create just one subuniverse per binder -- no need for more, really

src/librustc/infer/higher_ranked/mod.rs

@@ -591,9 +591,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                            -> (T, PlaceholderMap<'tcx>)
         where T : TypeFoldable<'tcx>
     {
+        let new_universe = self.create_subuniverse();
+
         let (result, map) = self.tcx.replace_late_bound_regions(binder, |br| {
-            self.universe.set(self.universe().subuniverse());
-            self.tcx.mk_region(ty::RePlaceholder(self.universe(), br))
+            self.tcx.mk_region(ty::RePlaceholder(new_universe, br))
         });
 
         debug!("skolemize_bound_regions(binder={:?}, result={:?}, map={:?})",
@@ -795,7 +796,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         let skol_regions: FxHashSet<_> = placeholder_map.values().cloned().collect();
         self.borrow_region_constraints()
             .pop_placeholders(
-                self.universe(),
                 &skol_regions,
                 &snapshot.region_constraints_snapshot,
             );

src/librustc/infer/region_constraints/mod.rs

@@ -10,19 +10,19 @@
 
 //! See README.md
 
-use self::UndoLogEntry::*;
 use self::CombineMapType::*;
+use self::UndoLogEntry::*;
 
-use super::{MiscVariable, RegionVariableOrigin, SubregionOrigin};
 use super::unify_key;
+use super::{MiscVariable, RegionVariableOrigin, SubregionOrigin};
 
-use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_data_structures::unify as ut;
-use ty::{self, Ty, TyCtxt};
-use ty::{Region, RegionVid};
 use ty::ReStatic;
+use ty::{self, Ty, TyCtxt};
 use ty::{BrFresh, ReLateBound, ReVar};
+use ty::{Region, RegionVid};
 
 use std::collections::BTreeMap;
 use std::{cmp, fmt, mem, u32};
@@ -495,13 +495,12 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         }
     }
 
-    pub fn new_region_var(&mut self,
-                          universe: ty::UniverseIndex,
-                          origin: RegionVariableOrigin) -> RegionVid {
-        let vid = self.var_infos.push(RegionVariableInfo {
-            origin,
-            universe,
-        });
+    pub fn new_region_var(
+        &mut self,
+        universe: ty::UniverseIndex,
+        origin: RegionVariableOrigin,
+    ) -> RegionVid {
+        let vid = self.var_infos.push(RegionVariableInfo { origin, universe });
 
         let u_vid = self.unification_table
             .new_key(unify_key::RegionVidKey { min_vid: vid });
@@ -511,8 +510,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         }
         debug!(
             "created new region variable {:?} with origin {:?}",
-            vid,
-            origin
+            vid, origin
         );
         return vid;
     }
@@ -533,45 +531,19 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     /// created in that time.
     pub fn pop_placeholders(
         &mut self,
-        skolemization_count: ty::UniverseIndex,
-        skols: &FxHashSet<ty::Region<'tcx>>,
+        placeholders: &FxHashSet<ty::Region<'tcx>>,
         snapshot: &RegionSnapshot,
     ) {
-        debug!("pop_placeholders(skols={:?})", skols);
+        debug!("pop_placeholders(placeholders={:?})", placeholders);
 
         assert!(self.in_snapshot());
         assert!(self.undo_log[snapshot.length] == OpenSnapshot);
-        assert!(
-            skolemization_count.as_usize() >= skols.len(),
-            "popping more placeholder variables than actually exist, \
-             sc now = {:?}, skols.len = {:?}",
-            skolemization_count,
-            skols.len()
-        );
-
-        let last_to_pop = skolemization_count.subuniverse();
-        let first_to_pop = ty::UniverseIndex::from(last_to_pop.as_u32() - skols.len() as u32);
-
-        debug_assert! {
-            skols.iter()
-                 .all(|&k| match *k {
-                     ty::RePlaceholder(universe, _) =>
-                         universe >= first_to_pop &&
-                         universe < last_to_pop,
-                     _ =>
-                         false
-                 }),
-            "invalid skolemization keys or keys out of range ({:?}..{:?}): {:?}",
-            first_to_pop,
-            last_to_pop,
-            skols
-        }
 
         let constraints_to_kill: Vec<usize> = self.undo_log
             .iter()
             .enumerate()
             .rev()
-            .filter(|&(_, undo_entry)| kill_constraint(skols, undo_entry))
+            .filter(|&(_, undo_entry)| kill_constraint(placeholders, undo_entry))
             .map(|(index, _)| index)
             .collect();
 
@@ -583,20 +555,20 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         return;
 
         fn kill_constraint<'tcx>(
-            skols: &FxHashSet<ty::Region<'tcx>>,
+            placeholders: &FxHashSet<ty::Region<'tcx>>,
             undo_entry: &UndoLogEntry<'tcx>,
         ) -> bool {
             match undo_entry {
                 &AddConstraint(Constraint::VarSubVar(..)) => false,
-                &AddConstraint(Constraint::RegSubVar(a, _)) => skols.contains(&a),
-                &AddConstraint(Constraint::VarSubReg(_, b)) => skols.contains(&b),
+                &AddConstraint(Constraint::RegSubVar(a, _)) => placeholders.contains(&a),
+                &AddConstraint(Constraint::VarSubReg(_, b)) => placeholders.contains(&b),
                 &AddConstraint(Constraint::RegSubReg(a, b)) => {
-                    skols.contains(&a) || skols.contains(&b)
+                    placeholders.contains(&a) || placeholders.contains(&b)
                 }
                 &AddGiven(..) => false,
                 &AddVerify(_) => false,
                 &AddCombination(_, ref two_regions) => {
-                    skols.contains(&two_regions.a) || skols.contains(&two_regions.b)
+                    placeholders.contains(&two_regions.a) || placeholders.contains(&two_regions.b)
                 }
                 &AddVar(..) | &OpenSnapshot | &Purged | &CommitedSnapshot => false,
             }
@@ -713,9 +685,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         // cannot add constraints once regions are resolved
         debug!(
             "RegionConstraintCollector: make_subregion({:?}, {:?}) due to {:?}",
-            sub,
-            sup,
-            origin
+            sub, sup, origin
         );
 
         match (sub, sup) {
@@ -854,19 +824,19 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
 
     fn universe(&self, region: Region<'tcx>) -> ty::UniverseIndex {
         match *region {
-            ty::ReScope(..) |
-            ty::ReStatic |
-            ty::ReEmpty |
-            ty::ReErased |
-            ty::ReFree(..) |
-            ty::ReEarlyBound(..) => ty::UniverseIndex::ROOT,
+            ty::ReScope(..)
+            | ty::ReStatic
+            | ty::ReEmpty
+            | ty::ReErased
+            | ty::ReFree(..)
+            | ty::ReEarlyBound(..) => ty::UniverseIndex::ROOT,
             ty::RePlaceholder(universe, _) => universe,
-            ty::ReClosureBound(vid) |
-            ty::ReVar(vid) => self.var_universe(vid),
-            ty::ReLateBound(..) =>
-                bug!("universe(): encountered bound region {:?}", region),
-            ty::ReCanonical(..) =>
-                bug!("region_universe(): encountered canonical region {:?}", region),
+            ty::ReClosureBound(vid) | ty::ReVar(vid) => self.var_universe(vid),
+            ty::ReLateBound(..) => bug!("universe(): encountered bound region {:?}", region),
+            ty::ReCanonical(..) => bug!(
+                "region_universe(): encountered canonical region {:?}",
+                region
+            ),
         }
     }
 
@@ -897,9 +867,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     ) -> FxHashSet<ty::Region<'tcx>> {
         debug!(
             "tainted(mark={:?}, r0={:?}, directions={:?})",
-            mark,
-            r0,
-            directions
+            mark, r0, directions
         );
 
         // `result_set` acts as a worklist: we explore all outgoing