add tuple_fields convenience method and use it in a few places

src/librustc/ty/mod.rs

@@ -2565,6 +2565,8 @@ impl<'tcx> AdtDef {
 }
 
 impl<'tcx> FieldDef {
+    /// Returns the type of this field. The `subst` is typically obtained
+    /// via the second field of `TyKind::AdtDef`.
     pub fn ty(&self, tcx: TyCtxt<'tcx>, subst: SubstsRef<'tcx>) -> Ty<'tcx> {
         tcx.type_of(self.did).subst(tcx, subst)
     }

src/librustc/ty/sty.rs

@@ -171,6 +171,7 @@ pub enum TyKind<'tcx> {
     Never,
 
     /// A tuple type. For example, `(i32, bool)`.
+    /// Use `TyS::tuple_fields` to iterate over the field types.
     Tuple(SubstsRef<'tcx>),
 
     /// The projection of an associated type. For example,
@@ -1723,8 +1724,8 @@ impl<'tcx> TyS<'tcx> {
                     })
                 })
             }
-            ty::Tuple(tys) => tys.iter().any(|ty| {
-                ty.expect_ty().conservative_is_privately_uninhabited(tcx)
+            ty::Tuple(..) => self.tuple_fields().any(|ty| {
+                ty.conservative_is_privately_uninhabited(tcx)
             }),
             ty::Array(ty, len) => {
                 match len.try_eval_usize(tcx, ParamEnv::empty()) {
@@ -2103,6 +2104,15 @@ impl<'tcx> TyS<'tcx> {
         }
     }
 
+    /// Iterates over tuple fields.
+    /// Panics when called on anything but a tuple.
+    pub fn tuple_fields(&self) -> impl DoubleEndedIterator<Item=Ty<'tcx>> {
+        match self.sty {
+            Tuple(substs) => substs.iter().map(|field| field.expect_ty()),
+            _ => bug!("tuple_fields called on non-tuple"),
+        }
+    }
+
     /// If the type contains variants, returns the valid range of variant indices.
     /// FIXME This requires the optimized MIR in the case of generators.
     #[inline]

src/librustc/ty/util.rs

@@ -845,15 +845,15 @@ impl<'tcx> ty::TyS<'tcx> {
             ty: Ty<'tcx>,
         ) -> Representability {
             match ty.sty {
-                Tuple(ref ts) => {
+                Tuple(..) => {
                     // Find non representable
-                    fold_repr(ts.iter().map(|ty| {
+                    fold_repr(ty.tuple_fields().map(|ty| {
                         is_type_structurally_recursive(
                             tcx,
                             sp,
                             seen,
                             representable_cache,
-                            ty.expect_ty(),
+                            ty,
                         )
                     }))
                 }
@@ -1095,7 +1095,7 @@ fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>
         // state transformation pass
         ty::Generator(..) => true,
 
-        ty::Tuple(ref tys) => tys.iter().map(|k| k.expect_ty()).any(needs_drop),
+        ty::Tuple(..) => ty.tuple_fields().any(needs_drop),
 
         // unions don't have destructors because of the child types,
         // only if they manually implement `Drop` (handled above).

src/librustc/ty/walk.rs

@@ -119,8 +119,8 @@ fn push_subtypes<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent_ty: Ty<'tcx>) {
         ty::GeneratorWitness(ts) => {
             stack.extend(ts.skip_binder().iter().cloned().rev());
         }
-        ty::Tuple(ts) => {
-            stack.extend(ts.iter().map(|k| k.expect_ty()).rev());
+        ty::Tuple(..) => {
+            stack.extend(parent_ty.tuple_fields().rev());
         }
         ty::FnDef(_, substs) => {
             stack.extend(substs.types().rev());

src/librustc_lint/builtin.rs

@@ -1903,14 +1903,9 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for InvalidValue {
                         _ => true, // Conservative fallback for multi-variant enum.
                     }
                 }
-                Tuple(substs) => {
+                Tuple(..) => {
                     // Proceed recursively, check all fields.
-                    substs.iter().all(|field| {
-                        ty_maybe_allows_zero_init(
-                            tcx,
-                            field.expect_ty(),
-                        )
-                    })
+                    ty.tuple_fields().all(|field| ty_maybe_allows_zero_init(tcx, field))
                 }
                 // FIXME: Would be nice to also warn for `NonNull`/`NonZero*`.
                 // Conservative fallback.

src/librustc_mir/shim.rs

@@ -324,7 +324,7 @@ fn build_clone_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, self_ty: Ty<'tcx>) -
                 substs.upvar_tys(def_id, tcx)
             )
         }
-        ty::Tuple(tys) => builder.tuple_like_shim(dest, src, tys.iter().map(|k| k.expect_ty())),
+        ty::Tuple(..) => builder.tuple_like_shim(dest, src, self_ty.tuple_fields()),
         _ => {
             bug!("clone shim for `{:?}` which is not `Copy` and is not an aggregate", self_ty)
         }

src/librustc_mir/util/elaborate_drops.rs

@@ -804,8 +804,8 @@ where
                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
                 self.open_drop_for_tuple(&tys)
             }
-            ty::Tuple(tys) => {
-                let tys: Vec<_> = tys.iter().map(|k| k.expect_ty()).collect();
+            ty::Tuple(..) => {
+                let tys: Vec<_> = ty.tuple_fields().collect();
                 self.open_drop_for_tuple(&tys)
             }
             ty::Adt(def, substs) => {