add universes to type inference variables

src/librustc/infer/anon_types/mod.rs

@@ -600,7 +600,8 @@ fn fold_anon_ty(
             return anon_defn.concrete_ty;
         }
         let span = tcx.def_span(def_id);
-        let ty_var = infcx.next_ty_var(TypeVariableOrigin::TypeInference(span));
+        let ty_var = infcx.next_ty_var(ty::UniverseIndex::ROOT,
+                                       TypeVariableOrigin::TypeInference(span));
 
         let predicates_of = tcx.predicates_of(def_id);
         let bounds = predicates_of.instantiate(tcx, substs);

src/librustc/infer/combine.rs

@@ -407,7 +407,7 @@ fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
                             drop(variables);
                             self.relate(&u, &u)
                         }
-                        TypeVariableValue::Unknown { .. } => {
+                        TypeVariableValue::Unknown { universe } => {
                             match self.ambient_variance {
                                 // Invariant: no need to make a fresh type variable.
                                 ty::Invariant => return Ok(t),
@@ -424,7 +424,7 @@ fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
                             }
 
                             let origin = *variables.var_origin(vid);
-                            let new_var_id = variables.new_var(false, origin);
+                            let new_var_id = variables.new_var(universe, false, origin);
                             let u = self.tcx().mk_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}",
                                    vid, u);

src/librustc/infer/fudge.rs

@@ -141,7 +141,11 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                         // This variable was created during the
                         // fudging. Recreate it with a fresh variable
                         // here.
-                        self.infcx.next_ty_var(origin)
+                        //
+                        // The ROOT universe is fine because we only
+                        // ever invoke this routine at the
+                        // "item-level" of inference.
+                        self.infcx.next_ty_var(ty::UniverseIndex::ROOT, origin)
                     }
                 }
             }

src/librustc/infer/lattice.rs

@@ -88,13 +88,17 @@ pub fn super_lattice_tys<'a, 'gcx, 'tcx, L>(this: &mut L,
         // is (e.g.) `Box<i32>`. A more obvious solution might be to
         // iterate on the subtype obligations that are returned, but I
         // think this suffices. -nmatsakis
-        (&ty::TyInfer(TyVar(..)), _) => {
-            let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span));
+        (&ty::TyInfer(TyVar(a_vid)), _) => {
+            let universe = infcx.type_variables.borrow_mut().probe(a_vid).universe().unwrap();
+            let v = infcx.next_ty_var(universe,
+                                      TypeVariableOrigin::LatticeVariable(this.cause().span));
             this.relate_bound(v, b, a)?;
             Ok(v)
         }
-        (_, &ty::TyInfer(TyVar(..))) => {
-            let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span));
+        (_, &ty::TyInfer(TyVar(b_vid))) => {
+            let universe = infcx.type_variables.borrow_mut().probe(b_vid).universe().unwrap();
+            let v = infcx.next_ty_var(universe,
+                                      TypeVariableOrigin::LatticeVariable(this.cause().span));
             this.relate_bound(v, a, b)?;
             Ok(v)
         }

src/librustc/infer/mod.rs

@@ -1015,18 +1015,22 @@ pub fn region_outlives_predicate(&self,
         })
     }
 
-    pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {
+    pub fn next_ty_var_id(&self,
+                          universe: ty::UniverseIndex,
+                          diverging: bool,
+                          origin: TypeVariableOrigin)
+                          -> TyVid {
         self.type_variables
             .borrow_mut()
-            .new_var(diverging, origin)
+            .new_var(universe, diverging, origin)
     }
 
-    pub fn next_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
-        self.tcx.mk_var(self.next_ty_var_id(false, origin))
+    pub fn next_ty_var(&self, universe: ty::UniverseIndex, origin: TypeVariableOrigin) -> Ty<'tcx> {
+        self.tcx.mk_var(self.next_ty_var_id(universe, false, origin))
     }
 
-    pub fn next_diverging_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
-        self.tcx.mk_var(self.next_ty_var_id(true, origin))
+    pub fn next_diverging_ty_var(&self, universe: ty::UniverseIndex, origin: TypeVariableOrigin) -> Ty<'tcx> {
+        self.tcx.mk_var(self.next_ty_var_id(universe, true, origin))
     }
 
     pub fn next_int_var_id(&self) -> IntVid {
@@ -1081,12 +1085,14 @@ pub fn region_var_for_def(&self,
     /// use an inference variable for `C` with `[T, U]`
     /// as the substitutions for the default, `(T, U)`.
     pub fn type_var_for_def(&self,
+                            universe: ty::UniverseIndex,
                             span: Span,
                             def: &ty::TypeParameterDef)
                             -> Ty<'tcx> {
         let ty_var_id = self.type_variables
                             .borrow_mut()
-                            .new_var(false,
+                            .new_var(universe,
+                                     false,
                                      TypeVariableOrigin::TypeParameterDefinition(span, def.name));
 
         self.tcx.mk_var(ty_var_id)
@@ -1095,13 +1101,14 @@ pub fn type_var_for_def(&self,
     /// Given a set of generics defined on a type or impl, returns a substitution mapping each
     /// type/region parameter to a fresh inference variable.
     pub fn fresh_substs_for_item(&self,
+                                 universe: ty::UniverseIndex,
                                  span: Span,
                                  def_id: DefId)
                                  -> &'tcx Substs<'tcx> {
         Substs::for_item(self.tcx, def_id, |def, _| {
             self.region_var_for_def(span, def)
         }, |def, _| {
-            self.type_var_for_def(span, def)
+            self.type_var_for_def(universe, span, def)
         })
     }
 

src/librustc/infer/type_variable.rs

@@ -12,6 +12,7 @@
 use syntax_pos::Span;
 use ty::{self, Ty};
 
+use std::cmp;
 use std::marker::PhantomData;
 use std::u32;
 use rustc_data_structures::fx::FxHashMap;
@@ -81,10 +82,18 @@ struct TypeVariableData {
 #[derive(Copy, Clone, Debug)]
 pub enum TypeVariableValue<'tcx> {
     Known { value: Ty<'tcx> },
-    Unknown,
+    Unknown { universe: ty::UniverseIndex },
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum ProbeTyValue<'tcx> {
+    Ty(Ty<'tcx>),
+    Vid(ty::TyVid),
 }
 
 impl<'tcx> TypeVariableValue<'tcx> {
+    /// If this value is known, returns the type it is known to be.
+    /// Otherwise, `None`.
     pub fn known(&self) -> Option<Ty<'tcx>> {
         match *self {
             TypeVariableValue::Unknown { .. } => None,
@@ -92,6 +101,14 @@ pub fn known(&self) -> Option<Ty<'tcx>> {
         }
     }
 
+    /// If this value is unknown, returns the universe, otherwise `None`.
+    pub fn universe(&self) -> Option<ty::UniverseIndex> {
+        match *self {
+            TypeVariableValue::Unknown { universe } => Some(universe),
+            TypeVariableValue::Known { .. } => None,
+        }
+    }
+
     pub fn is_unknown(&self) -> bool {
         match *self {
             TypeVariableValue::Unknown { .. } => true,
@@ -178,10 +195,11 @@ pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
     ///   The code in this module doesn't care, but it can be useful
     ///   for improving error messages.
     pub fn new_var(&mut self,
+                   universe: ty::UniverseIndex,
                    diverging: bool,
                    origin: TypeVariableOrigin)
                    -> ty::TyVid {
-        let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown);
+        let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
 
         let sub_key = self.sub_relations.new_key(());
         assert_eq!(eq_key.vid, sub_key);
@@ -388,8 +406,12 @@ fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
             (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
             (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
 
-            // If both sides are *unknown*, it hardly matters, does it?
-            (&TypeVariableValue::Unknown, &TypeVariableValue::Unknown) => Ok(*value1),
+            // If both sides are unknown, we need to pick the most restrictive universe.
+            (&TypeVariableValue::Unknown { universe: universe1 },
+             &TypeVariableValue::Unknown { universe: universe2 }) => {
+                let universe = cmp::min(universe1, universe2);
+                Ok(TypeVariableValue::Unknown { universe })
+            }
         }
     }
 }

src/librustc/traits/coherence.rs

@@ -92,7 +92,9 @@ fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, '
                                        -> ty::ImplHeader<'tcx>
 {
     let tcx = selcx.tcx();
-    let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
+    let impl_substs = selcx.infcx().fresh_substs_for_item(param_env.universe,
+                                                          DUMMY_SP,
+                                                          impl_def_id);
 
     let header = ty::ImplHeader {
         impl_def_id,

src/librustc/traits/error_reporting.rs

@@ -292,7 +292,9 @@ fn impl_similar_to(&self,
 
         self.tcx.for_each_relevant_impl(
             trait_ref.def_id, trait_self_ty, |def_id| {
-                let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
+                let impl_substs = self.fresh_substs_for_item(param_env.universe,
+                                                             obligation.cause.span,
+                                                             def_id);
                 let impl_trait_ref = tcx
                     .impl_trait_ref(def_id)
                     .unwrap()
@@ -1194,6 +1196,7 @@ fn predicate_can_apply(&self,
                            -> bool {
         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
+            param_env: ty::ParamEnv<'tcx>,
             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
         }
 
@@ -1203,9 +1206,14 @@ fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
                     let infcx = self.infcx;
-                    self.var_map.entry(ty).or_insert_with(||
-                        infcx.next_ty_var(
-                            TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
+                    let param_env = self.param_env;
+                    self.var_map
+                        .entry(ty)
+                        .or_insert_with(|| {
+                            let origin = TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP,
+                                                                                     name);
+                            infcx.next_ty_var(param_env.universe, origin)
+                        })
                 } else {
                     ty.super_fold_with(self)
                 }
@@ -1217,6 +1225,7 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 
             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
                 infcx: self,
+                param_env,
                 var_map: FxHashMap()
             });
 

src/librustc/traits/project.rs

@@ -469,6 +469,7 @@ pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
             let tcx = selcx.infcx().tcx;
             let def_id = projection_ty.item_def_id;
             let ty_var = selcx.infcx().next_ty_var(
+                param_env.universe,
                 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
             let projection = ty::Binder(ty::ProjectionPredicate {
                 projection_ty,
@@ -789,6 +790,7 @@ fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tc
     let tcx = selcx.infcx().tcx;
     let def_id = projection_ty.item_def_id;
     let new_value = selcx.infcx().next_ty_var(
+        param_env.universe,
         TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
     Normalized {
         value: new_value,

src/librustc/traits/select.rs

@@ -3111,7 +3111,8 @@ fn match_impl(&mut self,
             snapshot);
         let skol_obligation_trait_ref = skol_obligation.trait_ref;
 
-        let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
+        let impl_substs = self.infcx.fresh_substs_for_item(obligation.param_env.universe,
+                                                           obligation.cause.span,
                                                            impl_def_id);
 
         let impl_trait_ref = impl_trait_ref.subst(self.tcx(),

src/librustc/traits/specialize/mod.rs

@@ -221,7 +221,7 @@ fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
                                        target_impl: DefId)
                                        -> Result<&'tcx Substs<'tcx>, ()> {
     let selcx = &mut SelectionContext::new(&infcx);
-    let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
+    let target_substs = infcx.fresh_substs_for_item(param_env.universe, DUMMY_SP, target_impl);
     let (target_trait_ref, mut obligations) = impl_trait_ref_and_oblig(selcx,
                                                                        param_env,
                                                                        target_impl,

src/librustc_typeck/check/_match.rs

@@ -329,6 +329,7 @@ pub fn check_pat_walk(
                 let element_tys_iter = (0..max_len).map(|_| self.next_ty_var(
                     // FIXME: MiscVariable for now, obtaining the span and name information
                     //       from all tuple elements isn't trivial.
+                    ty::UniverseIndex::ROOT,
                     TypeVariableOrigin::TypeInference(pat.span)));
                 let element_tys = tcx.mk_type_list(element_tys_iter);
                 let pat_ty = tcx.mk_ty(ty::TyTuple(element_tys, false));
@@ -339,7 +340,8 @@ pub fn check_pat_walk(
                 pat_ty
             }
             PatKind::Box(ref inner) => {
-                let inner_ty = self.next_ty_var(TypeVariableOrigin::TypeInference(inner.span));
+                let inner_ty = self.next_ty_var(ty::UniverseIndex::ROOT,
+                                                TypeVariableOrigin::TypeInference(inner.span));
                 let uniq_ty = tcx.mk_box(inner_ty);
 
                 if self.check_dereferencable(pat.span, expected, &inner) {
@@ -372,6 +374,7 @@ pub fn check_pat_walk(
                         }
                         _ => {
                             let inner_ty = self.next_ty_var(
+                                ty::UniverseIndex::ROOT,
                                 TypeVariableOrigin::TypeInference(inner.span));
                             let mt = ty::TypeAndMut { ty: inner_ty, mutbl: mutbl };
                             let region = self.next_region_var(infer::PatternRegion(pat.span));
@@ -630,7 +633,8 @@ pub fn check_match(&self,
             // ...but otherwise we want to use any supertype of the
             // discriminant. This is sort of a workaround, see note (*) in
             // `check_pat` for some details.
-            discrim_ty = self.next_ty_var(TypeVariableOrigin::TypeInference(discrim.span));
+            discrim_ty = self.next_ty_var(ty::UniverseIndex::ROOT,
+                                          TypeVariableOrigin::TypeInference(discrim.span));
             self.check_expr_has_type_or_error(discrim, discrim_ty);
         };
 
@@ -691,7 +695,8 @@ pub fn check_match(&self,
                 // arm for inconsistent arms or to the whole match when a `()` type
                 // is required).
                 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_nil() => ety,
-                _ => self.next_ty_var(TypeVariableOrigin::MiscVariable(expr.span)),
+                _ => self.next_ty_var(ty::UniverseIndex::ROOT,
+                                      TypeVariableOrigin::MiscVariable(expr.span)),
             };
             CoerceMany::with_coercion_sites(coerce_first, arms)
         };

src/librustc_typeck/check/closure.rs

@@ -110,7 +110,8 @@ fn check_closure(
             |_, _| span_bug!(expr.span, "closure has region param"),
             |_, _| {
                 self.infcx
-                    .next_ty_var(TypeVariableOrigin::ClosureSynthetic(expr.span))
+                    .next_ty_var(ty::UniverseIndex::ROOT,
+                                 TypeVariableOrigin::ClosureSynthetic(expr.span))
             },
         );
         let substs = ty::ClosureSubsts { substs };

src/librustc_typeck/check/coercion.rs

@@ -177,6 +177,7 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
                 // micro-optimization: no need for this if `b` is
                 // already resolved in some way.
                 let diverging_ty = self.next_diverging_ty_var(
+                    ty::UniverseIndex::ROOT,
                     TypeVariableOrigin::AdjustmentType(self.cause.span));
                 self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
             } else {
@@ -510,7 +511,7 @@ fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tc
         // We only have the latter, so we use an inference variable
         // for the former and let type inference do the rest.
         let origin = TypeVariableOrigin::MiscVariable(self.cause.span);
-        let coerce_target = self.next_ty_var(origin);
+        let coerce_target = self.next_ty_var(ty::UniverseIndex::ROOT, origin);
         let mut coercion = self.unify_and(coerce_target, target, |target| {
             let unsize = Adjustment {
                 kind: Adjust::Unsize,

src/librustc_typeck/check/dropck.rs

@@ -90,7 +90,7 @@ fn ensure_drop_params_and_item_params_correspond<'a, 'tcx>(
 
         let drop_impl_span = tcx.def_span(drop_impl_did);
         let fresh_impl_substs =
-            infcx.fresh_substs_for_item(drop_impl_span, drop_impl_did);
+            infcx.fresh_substs_for_item(ty::UniverseIndex::ROOT, drop_impl_span, drop_impl_did);
         let fresh_impl_self_ty = drop_impl_ty.subst(tcx, fresh_impl_substs);
 
         let cause = &ObligationCause::misc(drop_impl_span, drop_impl_node_id);

src/librustc_typeck/check/method/confirm.rs

@@ -259,7 +259,7 @@ fn fresh_receiver_substs(&mut self,
                 // the process we will unify the transformed-self-type
                 // of the method with the actual type in order to
                 // unify some of these variables.
-                self.fresh_substs_for_item(self.span, trait_def_id)
+                self.fresh_substs_for_item(ty::UniverseIndex::ROOT, self.span, trait_def_id)
             }
 
             probe::WhereClausePick(ref poly_trait_ref) => {
@@ -336,7 +336,7 @@ fn instantiate_method_substs(&mut self,
             {
                 self.to_ty(ast_ty)
             } else {
-                self.type_var_for_def(self.span, def)
+                self.type_var_for_def(ty::UniverseIndex::ROOT, self.span, def)
             }
         })
     }

src/librustc_typeck/check/method/mod.rs

@@ -255,7 +255,7 @@ pub fn lookup_method_in_trait(&self,
             } else if let Some(ref input_types) = opt_input_types {
                 input_types[def.index as usize - 1]
             } else {
-                self.type_var_for_def(span, def)
+                self.type_var_for_def(ty::UniverseIndex::ROOT, span, def)
             }
         });
 

src/librustc_typeck/check/method/probe.rs

@@ -730,7 +730,9 @@ pub fn matches_return_type(&self,
             Def::Method(def_id) => {
                 let fty = self.tcx.fn_sig(def_id);
                 self.probe(|_| {
-                    let substs = self.fresh_substs_for_item(self.span, method.def_id);
+                    let substs = self.fresh_substs_for_item(ty::UniverseIndex::ROOT,
+                                                            self.span,
+                                                            method.def_id);
                     let fty = fty.subst(self.tcx, substs);
                     let (fty, _) = self.replace_late_bound_regions_with_fresh_var(
                         self.span, infer::FnCall, &fty);
@@ -1309,7 +1311,7 @@ fn xform_method_sig(&self,
                 if i < substs.len() {
                     substs.type_at(i)
                 } else {
-                    self.type_var_for_def(self.span, def)
+                    self.type_var_for_def(ty::UniverseIndex::ROOT, self.span, def)
                 }
             });
             xform_fn_sig.subst(self.tcx, substs)
@@ -1326,6 +1328,7 @@ fn fresh_item_substs(&self, def_id: DefId) -> &'tcx Substs<'tcx> {
                          def_id,
                          |_, _| self.tcx.types.re_erased,
                          |_, _| self.next_ty_var(
+                             ty::UniverseIndex::ROOT,
                              TypeVariableOrigin::SubstitutionPlaceholder(
                                  self.tcx.def_span(def_id))))
     }

src/librustc_typeck/check/method/suggest.rs

@@ -54,7 +54,8 @@ fn is_fn_ty(&self, ty: &Ty<'tcx>, span: Span) -> bool {
                 self.autoderef(span, ty).any(|(ty, _)| {
                     self.probe(|_| {
                         let fn_once_substs = tcx.mk_substs_trait(ty,
-                            &[self.next_ty_var(TypeVariableOrigin::MiscVariable(span))]);
+                            &[self.next_ty_var(ty::UniverseIndex::ROOT,
+                                               TypeVariableOrigin::MiscVariable(span))]);
                         let trait_ref = ty::TraitRef::new(fn_once, fn_once_substs);
                         let poly_trait_ref = trait_ref.to_poly_trait_ref();
                         let obligation =

src/librustc_typeck/check/mod.rs

@@ -362,7 +362,8 @@ fn only_has_type(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Option<Ty<'tcx>> {
     /// hard constraint exists, creates a fresh type variable.
     fn coercion_target_type(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>, span: Span) -> Ty<'tcx> {
         self.only_has_type(fcx)
-            .unwrap_or_else(|| fcx.next_ty_var(TypeVariableOrigin::MiscVariable(span)))
+            .unwrap_or_else(|| fcx.next_ty_var(ty::UniverseIndex::ROOT,
+                                               TypeVariableOrigin::MiscVariable(span)))
     }
 }
 
@@ -921,7 +922,8 @@ fn assign(&mut self, span: Span, nid: ast::NodeId, ty_opt: Option<Ty<'tcx>>) ->
         match ty_opt {
             None => {
                 // infer the variable's type
-                let var_ty = self.fcx.next_ty_var(TypeVariableOrigin::TypeInference(span));
+                let var_ty = self.fcx.next_ty_var(ty::UniverseIndex::ROOT,
+                                                  TypeVariableOrigin::TypeInference(span));
                 self.fcx.locals.borrow_mut().insert(nid, var_ty);
                 var_ty
             }
@@ -1025,7 +1027,8 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     let span = body.value.span;
 
     if body.is_generator && can_be_generator.is_some() {
-        let yield_ty = fcx.next_ty_var(TypeVariableOrigin::TypeInference(span));
+        let yield_ty = fcx.next_ty_var(fcx.next_ty_var(ty::UniverseIndex::ROOT,
+                                                       TypeVariableOrigin::TypeInference(span)));
         fcx.require_type_is_sized(yield_ty, span, traits::SizedYieldType);
         fcx.yield_ty = Some(yield_ty);
     }
@@ -1058,7 +1061,8 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     // This ensures that all nested generators appear before the entry of this generator.
     // resolve_generator_interiors relies on this property.
     let gen_ty = if can_be_generator.is_some() && body.is_generator {
-        let witness = fcx.next_ty_var(TypeVariableOrigin::MiscVariable(span));
+        let witness = fcx.next_ty_var(ty::UniverseIndex::ROOT,
+                                      TypeVariableOrigin::MiscVariable(span));
         let interior = ty::GeneratorInterior {
             witness,
             movable: can_be_generator.unwrap() == hir::GeneratorMovability::Movable,
@@ -1096,6 +1100,7 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     let mut actual_return_ty = coercion.complete(&fcx);
     if actual_return_ty.is_never() {
         actual_return_ty = fcx.next_diverging_ty_var(
+            ty::UniverseIndex::ROOT,
             TypeVariableOrigin::DivergingFn(span));
     }
     fcx.demand_suptype(span, ret_ty, actual_return_ty);
@@ -1687,13 +1692,14 @@ fn re_infer(&self, span: Span, def: Option<&ty::RegionParameterDef>)
     }
 
     fn ty_infer(&self, span: Span) -> Ty<'tcx> {
-        self.next_ty_var(TypeVariableOrigin::TypeInference(span))
+        self.next_ty_var(ty::UniverseIndex::ROOT,
+                         TypeVariableOrigin::TypeInference(span))
     }
 
     fn ty_infer_for_def(&self,
                         ty_param_def: &ty::TypeParameterDef,
                         span: Span) -> Ty<'tcx> {
-        self.type_var_for_def(span, ty_param_def)
+        self.type_var_for_def(ty::UniverseIndex::ROOT, span, ty_param_def)
     }
 
     fn projected_ty_from_poly_trait_ref(&self,
@@ -2315,7 +2321,8 @@ fn try_index_step(&self,
             // If some lookup succeeds, write callee into table and extract index/element
             // type from the method signature.
             // If some lookup succeeded, install method in table
-            let input_ty = self.next_ty_var(TypeVariableOrigin::AutoDeref(base_expr.span));
+            let input_ty = self.next_ty_var(ty::UniverseIndex::ROOT,
+                                            TypeVariableOrigin::AutoDeref(base_expr.span));
             let method = self.try_overloaded_place_op(
                 expr.span, self_ty, &[input_ty], needs, PlaceOp::Index);
 
@@ -2754,6 +2761,7 @@ fn check_expr_meets_expectation_or_error(&self,
             assert!(!self.tables.borrow().adjustments().contains_key(expr.hir_id),
                     "expression with never type wound up being adjusted");
             let adj_ty = self.next_diverging_ty_var(
+                ty::UniverseIndex::ROOT,
                 TypeVariableOrigin::AdjustmentType(expr.span));
             self.apply_adjustments(expr, vec![Adjustment {
                 kind: Adjust::NeverToAny,
@@ -2831,7 +2839,7 @@ pub fn impl_self_ty(&self,
         let ity = self.tcx.type_of(did);
         debug!("impl_self_ty: ity={:?}", ity);
 
-        let substs = self.fresh_substs_for_item(span, did);
+        let substs = self.fresh_substs_for_item(ty::UniverseIndex::ROOT, span, did);
         let substd_ty = self.instantiate_type_scheme(span, &substs, &ity);
 
         TypeAndSubsts { substs: substs, ty: substd_ty }
@@ -3971,7 +3979,8 @@ fn check_expr_kind(&self,
 
               let element_ty = if !args.is_empty() {
                   let coerce_to = uty.unwrap_or_else(
-                      || self.next_ty_var(TypeVariableOrigin::TypeInference(expr.span)));
+                      || self.next_ty_var(ty::UniverseIndex::ROOT,
+                                          TypeVariableOrigin::TypeInference(expr.span)));
                   let mut coerce = CoerceMany::with_coercion_sites(coerce_to, args);
                   assert_eq!(self.diverges.get(), Diverges::Maybe);
                   for e in args {
@@ -3981,7 +3990,8 @@ fn check_expr_kind(&self,
                   }
                   coerce.complete(self)
               } else {
-                  self.next_ty_var(TypeVariableOrigin::TypeInference(expr.span))
+                  self.next_ty_var(ty::UniverseIndex::ROOT,
+                                   TypeVariableOrigin::TypeInference(expr.span))
               };
               tcx.mk_array(element_ty, args.len() as u64)
           }
@@ -4011,7 +4021,8 @@ fn check_expr_kind(&self,
                     (uty, uty)
                 }
                 None => {
-                    let t: Ty = self.next_ty_var(TypeVariableOrigin::MiscVariable(element.span));
+                    let t: Ty = self.next_ty_var(ty::UniverseIndex::ROOT,
+                                                 TypeVariableOrigin::MiscVariable(element.span));
                     let element_ty = self.check_expr_has_type_or_error(&element, t);
                     (element_ty, t)
                 }
@@ -4792,7 +4803,7 @@ pub fn instantiate_value_path(&self,
                 // Handle Self first, so we can adjust the index to match the AST.
                 if has_self && i == 0 {
                     return opt_self_ty.unwrap_or_else(|| {
-                        self.type_var_for_def(span, def)
+                        self.type_var_for_def(ty::UniverseIndex::ROOT, span, def)
                     });
                 }
                 i -= has_self as usize;
@@ -4825,7 +4836,7 @@ pub fn instantiate_value_path(&self,
                 // This can also be reached in some error cases:
                 // We prefer to use inference variables instead of
                 // TyError to let type inference recover somewhat.
-                self.type_var_for_def(span, def)
+                self.type_var_for_def(ty::UniverseIndex::ROOT, span, def)
             }
         });
 

src/librustc_typeck/check/op.rs

@@ -174,8 +174,10 @@ fn check_overloaded_binop(&self,
         // trait matching creating lifetime constraints that are too strict.
         // E.g. adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
         // in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
-        let lhs_ty = self.check_expr_coercable_to_type_with_needs(lhs_expr,
-            self.next_ty_var(TypeVariableOrigin::MiscVariable(lhs_expr.span)),
+        let lhs_ty = self.check_expr_coercable_to_type_with_needs(
+            lhs_expr,
+            self.next_ty_var(ty::UniverseIndex::ROOT,
+                             TypeVariableOrigin::MiscVariable(lhs_expr.span)),
             lhs_needs);
         let lhs_ty = self.resolve_type_vars_with_obligations(lhs_ty);
 
@@ -185,7 +187,8 @@ fn check_overloaded_binop(&self,
         // using this variable as the expected type, which sometimes lets
         // us do better coercions than we would be able to do otherwise,
         // particularly for things like `String + &String`.
-        let rhs_ty_var = self.next_ty_var(TypeVariableOrigin::MiscVariable(rhs_expr.span));
+        let rhs_ty_var = self.next_ty_var(ty::UniverseIndex::ROOT,
+                         TypeVariableOrigin::MiscVariable(rhs_expr.span));
 
         let result = self.lookup_op_method(lhs_ty, &[rhs_ty_var], Op::Binary(op, is_assign));