRef<'tcx>> {
-        match self.get(0) {
-            Some(&ExistentialPredicate::Trait(tr)) => Some(tr),
-            _ => None,
+    pub fn principal(&self) -> ExistentialTraitRef<'tcx> {
+        match self[0] {
+            ExistentialPredicate::Trait(tr) => tr,
+            other => bug!("first predicate is {:?}", other),
         }
     }
 
@@ -589,8 +589,8 @@ impl<'tcx> List<ExistentialPredicate<'tcx>> {
 }
 
 impl<'tcx> Binder<&'tcx List<ExistentialPredicate<'tcx>>> {
-    pub fn principal(&self) -> Option<PolyExistentialTraitRef<'tcx>> {
-        self.skip_binder().principal().map(Binder::bind)
+    pub fn principal(&self) -> PolyExistentialTraitRef<'tcx> {
+        Binder::bind(self.skip_binder().principal())
     }
 
     #[inline]
@@ -1825,9 +1825,7 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
             }
             Dynamic(ref obj, region) => {
                 let mut v = vec![region];
-                if let Some(p) = obj.principal() {
-                    v.extend(p.skip_binder().substs.regions());
-                }
+                v.extend(obj.principal().skip_binder().substs.regions());
                 v
             }
             Adt(_, substs) | Opaque(_, substs) => {
diff --git a/src/librustc/ty/wf.rs b/src/librustc/ty/wf.rs
index 7af838845cd..27747970f76 100644
--- a/src/librustc/ty/wf.rs
+++ b/src/librustc/ty/wf.rs
@@ -387,7 +387,7 @@ impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 
                     let cause = self.cause(traits::MiscObligation);
                     let component_traits =
-                        data.auto_traits().chain(data.principal().map(|p| p.def_id()));
+                        data.auto_traits().chain(once(data.principal().def_id()));
                     self.out.extend(
                         component_traits.map(|did| traits::Obligation::new(
                             cause.clone(),