diff --git a/Cargo.lock b/Cargo.lock
index 16bb501d312..83d7df889f9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -993,6 +993,15 @@ dependencies = [
  "log",
 ]
 
+[[package]]
+name = "ena"
+version = "0.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d7402b94a93c24e742487327a7cd839dc9d36fec9de9fb25b09f2dae459f36c3"
+dependencies = [
+ "log",
+]
+
 [[package]]
 name = "encoding_rs"
 version = "0.8.17"
@@ -3234,7 +3243,7 @@ dependencies = [
  "bitflags",
  "cfg-if",
  "crossbeam-utils 0.7.2",
- "ena",
+ "ena 0.13.1",
  "indexmap",
  "jobserver",
  "lazy_static 1.4.0",
@@ -3683,7 +3692,7 @@ dependencies = [
  "bitflags",
  "cfg-if",
  "crossbeam-utils 0.7.2",
- "ena",
+ "ena 0.14.0",
  "graphviz",
  "indexmap",
  "jobserver",
diff --git a/src/librustc_data_structures/Cargo.toml b/src/librustc_data_structures/Cargo.toml
index 6d7022acc78..e257ada0629 100644
--- a/src/librustc_data_structures/Cargo.toml
+++ b/src/librustc_data_structures/Cargo.toml
@@ -10,7 +10,7 @@ path = "lib.rs"
 doctest = false
 
 [dependencies]
-ena = "0.13.1"
+ena = "0.14"
 indexmap = "1"
 log = "0.4"
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index bc2da535fd3..a7bee8a067c 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -84,6 +84,7 @@ pub mod sync;
 pub mod thin_vec;
 pub mod tiny_list;
 pub mod transitive_relation;
+pub use ena::undo_log;
 pub use ena::unify;
 mod atomic_ref;
 pub mod fingerprint;
diff --git a/src/librustc_data_structures/snapshot_map/mod.rs b/src/librustc_data_structures/snapshot_map/mod.rs
index b71163a8f94..52865f55f78 100644
--- a/src/librustc_data_structures/snapshot_map/mod.rs
+++ b/src/librustc_data_structures/snapshot_map/mod.rs
@@ -1,77 +1,75 @@
 use crate::fx::FxHashMap;
+use crate::undo_log::{Rollback, Snapshots, UndoLogs, VecLog};
+use std::borrow::{Borrow, BorrowMut};
 use std::hash::Hash;
-use std::mem;
+use std::marker::PhantomData;
 use std::ops;
 
+pub use crate::undo_log::Snapshot;
+
 #[cfg(test)]
 mod tests;
 
-pub struct SnapshotMap<K, V>
-where
-    K: Clone + Eq,
-{
-    map: FxHashMap<K, V>,
-    undo_log: Vec<UndoLog<K, V>>,
-    num_open_snapshots: usize,
+pub type SnapshotMapStorage<K, V> = SnapshotMap<K, V, FxHashMap<K, V>, ()>;
+pub type SnapshotMapRef<'a, K, V, L> = SnapshotMap<K, V, &'a mut FxHashMap<K, V>, &'a mut L>;
+
+pub struct SnapshotMap<K, V, M = FxHashMap<K, V>, L = VecLog<UndoLog<K, V>>> {
+    map: M,
+    undo_log: L,
+    _marker: PhantomData<(K, V)>,
 }
 
 // HACK(eddyb) manual impl avoids `Default` bounds on `K` and `V`.
-impl<K, V> Default for SnapshotMap<K, V>
+impl<K, V, M, L> Default for SnapshotMap<K, V, M, L>
 where
-    K: Hash + Clone + Eq,
+    M: Default,
+    L: Default,
 {
     fn default() -> Self {
-        SnapshotMap { map: Default::default(), undo_log: Default::default(), num_open_snapshots: 0 }
+        SnapshotMap { map: Default::default(), undo_log: Default::default(), _marker: PhantomData }
     }
 }
 
-pub struct Snapshot {
-    len: usize,
-}
-
-enum UndoLog<K, V> {
+pub enum UndoLog<K, V> {
     Inserted(K),
     Overwrite(K, V),
     Purged,
 }
 
-impl<K, V> SnapshotMap<K, V>
+impl<K, V, M, L> SnapshotMap<K, V, M, L> {
+    pub fn with_log<L2>(&mut self, undo_log: L2) -> SnapshotMap<K, V, &mut M, L2> {
+        SnapshotMap { map: &mut self.map, undo_log, _marker: PhantomData }
+    }
+}
+
+impl<K, V, M, L> SnapshotMap<K, V, M, L>
 where
     K: Hash + Clone + Eq,
+    M: BorrowMut<FxHashMap<K, V>> + Borrow<FxHashMap<K, V>>,
+    L: UndoLogs<UndoLog<K, V>>,
 {
     pub fn clear(&mut self) {
-        self.map.clear();
+        self.map.borrow_mut().clear();
         self.undo_log.clear();
-        self.num_open_snapshots = 0;
-    }
-
-    fn in_snapshot(&self) -> bool {
-        self.num_open_snapshots > 0
     }
 
     pub fn insert(&mut self, key: K, value: V) -> bool {
-        match self.map.insert(key.clone(), value) {
+        match self.map.borrow_mut().insert(key.clone(), value) {
             None => {
-                if self.in_snapshot() {
-                    self.undo_log.push(UndoLog::Inserted(key));
-                }
+                self.undo_log.push(UndoLog::Inserted(key));
                 true
             }
             Some(old_value) => {
-                if self.in_snapshot() {
-                    self.undo_log.push(UndoLog::Overwrite(key, old_value));
-                }
+                self.undo_log.push(UndoLog::Overwrite(key, old_value));
                 false
             }
         }
     }
 
     pub fn remove(&mut self, key: K) -> bool {
-        match self.map.remove(&key) {
+        match self.map.borrow_mut().remove(&key) {
             Some(old_value) => {
-                if self.in_snapshot() {
-                    self.undo_log.push(UndoLog::Overwrite(key, old_value));
-                }
+                self.undo_log.push(UndoLog::Overwrite(key, old_value));
                 true
             }
             None => false,
@@ -79,83 +77,64 @@ where
     }
 
     pub fn get(&self, key: &K) -> Option<&V> {
-        self.map.get(key)
+        self.map.borrow().get(key)
     }
+}
 
+impl<K, V> SnapshotMap<K, V>
+where
+    K: Hash + Clone + Eq,
+{
     pub fn snapshot(&mut self) -> Snapshot {
-        let len = self.undo_log.len();
-        self.num_open_snapshots += 1;
-        Snapshot { len }
-    }
-
-    fn assert_open_snapshot(&self, snapshot: &Snapshot) {
-        assert!(self.undo_log.len() >= snapshot.len);
-        assert!(self.num_open_snapshots > 0);
+        self.undo_log.start_snapshot()
     }
 
     pub fn commit(&mut self, snapshot: Snapshot) {
-        self.assert_open_snapshot(&snapshot);
-        if self.num_open_snapshots == 1 {
-            // The root snapshot. It's safe to clear the undo log because
-            // there's no snapshot further out that we might need to roll back
-            // to.
-            assert!(snapshot.len == 0);
-            self.undo_log.clear();
-        }
-
-        self.num_open_snapshots -= 1;
-    }
-
-    pub fn partial_rollback<F>(&mut self, snapshot: &Snapshot, should_revert_key: &F)
-    where
-        F: Fn(&K) -> bool,
-    {
-        self.assert_open_snapshot(snapshot);
-        for i in (snapshot.len..self.undo_log.len()).rev() {
-            let reverse = match self.undo_log[i] {
-                UndoLog::Purged => false,
-                UndoLog::Inserted(ref k) => should_revert_key(k),
-                UndoLog::Overwrite(ref k, _) => should_revert_key(k),
-            };
-
-            if reverse {
-                let entry = mem::replace(&mut self.undo_log[i], UndoLog::Purged);
-                self.reverse(entry);
-            }
-        }
+        self.undo_log.commit(snapshot)
     }
 
     pub fn rollback_to(&mut self, snapshot: Snapshot) {
-        self.assert_open_snapshot(&snapshot);
-        while self.undo_log.len() > snapshot.len {
-            let entry = self.undo_log.pop().unwrap();
-            self.reverse(entry);
-        }
-
-        self.num_open_snapshots -= 1;
+        let map = &mut self.map;
+        self.undo_log.rollback_to(|| map, snapshot)
     }
+}
 
-    fn reverse(&mut self, entry: UndoLog<K, V>) {
-        match entry {
+impl<'k, K, V, M, L> ops::Index<&'k K> for SnapshotMap<K, V, M, L>
+where
+    K: Hash + Clone + Eq,
+    M: Borrow<FxHashMap<K, V>>,
+{
+    type Output = V;
+    fn index(&self, key: &'k K) -> &V {
+        &self.map.borrow()[key]
+    }
+}
+
+impl<K, V, M, L> Rollback<UndoLog<K, V>> for SnapshotMap<K, V, M, L>
+where
+    K: Eq + Hash,
+    M: Rollback<UndoLog<K, V>>,
+{
+    fn reverse(&mut self, undo: UndoLog<K, V>) {
+        self.map.reverse(undo)
+    }
+}
+
+impl<K, V> Rollback<UndoLog<K, V>> for FxHashMap<K, V>
+where
+    K: Eq + Hash,
+{
+    fn reverse(&mut self, undo: UndoLog<K, V>) {
+        match undo {
             UndoLog::Inserted(key) => {
-                self.map.remove(&key);
+                self.remove(&key);
             }
 
             UndoLog::Overwrite(key, old_value) => {
-                self.map.insert(key, old_value);
+                self.insert(key, old_value);
             }
 
             UndoLog::Purged => {}
         }
     }
 }
-
-impl<'k, K, V> ops::Index<&'k K> for SnapshotMap<K, V>
-where
-    K: Hash + Clone + Eq,
-{
-    type Output = V;
-    fn index(&self, key: &'k K) -> &V {
-        &self.map[key]
-    }
-}
diff --git a/src/librustc_infer/infer/combine.rs b/src/librustc_infer/infer/combine.rs
index b03044b72da..d4af4704996 100644
--- a/src/librustc_infer/infer/combine.rs
+++ b/src/librustc_infer/infer/combine.rs
@@ -76,7 +76,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
                 self.inner
                     .borrow_mut()
-                    .int_unification_table
+                    .int_unification_table()
                     .unify_var_var(a_id, b_id)
                     .map_err(|e| int_unification_error(a_is_expected, e))?;
                 Ok(a)
@@ -98,7 +98,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
                 self.inner
                     .borrow_mut()
-                    .float_unification_table
+                    .float_unification_table()
                     .unify_var_var(a_id, b_id)
                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
                 Ok(a)
@@ -133,8 +133,8 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
             return Ok(a);
         }
 
-        let a = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table, a);
-        let b = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table, b);
+        let a = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table(), a);
+        let b = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table(), b);
 
         let a_is_expected = relation.a_is_expected();
 
@@ -145,7 +145,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
             ) => {
                 self.inner
                     .borrow_mut()
-                    .const_unification_table
+                    .const_unification_table()
                     .unify_var_var(a_vid, b_vid)
                     .map_err(|e| const_unification_error(a_is_expected, e))?;
                 return Ok(a);
@@ -179,7 +179,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
         self.inner
             .borrow_mut()
-            .const_unification_table
+            .const_unification_table()
             .unify_var_value(
                 vid,
                 ConstVarValue {
@@ -202,7 +202,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
     ) -> RelateResult<'tcx, Ty<'tcx>> {
         self.inner
             .borrow_mut()
-            .int_unification_table
+            .int_unification_table()
             .unify_var_value(vid, Some(val))
             .map_err(|e| int_unification_error(vid_is_expected, e))?;
         match val {
@@ -219,7 +219,7 @@ impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
     ) -> RelateResult<'tcx, Ty<'tcx>> {
         self.inner
             .borrow_mut()
-            .float_unification_table
+            .float_unification_table()
             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
             .map_err(|e| float_unification_error(vid_is_expected, e))?;
         Ok(self.tcx.mk_mach_float(val))
@@ -266,7 +266,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         use self::RelationDir::*;
 
         // Get the actual variable that b_vid has been inferred to
-        debug_assert!(self.infcx.inner.borrow_mut().type_variables.probe(b_vid).is_unknown());
+        debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
 
         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 
@@ -286,7 +286,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             "instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
             a_ty, dir, b_vid, b_ty
         );
-        self.infcx.inner.borrow_mut().type_variables.instantiate(b_vid, b_ty);
+        self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 
         if needs_wf {
             self.obligations.push(Obligation::new(
@@ -344,7 +344,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 
         debug!("generalize: ambient_variance = {:?}", ambient_variance);
 
-        let for_universe = match self.infcx.inner.borrow_mut().type_variables.probe(for_vid) {
+        let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
             v @ TypeVariableValue::Known { .. } => {
                 panic!("instantiating {:?} which has a known value {:?}", for_vid, v,)
             }
@@ -356,7 +356,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         let mut generalize = Generalizer {
             infcx: self.infcx,
             span: self.trace.cause.span,
-            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables.sub_root_var(for_vid),
+            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
             for_universe,
             ambient_variance,
             needs_wf: false,
@@ -508,14 +508,14 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         // us from creating infinitely sized types.
         match t.kind {
             ty::Infer(ty::TyVar(vid)) => {
-                let vid = self.infcx.inner.borrow_mut().type_variables.root_var(vid);
-                let sub_vid = self.infcx.inner.borrow_mut().type_variables.sub_root_var(vid);
+                let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
+                let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
                 if sub_vid == self.for_vid_sub_root {
                     // If sub-roots are equal, then `for_vid` and
                     // `vid` are related via subtyping.
                     Err(TypeError::CyclicTy(self.root_ty))
                 } else {
-                    let probe = self.infcx.inner.borrow_mut().type_variables.probe(vid);
+                    let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
                     match probe {
                         TypeVariableValue::Known { value: u } => {
                             debug!("generalize: known value {:?}", u);
@@ -542,12 +542,13 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                             }
 
                             let origin =
-                                *self.infcx.inner.borrow_mut().type_variables.var_origin(vid);
-                            let new_var_id = self.infcx.inner.borrow_mut().type_variables.new_var(
-                                self.for_universe,
-                                false,
-                                origin,
-                            );
+                                *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
+                            let new_var_id = self
+                                .infcx
+                                .inner
+                                .borrow_mut()
+                                .type_variables()
+                                .new_var(self.for_universe, false, origin);
                             let u = self.tcx().mk_ty_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
                             Ok(u)
@@ -618,7 +619,8 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 
         match c.val {
             ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                let variable_table = &mut self.infcx.inner.borrow_mut().const_unification_table;
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variable_table = &mut inner.const_unification_table();
                 let var_value = variable_table.probe_value(vid);
                 match var_value.val {
                     ConstVariableValue::Known { value: u } => self.relate(&u, &u),
diff --git a/src/librustc_infer/infer/equate.rs b/src/librustc_infer/infer/equate.rs
index e05094cda27..d054070e292 100644
--- a/src/librustc_infer/infer/equate.rs
+++ b/src/librustc_infer/infer/equate.rs
@@ -72,14 +72,14 @@ impl TypeRelation<'tcx> for Equate<'combine, 'infcx, 'tcx> {
         }
 
         let infcx = self.fields.infcx;
-        let a = infcx.inner.borrow_mut().type_variables.replace_if_possible(a);
-        let b = infcx.inner.borrow_mut().type_variables.replace_if_possible(b);
+        let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
+        let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
 
         debug!("{}.tys: replacements ({:?}, {:?})", self.tag(), a, b);
 
         match (&a.kind, &b.kind) {
             (&ty::Infer(TyVar(a_id)), &ty::Infer(TyVar(b_id))) => {
-                infcx.inner.borrow_mut().type_variables.equate(a_id, b_id);
+                infcx.inner.borrow_mut().type_variables().equate(a_id, b_id);
             }
 
             (&ty::Infer(TyVar(a_id)), _) => {
diff --git a/src/librustc_infer/infer/error_reporting/need_type_info.rs b/src/librustc_infer/infer/error_reporting/need_type_info.rs
index 93c8e505697..d8133c58df7 100644
--- a/src/librustc_infer/infer/error_reporting/need_type_info.rs
+++ b/src/librustc_infer/infer/error_reporting/need_type_info.rs
@@ -59,7 +59,7 @@ impl<'a, 'tcx> FindHirNodeVisitor<'a, 'tcx> {
                                         .infcx
                                         .inner
                                         .borrow_mut()
-                                        .type_variables
+                                        .type_variables()
                                         .sub_unified(a_vid, b_vid),
                                     _ => false,
                                 }
@@ -194,7 +194,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         highlight: Option<ty::print::RegionHighlightMode>,
     ) -> (String, Option<Span>, Cow<'static, str>, Option<String>, Option<&'static str>) {
         if let ty::Infer(ty::TyVar(ty_vid)) = ty.kind {
-            let ty_vars = &self.inner.borrow().type_variables;
+            let mut inner = self.inner.borrow_mut();
+            let ty_vars = &inner.type_variables();
             let var_origin = ty_vars.var_origin(ty_vid);
             if let TypeVariableOriginKind::TypeParameterDefinition(name, def_id) = var_origin.kind {
                 let parent_def_id = def_id.and_then(|def_id| self.tcx.parent(def_id));
@@ -248,7 +249,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let ty_to_string = |ty: Ty<'tcx>| -> String {
             let mut s = String::new();
             let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
-            let ty_vars = &self.inner.borrow().type_variables;
+            let mut inner = self.inner.borrow_mut();
+            let ty_vars = inner.type_variables();
             let getter = move |ty_vid| {
                 let var_origin = ty_vars.var_origin(ty_vid);
                 if let TypeVariableOriginKind::TypeParameterDefinition(name, _) = var_origin.kind {
diff --git a/src/librustc_infer/infer/freshen.rs b/src/librustc_infer/infer/freshen.rs
index 636cf42198b..47346c3a856 100644
--- a/src/librustc_infer/infer/freshen.rs
+++ b/src/librustc_infer/infer/freshen.rs
@@ -147,7 +147,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
 
         match t.kind {
             ty::Infer(ty::TyVar(v)) => {
-                let opt_ty = self.infcx.inner.borrow_mut().type_variables.probe(v).known();
+                let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
                 self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
             }
 
@@ -155,7 +155,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                 self.infcx
                     .inner
                     .borrow_mut()
-                    .int_unification_table
+                    .int_unification_table()
                     .probe_value(v)
                     .map(|v| v.to_type(tcx)),
                 ty::IntVar(v),
@@ -166,7 +166,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                 self.infcx
                     .inner
                     .borrow_mut()
-                    .float_unification_table
+                    .float_unification_table()
                     .probe_value(v)
                     .map(|v| v.to_type(tcx)),
                 ty::FloatVar(v),
@@ -222,7 +222,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                     .infcx
                     .inner
                     .borrow_mut()
-                    .const_unification_table
+                    .const_unification_table()
                     .probe_value(v)
                     .val
                     .known();
diff --git a/src/librustc_infer/infer/fudge.rs b/src/librustc_infer/infer/fudge.rs
index 1a58e100fb3..c6651108df5 100644
--- a/src/librustc_infer/infer/fudge.rs
+++ b/src/librustc_infer/infer/fudge.rs
@@ -3,18 +3,30 @@ use rustc_middle::ty::{self, ConstVid, FloatVid, IntVid, RegionVid, Ty, TyCtxt,
 
 use super::type_variable::TypeVariableOrigin;
 use super::InferCtxt;
-use super::{ConstVariableOrigin, RegionVariableOrigin};
+use super::{ConstVariableOrigin, RegionVariableOrigin, UnificationTable};
 
+use rustc_data_structures::snapshot_vec as sv;
 use rustc_data_structures::unify as ut;
 use ut::UnifyKey;
 
 use std::ops::Range;
 
+fn vars_since_snapshot<'tcx, T>(
+    table: &mut UnificationTable<'_, 'tcx, T>,
+    snapshot_var_len: usize,
+) -> Range<T>
+where
+    T: UnifyKey,
+    super::UndoLog<'tcx>: From<sv::UndoLog<ut::Delegate<T>>>,
+{
+    T::from_index(snapshot_var_len as u32)..T::from_index(table.len() as u32)
+}
+
 fn const_vars_since_snapshot<'tcx>(
-    table: &mut ut::UnificationTable<ut::InPlace<ConstVid<'tcx>>>,
-    snapshot: &ut::Snapshot<ut::InPlace<ConstVid<'tcx>>>,
+    table: &mut UnificationTable<'_, 'tcx, ConstVid<'tcx>>,
+    snapshot_var_len: usize,
 ) -> (Range<ConstVid<'tcx>>, Vec<ConstVariableOrigin>) {
-    let range = table.vars_since_snapshot(snapshot);
+    let range = vars_since_snapshot(table, snapshot_var_len);
     (
         range.start..range.end,
         (range.start.index..range.end.index)
@@ -23,7 +35,26 @@ fn const_vars_since_snapshot<'tcx>(
     )
 }
 
+struct VariableLengths {
+    type_var_len: usize,
+    const_var_len: usize,
+    int_var_len: usize,
+    float_var_len: usize,
+    region_constraints_len: usize,
+}
+
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    fn variable_lengths(&self) -> VariableLengths {
+        let mut inner = self.inner.borrow_mut();
+        VariableLengths {
+            type_var_len: inner.type_variables().num_vars(),
+            const_var_len: inner.const_unification_table().len(),
+            int_var_len: inner.int_unification_table().len(),
+            float_var_len: inner.float_unification_table().len(),
+            region_constraints_len: inner.unwrap_region_constraints().num_region_vars(),
+        }
+    }
+
     /// This rather funky routine is used while processing expected
     /// types. What happens here is that we want to propagate a
     /// coercion through the return type of a fn to its
@@ -70,7 +101,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     {
         debug!("fudge_inference_if_ok()");
 
-        let (mut fudger, value) = self.probe(|snapshot| {
+        let variable_lengths = self.variable_lengths();
+        let (mut fudger, value) = self.probe(|_| {
             match f() {
                 Ok(value) => {
                     let value = self.resolve_vars_if_possible(&value);
@@ -83,17 +115,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
                     let mut inner = self.inner.borrow_mut();
                     let type_vars =
-                        inner.type_variables.vars_since_snapshot(&snapshot.type_snapshot);
-                    let int_vars =
-                        inner.int_unification_table.vars_since_snapshot(&snapshot.int_snapshot);
-                    let float_vars =
-                        inner.float_unification_table.vars_since_snapshot(&snapshot.float_snapshot);
+                        inner.type_variables().vars_since_snapshot(variable_lengths.type_var_len);
+                    let int_vars = vars_since_snapshot(
+                        &mut inner.int_unification_table(),
+                        variable_lengths.int_var_len,
+                    );
+                    let float_vars = vars_since_snapshot(
+                        &mut inner.float_unification_table(),
+                        variable_lengths.float_var_len,
+                    );
                     let region_vars = inner
                         .unwrap_region_constraints()
-                        .vars_since_snapshot(&snapshot.region_constraints_snapshot);
+                        .vars_since_snapshot(variable_lengths.region_constraints_len);
                     let const_vars = const_vars_since_snapshot(
-                        &mut inner.const_unification_table,
-                        &snapshot.const_snapshot,
+                        &mut inner.const_unification_table(),
+                        variable_lengths.const_var_len,
                     );
 
                     let fudger = InferenceFudger {
@@ -161,7 +197,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for InferenceFudger<'a, 'tcx> {
                     // that it is unbound, so we can just return
                     // it.
                     debug_assert!(
-                        self.infcx.inner.borrow_mut().type_variables.probe(vid).is_unknown()
+                        self.infcx.inner.borrow_mut().type_variables().probe(vid).is_unknown()
                     );
                     ty
                 }
diff --git a/src/librustc_infer/infer/lattice.rs b/src/librustc_infer/infer/lattice.rs
index c29614b8556..1bf43e74dcd 100644
--- a/src/librustc_infer/infer/lattice.rs
+++ b/src/librustc_infer/infer/lattice.rs
@@ -56,8 +56,8 @@ where
     }
 
     let infcx = this.infcx();
-    let a = infcx.inner.borrow_mut().type_variables.replace_if_possible(a);
-    let b = infcx.inner.borrow_mut().type_variables.replace_if_possible(b);
+    let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
+    let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
     match (&a.kind, &b.kind) {
         // If one side is known to be a variable and one is not,
         // create a variable (`v`) to represent the LUB. Make sure to
diff --git a/src/librustc_infer/infer/mod.rs b/src/librustc_infer/infer/mod.rs
index 54f80e8f388..2cf9dd882e4 100644
--- a/src/librustc_infer/infer/mod.rs
+++ b/src/librustc_infer/infer/mod.rs
@@ -6,11 +6,14 @@ pub use self::RegionVariableOrigin::*;
 pub use self::SubregionOrigin::*;
 pub use self::ValuePairs::*;
 
+pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
+
 use crate::traits::{self, ObligationCause, PredicateObligations, TraitEngine};
 
 use rustc_ast::ast;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::undo_log::Rollback;
 use rustc_data_structures::unify as ut;
 use rustc_errors::DiagnosticBuilder;
 use rustc_hir as hir;
@@ -42,7 +45,9 @@ use self::free_regions::RegionRelations;
 use self::lexical_region_resolve::LexicalRegionResolutions;
 use self::outlives::env::OutlivesEnvironment;
 use self::region_constraints::{GenericKind, RegionConstraintData, VarInfos, VerifyBound};
-use self::region_constraints::{RegionConstraintCollector, RegionSnapshot};
+use self::region_constraints::{
+    RegionConstraintCollector, RegionConstraintStorage, RegionSnapshot,
+};
 use self::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 
 pub mod at;
@@ -64,6 +69,7 @@ pub mod region_constraints;
 pub mod resolve;
 mod sub;
 pub mod type_variable;
+mod undo_log;
 
 use crate::infer::canonical::OriginalQueryValues;
 pub use rustc_middle::infer::unify_key;
@@ -80,6 +86,10 @@ pub type Bound<T> = Option<T>;
 pub type UnitResult<'tcx> = RelateResult<'tcx, ()>; // "unify result"
 pub type FixupResult<'tcx, T> = Result<T, FixupError<'tcx>>; // "fixup result"
 
+pub(crate) type UnificationTable<'a, 'tcx, T> = ut::UnificationTable<
+    ut::InPlace<T, &'a mut ut::UnificationStorage<T>, &'a mut InferCtxtUndoLogs<'tcx>>,
+>;
+
 /// How we should handle region solving.
 ///
 /// This is used so that the region values inferred by HIR region solving are
@@ -136,28 +146,28 @@ pub struct InferCtxtInner<'tcx> {
     /// Cache for projections. This cache is snapshotted along with the infcx.
     ///
     /// Public so that `traits::project` can use it.
-    pub projection_cache: traits::ProjectionCache<'tcx>,
+    pub projection_cache: traits::ProjectionCacheStorage<'tcx>,
 
     /// We instantiate `UnificationTable` with `bounds<Ty>` because the types
     /// that might instantiate a general type variable have an order,
     /// represented by its upper and lower bounds.
-    type_variables: type_variable::TypeVariableTable<'tcx>,
+    type_variable_storage: type_variable::TypeVariableStorage<'tcx>,
 
     /// Map from const parameter variable to the kind of const it represents.
-    const_unification_table: ut::UnificationTable<ut::InPlace<ty::ConstVid<'tcx>>>,
+    const_unification_storage: ut::UnificationTableStorage<ty::ConstVid<'tcx>>,
 
     /// Map from integral variable to the kind of integer it represents.
-    int_unification_table: ut::UnificationTable<ut::InPlace<ty::IntVid>>,
+    int_unification_storage: ut::UnificationTableStorage<ty::IntVid>,
 
     /// Map from floating variable to the kind of float it represents.
-    float_unification_table: ut::UnificationTable<ut::InPlace<ty::FloatVid>>,
+    float_unification_storage: ut::UnificationTableStorage<ty::FloatVid>,
 
     /// Tracks the set of region variables and the constraints between them.
     /// This is initially `Some(_)` but when
     /// `resolve_regions_and_report_errors` is invoked, this gets set to `None`
     /// -- further attempts to perform unification, etc., may fail if new
     /// region constraints would've been added.
-    region_constraints: Option<RegionConstraintCollector<'tcx>>,
+    region_constraint_storage: Option<RegionConstraintStorage<'tcx>>,
 
     /// A set of constraints that regionck must validate. Each
     /// constraint has the form `T:'a`, meaning "some type `T` must
@@ -190,24 +200,78 @@ pub struct InferCtxtInner<'tcx> {
     /// for each body-id in this map, which will process the
     /// obligations within. This is expected to be done 'late enough'
     /// that all type inference variables have been bound and so forth.
-    pub region_obligations: Vec<(hir::HirId, RegionObligation<'tcx>)>,
+    region_obligations: Vec<(hir::HirId, RegionObligation<'tcx>)>,
+
+    undo_log: InferCtxtUndoLogs<'tcx>,
 }
 
 impl<'tcx> InferCtxtInner<'tcx> {
     fn new() -> InferCtxtInner<'tcx> {
         InferCtxtInner {
             projection_cache: Default::default(),
-            type_variables: type_variable::TypeVariableTable::new(),
-            const_unification_table: ut::UnificationTable::new(),
-            int_unification_table: ut::UnificationTable::new(),
-            float_unification_table: ut::UnificationTable::new(),
-            region_constraints: Some(RegionConstraintCollector::new()),
+            type_variable_storage: type_variable::TypeVariableStorage::new(),
+            undo_log: InferCtxtUndoLogs::default(),
+            const_unification_storage: ut::UnificationTableStorage::new(),
+            int_unification_storage: ut::UnificationTableStorage::new(),
+            float_unification_storage: ut::UnificationTableStorage::new(),
+            region_constraint_storage: Some(RegionConstraintStorage::new()),
             region_obligations: vec![],
         }
     }
 
-    pub fn unwrap_region_constraints(&mut self) -> &mut RegionConstraintCollector<'tcx> {
-        self.region_constraints.as_mut().expect("region constraints already solved")
+    pub fn region_obligations(&self) -> &[(hir::HirId, RegionObligation<'tcx>)] {
+        &self.region_obligations
+    }
+
+    pub fn projection_cache(&mut self) -> traits::ProjectionCache<'_, 'tcx> {
+        self.projection_cache.with_log(&mut self.undo_log)
+    }
+
+    fn type_variables(&mut self) -> type_variable::TypeVariableTable<'_, 'tcx> {
+        self.type_variable_storage.with_log(&mut self.undo_log)
+    }
+
+    fn int_unification_table(
+        &mut self,
+    ) -> ut::UnificationTable<
+        ut::InPlace<
+            ty::IntVid,
+            &mut ut::UnificationStorage<ty::IntVid>,
+            &mut InferCtxtUndoLogs<'tcx>,
+        >,
+    > {
+        self.int_unification_storage.with_log(&mut self.undo_log)
+    }
+
+    fn float_unification_table(
+        &mut self,
+    ) -> ut::UnificationTable<
+        ut::InPlace<
+            ty::FloatVid,
+            &mut ut::UnificationStorage<ty::FloatVid>,
+            &mut InferCtxtUndoLogs<'tcx>,
+        >,
+    > {
+        self.float_unification_storage.with_log(&mut self.undo_log)
+    }
+
+    fn const_unification_table(
+        &mut self,
+    ) -> ut::UnificationTable<
+        ut::InPlace<
+            ty::ConstVid<'tcx>,
+            &mut ut::UnificationStorage<ty::ConstVid<'tcx>>,
+            &mut InferCtxtUndoLogs<'tcx>,
+        >,
+    > {
+        self.const_unification_storage.with_log(&mut self.undo_log)
+    }
+
+    pub fn unwrap_region_constraints(&mut self) -> RegionConstraintCollector<'_, 'tcx> {
+        self.region_constraint_storage
+            .as_mut()
+            .expect("region constraints already solved")
+            .with_log(&mut self.undo_log)
     }
 }
 
@@ -643,16 +707,10 @@ impl<'tcx> InferOk<'tcx, ()> {
 
 #[must_use = "once you start a snapshot, you should always consume it"]
 pub struct CombinedSnapshot<'a, 'tcx> {
-    projection_cache_snapshot: traits::ProjectionCacheSnapshot,
-    type_snapshot: type_variable::Snapshot<'tcx>,
-    const_snapshot: ut::Snapshot<ut::InPlace<ty::ConstVid<'tcx>>>,
-    int_snapshot: ut::Snapshot<ut::InPlace<ty::IntVid>>,
-    float_snapshot: ut::Snapshot<ut::InPlace<ty::FloatVid>>,
+    undo_snapshot: Snapshot<'tcx>,
     region_constraints_snapshot: RegionSnapshot,
-    region_obligations_snapshot: usize,
     universe: ty::UniverseIndex,
     was_in_snapshot: bool,
-    was_skip_leak_check: bool,
     _in_progress_tables: Option<Ref<'a, ty::TypeckTables<'tcx>>>,
 }
 
@@ -667,7 +725,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
     pub fn type_var_diverges(&'a self, ty: Ty<'_>) -> bool {
         match ty.kind {
-            ty::Infer(ty::TyVar(vid)) => self.inner.borrow().type_variables.var_diverges(vid),
+            ty::Infer(ty::TyVar(vid)) => self.inner.borrow_mut().type_variables().var_diverges(vid),
             _ => false,
         }
     }
@@ -681,14 +739,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         use rustc_middle::ty::error::UnconstrainedNumeric::{UnconstrainedFloat, UnconstrainedInt};
         match ty.kind {
             ty::Infer(ty::IntVar(vid)) => {
-                if self.inner.borrow_mut().int_unification_table.probe_value(vid).is_some() {
+                if self.inner.borrow_mut().int_unification_table().probe_value(vid).is_some() {
                     Neither
                 } else {
                     UnconstrainedInt
                 }
             }
             ty::Infer(ty::FloatVar(vid)) => {
-                if self.inner.borrow_mut().float_unification_table.probe_value(vid).is_some() {
+                if self.inner.borrow_mut().float_unification_table().probe_value(vid).is_some() {
                     Neither
                 } else {
                     UnconstrainedFloat
@@ -703,21 +761,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         // FIXME(const_generics): should there be an equivalent function for const variables?
 
         let mut vars: Vec<Ty<'_>> = inner
-            .type_variables
+            .type_variables()
             .unsolved_variables()
             .into_iter()
             .map(|t| self.tcx.mk_ty_var(t))
             .collect();
         vars.extend(
-            (0..inner.int_unification_table.len())
+            (0..inner.int_unification_table().len())
                 .map(|i| ty::IntVid { index: i as u32 })
-                .filter(|&vid| inner.int_unification_table.probe_value(vid).is_none())
+                .filter(|&vid| inner.int_unification_table().probe_value(vid).is_none())
                 .map(|v| self.tcx.mk_int_var(v)),
         );
         vars.extend(
-            (0..inner.float_unification_table.len())
+            (0..inner.float_unification_table().len())
                 .map(|i| ty::FloatVid { index: i as u32 })
-                .filter(|&vid| inner.float_unification_table.probe_value(vid).is_none())
+                .filter(|&vid| inner.float_unification_table().probe_value(vid).is_none())
                 .map(|v| self.tcx.mk_float_var(v)),
         );
         vars
@@ -769,17 +827,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let in_snapshot = self.in_snapshot.replace(true);
 
         let mut inner = self.inner.borrow_mut();
+
         CombinedSnapshot {
-            projection_cache_snapshot: inner.projection_cache.snapshot(),
-            type_snapshot: inner.type_variables.snapshot(),
-            const_snapshot: inner.const_unification_table.snapshot(),
-            int_snapshot: inner.int_unification_table.snapshot(),
-            float_snapshot: inner.float_unification_table.snapshot(),
+            undo_snapshot: inner.undo_log.start_snapshot(),
             region_constraints_snapshot: inner.unwrap_region_constraints().start_snapshot(),
-            region_obligations_snapshot: inner.region_obligations.len(),
             universe: self.universe(),
             was_in_snapshot: in_snapshot,
-            was_skip_leak_check: self.skip_leak_check.get(),
             // Borrow tables "in progress" (i.e., during typeck)
             // to ban writes from within a snapshot to them.
             _in_progress_tables: self.in_progress_tables.map(|tables| tables.borrow()),
@@ -789,59 +842,34 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     fn rollback_to(&self, cause: &str, snapshot: CombinedSnapshot<'a, 'tcx>) {
         debug!("rollback_to(cause={})", cause);
         let CombinedSnapshot {
-            projection_cache_snapshot,
-            type_snapshot,
-            const_snapshot,
-            int_snapshot,
-            float_snapshot,
+            undo_snapshot,
             region_constraints_snapshot,
-            region_obligations_snapshot,
             universe,
             was_in_snapshot,
-            was_skip_leak_check,
             _in_progress_tables,
         } = snapshot;
 
         self.in_snapshot.set(was_in_snapshot);
         self.universe.set(universe);
-        self.skip_leak_check.set(was_skip_leak_check);
 
         let mut inner = self.inner.borrow_mut();
-        inner.projection_cache.rollback_to(projection_cache_snapshot);
-        inner.type_variables.rollback_to(type_snapshot);
-        inner.const_unification_table.rollback_to(const_snapshot);
-        inner.int_unification_table.rollback_to(int_snapshot);
-        inner.float_unification_table.rollback_to(float_snapshot);
+        inner.rollback_to(undo_snapshot);
         inner.unwrap_region_constraints().rollback_to(region_constraints_snapshot);
-        inner.region_obligations.truncate(region_obligations_snapshot);
     }
 
     fn commit_from(&self, snapshot: CombinedSnapshot<'a, 'tcx>) {
         debug!("commit_from()");
         let CombinedSnapshot {
-            projection_cache_snapshot,
-            type_snapshot,
-            const_snapshot,
-            int_snapshot,
-            float_snapshot,
-            region_constraints_snapshot,
-            region_obligations_snapshot: _,
+            undo_snapshot,
+            region_constraints_snapshot: _,
             universe: _,
             was_in_snapshot,
-            was_skip_leak_check,
             _in_progress_tables,
         } = snapshot;
 
         self.in_snapshot.set(was_in_snapshot);
-        self.skip_leak_check.set(was_skip_leak_check);
 
-        let mut inner = self.inner.borrow_mut();
-        inner.projection_cache.commit(projection_cache_snapshot);
-        inner.type_variables.commit(type_snapshot);
-        inner.const_unification_table.commit(const_snapshot);
-        inner.int_unification_table.commit(int_snapshot);
-        inner.float_unification_table.commit(float_snapshot);
-        inner.unwrap_region_constraints().commit(region_constraints_snapshot);
+        self.inner.borrow_mut().commit(undo_snapshot);
     }
 
     /// Executes `f` and commit the bindings.
@@ -895,10 +923,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     {
         debug!("probe()");
         let snapshot = self.start_snapshot();
-        let skip_leak_check = should_skip || self.skip_leak_check.get();
-        self.skip_leak_check.set(skip_leak_check);
+        let was_skip_leak_check = self.skip_leak_check.get();
+        if should_skip {
+            self.skip_leak_check.set(true);
+        }
         let r = f(&snapshot);
         self.rollback_to("probe", snapshot);
+        self.skip_leak_check.set(was_skip_leak_check);
         r
     }
 
@@ -914,7 +945,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.inner
             .borrow_mut()
             .unwrap_region_constraints()
-            .region_constraints_added_in_snapshot(&snapshot.region_constraints_snapshot)
+            .region_constraints_added_in_snapshot(&snapshot.undo_snapshot)
     }
 
     pub fn add_given(&self, sub: ty::Region<'tcx>, sup: ty::RegionVid) {
@@ -1032,7 +1063,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {
-        self.inner.borrow_mut().type_variables.new_var(self.universe(), diverging, origin)
+        self.inner.borrow_mut().type_variables().new_var(self.universe(), diverging, origin)
     }
 
     pub fn next_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
@@ -1044,7 +1075,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         origin: TypeVariableOrigin,
         universe: ty::UniverseIndex,
     ) -> Ty<'tcx> {
-        let vid = self.inner.borrow_mut().type_variables.new_var(universe, false, origin);
+        let vid = self.inner.borrow_mut().type_variables().new_var(universe, false, origin);
         self.tcx.mk_ty_var(vid)
     }
 
@@ -1069,20 +1100,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let vid = self
             .inner
             .borrow_mut()
-            .const_unification_table
+            .const_unification_table()
             .new_key(ConstVarValue { origin, val: ConstVariableValue::Unknown { universe } });
         self.tcx.mk_const_var(vid, ty)
     }
 
     pub fn next_const_var_id(&self, origin: ConstVariableOrigin) -> ConstVid<'tcx> {
-        self.inner.borrow_mut().const_unification_table.new_key(ConstVarValue {
+        self.inner.borrow_mut().const_unification_table().new_key(ConstVarValue {
             origin,
             val: ConstVariableValue::Unknown { universe: self.universe() },
         })
     }
 
     fn next_int_var_id(&self) -> IntVid {
-        self.inner.borrow_mut().int_unification_table.new_key(None)
+        self.inner.borrow_mut().int_unification_table().new_key(None)
     }
 
     pub fn next_int_var(&self) -> Ty<'tcx> {
@@ -1090,7 +1121,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     fn next_float_var_id(&self) -> FloatVid {
-        self.inner.borrow_mut().float_unification_table.new_key(None)
+        self.inner.borrow_mut().float_unification_table().new_key(None)
     }
 
     pub fn next_float_var(&self) -> Ty<'tcx> {
@@ -1161,7 +1192,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // used in a path such as `Foo::<T, U>::new()` will
                 // use an inference variable for `C` with `[T, U]`
                 // as the substitutions for the default, `(T, U)`.
-                let ty_var_id = self.inner.borrow_mut().type_variables.new_var(
+                let ty_var_id = self.inner.borrow_mut().type_variables().new_var(
                     self.universe(),
                     false,
                     TypeVariableOrigin {
@@ -1181,7 +1212,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     span,
                 };
                 let const_var_id =
-                    self.inner.borrow_mut().const_unification_table.new_key(ConstVarValue {
+                    self.inner.borrow_mut().const_unification_table().new_key(ConstVarValue {
                         origin,
                         val: ConstVariableValue::Unknown { universe: self.universe() },
                     });
@@ -1234,18 +1265,21 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         outlives_env: &OutlivesEnvironment<'tcx>,
         mode: RegionckMode,
     ) {
-        assert!(
-            self.is_tainted_by_errors() || self.inner.borrow().region_obligations.is_empty(),
-            "region_obligations not empty: {:#?}",
-            self.inner.borrow().region_obligations
-        );
-        let (var_infos, data) = self
-            .inner
-            .borrow_mut()
-            .region_constraints
-            .take()
-            .expect("regions already resolved")
-            .into_infos_and_data();
+        let (var_infos, data) = {
+            let mut inner = self.inner.borrow_mut();
+            let inner = &mut *inner;
+            assert!(
+                self.is_tainted_by_errors() || inner.region_obligations.is_empty(),
+                "region_obligations not empty: {:#?}",
+                inner.region_obligations
+            );
+            inner
+                .region_constraint_storage
+                .take()
+                .expect("regions already resolved")
+                .with_log(&mut inner.undo_log)
+                .into_infos_and_data()
+        };
 
         let region_rels = &RegionRelations::new(
             self.tcx,
@@ -1306,12 +1340,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// called. This is used only during NLL processing to "hand off" ownership
     /// of the set of region variables into the NLL region context.
     pub fn take_region_var_origins(&self) -> VarInfos {
-        let (var_infos, data) = self
-            .inner
-            .borrow_mut()
-            .region_constraints
+        let mut inner = self.inner.borrow_mut();
+        let (var_infos, data) = inner
+            .region_constraint_storage
             .take()
             .expect("regions already resolved")
+            .with_log(&mut inner.undo_log)
             .into_infos_and_data();
         assert!(data.is_empty());
         var_infos
@@ -1335,7 +1369,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn probe_ty_var(&self, vid: TyVid) -> Result<Ty<'tcx>, ty::UniverseIndex> {
         use self::type_variable::TypeVariableValue;
 
-        match self.inner.borrow_mut().type_variables.probe(vid) {
+        match self.inner.borrow_mut().type_variables().probe(vid) {
             TypeVariableValue::Known { value } => Ok(value),
             TypeVariableValue::Unknown { universe } => Err(universe),
         }
@@ -1357,7 +1391,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
-        self.inner.borrow_mut().type_variables.root_var(var)
+        self.inner.borrow_mut().type_variables().root_var(var)
     }
 
     /// Where possible, replaces type/const variables in
@@ -1395,7 +1429,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         &self,
         vid: ty::ConstVid<'tcx>,
     ) -> Result<&'tcx ty::Const<'tcx>, ty::UniverseIndex> {
-        match self.inner.borrow_mut().const_unification_table.probe_value(vid).val {
+        match self.inner.borrow_mut().const_unification_table().probe_value(vid).val {
             ConstVariableValue::Known { value } => Ok(value),
             ConstVariableValue::Unknown { universe } => Err(universe),
         }
@@ -1513,7 +1547,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn clear_caches(&self) {
         self.selection_cache.clear();
         self.evaluation_cache.clear();
-        self.inner.borrow_mut().projection_cache.clear();
+        self.inner.borrow_mut().projection_cache().clear();
     }
 
     fn universe(&self) -> ty::UniverseIndex {
@@ -1576,14 +1610,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 //
                 // Note: if these two lines are combined into one we get
                 // dynamic borrow errors on `self.inner`.
-                let known = self.inner.borrow_mut().type_variables.probe(v).known();
+                let known = self.inner.borrow_mut().type_variables().probe(v).known();
                 known.map(|t| self.shallow_resolve_ty(t)).unwrap_or(typ)
             }
 
             ty::Infer(ty::IntVar(v)) => self
                 .inner
                 .borrow_mut()
-                .int_unification_table
+                .int_unification_table()
                 .probe_value(v)
                 .map(|v| v.to_type(self.tcx))
                 .unwrap_or(typ),
@@ -1591,7 +1625,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             ty::Infer(ty::FloatVar(v)) => self
                 .inner
                 .borrow_mut()
-                .float_unification_table
+                .float_unification_table()
                 .probe_value(v)
                 .map(|v| v.to_type(self.tcx))
                 .unwrap_or(typ),
@@ -1611,13 +1645,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// having to resort to storing full `GenericArg`s in `stalled_on`.
     #[inline(always)]
     pub fn ty_or_const_infer_var_changed(&self, infer_var: TyOrConstInferVar<'tcx>) -> bool {
+        let mut inner = self.inner.borrow_mut();
         match infer_var {
             TyOrConstInferVar::Ty(v) => {
                 use self::type_variable::TypeVariableValue;
 
                 // If `inlined_probe` returns a `Known` value, it never equals
                 // `ty::Infer(ty::TyVar(v))`.
-                match self.inner.borrow_mut().type_variables.inlined_probe(v) {
+                match inner.type_variables().inlined_probe(v) {
                     TypeVariableValue::Unknown { .. } => false,
                     TypeVariableValue::Known { .. } => true,
                 }
@@ -1627,7 +1662,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // If `inlined_probe_value` returns a value it's always a
                 // `ty::Int(_)` or `ty::UInt(_)`, which never matches a
                 // `ty::Infer(_)`.
-                self.inner.borrow_mut().int_unification_table.inlined_probe_value(v).is_some()
+                inner.int_unification_table().inlined_probe_value(v).is_some()
             }
 
             TyOrConstInferVar::TyFloat(v) => {
@@ -1635,7 +1670,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // `ty::Float(_)`, which never matches a `ty::Infer(_)`.
                 //
                 // Not `inlined_probe_value(v)` because this call site is colder.
-                self.inner.borrow_mut().float_unification_table.probe_value(v).is_some()
+                inner.float_unification_table().probe_value(v).is_some()
             }
 
             TyOrConstInferVar::Const(v) => {
@@ -1643,7 +1678,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // `ty::ConstKind::Infer(ty::InferConst::Var(v))`.
                 //
                 // Not `inlined_probe_value(v)` because this call site is colder.
-                match self.inner.borrow_mut().const_unification_table.probe_value(v).val {
+                match inner.const_unification_table().probe_value(v).val {
                     ConstVariableValue::Unknown { .. } => false,
                     ConstVariableValue::Known { .. } => true,
                 }
@@ -1718,7 +1753,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
             self.infcx
                 .inner
                 .borrow_mut()
-                .const_unification_table
+                .const_unification_table()
                 .probe_value(*vid)
                 .val
                 .known()
diff --git a/src/librustc_infer/infer/nll_relate/mod.rs b/src/librustc_infer/infer/nll_relate/mod.rs
index a2907e6e373..7aea26987a2 100644
--- a/src/librustc_infer/infer/nll_relate/mod.rs
+++ b/src/librustc_infer/infer/nll_relate/mod.rs
@@ -311,7 +311,7 @@ where
         match value_ty.kind {
             ty::Infer(ty::TyVar(value_vid)) => {
                 // Two type variables: just equate them.
-                self.infcx.inner.borrow_mut().type_variables.equate(vid, value_vid);
+                self.infcx.inner.borrow_mut().type_variables().equate(vid, value_vid);
                 return Ok(value_ty);
             }
 
@@ -332,7 +332,7 @@ where
             assert!(!generalized_ty.has_infer_types_or_consts());
         }
 
-        self.infcx.inner.borrow_mut().type_variables.instantiate(vid, generalized_ty);
+        self.infcx.inner.borrow_mut().type_variables().instantiate(vid, generalized_ty);
 
         // The generalized values we extract from `canonical_var_values` have
         // been fully instantiated and hence the set of scopes we have
@@ -362,7 +362,7 @@ where
             delegate: &mut self.delegate,
             first_free_index: ty::INNERMOST,
             ambient_variance: self.ambient_variance,
-            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables.sub_root_var(for_vid),
+            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
             universe,
         };
 
@@ -859,7 +859,8 @@ where
             }
 
             ty::Infer(ty::TyVar(vid)) => {
-                let variables = &mut self.infcx.inner.borrow_mut().type_variables;
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variables = &mut inner.type_variables();
                 let vid = variables.root_var(vid);
                 let sub_vid = variables.sub_root_var(vid);
                 if sub_vid == self.for_vid_sub_root {
@@ -961,7 +962,8 @@ where
                 bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
             }
             ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                let variable_table = &mut self.infcx.inner.borrow_mut().const_unification_table;
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variable_table = &mut inner.const_unification_table();
                 let var_value = variable_table.probe_value(vid);
                 match var_value.val.known() {
                     Some(u) => self.relate(&u, &u),
diff --git a/src/librustc_infer/infer/outlives/obligations.rs b/src/librustc_infer/infer/outlives/obligations.rs
index c904926e9d9..48f6d937f2f 100644
--- a/src/librustc_infer/infer/outlives/obligations.rs
+++ b/src/librustc_infer/infer/outlives/obligations.rs
@@ -61,13 +61,16 @@
 
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::outlives::verify::VerifyBoundCx;
-use crate::infer::{self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, VerifyBound};
+use crate::infer::{
+    self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, UndoLog, VerifyBound,
+};
 use crate::traits::ObligationCause;
 use rustc_middle::ty::outlives::Component;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, Region, Ty, TyCtxt, TypeFoldable};
 
 use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::undo_log::UndoLogs;
 use rustc_hir as hir;
 use smallvec::smallvec;
 
@@ -84,7 +87,9 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     ) {
         debug!("register_region_obligation(body_id={:?}, obligation={:?})", body_id, obligation);
 
-        self.inner.borrow_mut().region_obligations.push((body_id, obligation));
+        let mut inner = self.inner.borrow_mut();
+        inner.undo_log.push(UndoLog::PushRegionObligation);
+        inner.region_obligations.push((body_id, obligation));
     }
 
     pub fn register_region_obligation_with_cause(
diff --git a/src/librustc_infer/infer/region_constraints/leak_check.rs b/src/librustc_infer/infer/region_constraints/leak_check.rs
index 18e86162eb5..473550d5433 100644
--- a/src/librustc_infer/infer/region_constraints/leak_check.rs
+++ b/src/librustc_infer/infer/region_constraints/leak_check.rs
@@ -1,9 +1,10 @@
 use super::*;
 use crate::infer::{CombinedSnapshot, PlaceholderMap};
+use rustc_data_structures::undo_log::UndoLogs;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::RelateResult;
 
-impl<'tcx> RegionConstraintCollector<'tcx> {
+impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     /// Searches region constraints created since `snapshot` that
     /// affect one of the placeholders in `placeholder_map`, returning
     /// an error if any of the placeholders are related to another
@@ -31,7 +32,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     ) -> RelateResult<'tcx, ()> {
         debug!("leak_check(placeholders={:?})", placeholder_map);
 
-        assert!(self.in_snapshot());
+        assert!(UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
 
         // Go through each placeholder that we created.
         for &placeholder_region in placeholder_map.values() {
@@ -45,7 +46,11 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             // in some way. This means any region that either outlives
             // or is outlived by a placeholder.
             let mut taint_set = TaintSet::new(TaintDirections::both(), placeholder_region);
-            taint_set.fixed_point(tcx, &self.undo_log, &self.data.verifys);
+            taint_set.fixed_point(
+                tcx,
+                self.undo_log.region_constraints(),
+                &self.storage.data.verifys,
+            );
             let tainted_regions = taint_set.into_set();
 
             // Report an error if two placeholders in the same universe
@@ -88,19 +93,21 @@ impl<'tcx> TaintSet<'tcx> {
         TaintSet { directions, regions }
     }
 
-    fn fixed_point(
+    fn fixed_point<'a>(
         &mut self,
         tcx: TyCtxt<'tcx>,
-        undo_log: &[UndoLog<'tcx>],
+        undo_log: impl IntoIterator<Item = &'a UndoLog<'tcx>> + Clone,
         verifys: &[Verify<'tcx>],
-    ) {
+    ) where
+        'tcx: 'a,
+    {
         let mut prev_len = 0;
         while prev_len < self.len() {
             debug!("tainted: prev_len = {:?} new_len = {:?}", prev_len, self.len());
 
             prev_len = self.len();
 
-            for undo_entry in undo_log {
+            for undo_entry in undo_log.clone() {
                 match undo_entry {
                     &AddConstraint(Constraint::VarSubVar(a, b)) => {
                         self.add_edge(tcx.mk_region(ReVar(a)), tcx.mk_region(ReVar(b)));
diff --git a/src/librustc_infer/infer/region_constraints/mod.rs b/src/librustc_infer/infer/region_constraints/mod.rs
index 2be6ec4481c..0c9f002a2a2 100644
--- a/src/librustc_infer/infer/region_constraints/mod.rs
+++ b/src/librustc_infer/infer/region_constraints/mod.rs
@@ -4,11 +4,15 @@ use self::CombineMapType::*;
 use self::UndoLog::*;
 
 use super::unify_key;
-use super::{MiscVariable, RegionVariableOrigin, SubregionOrigin};
+use super::{
+    InferCtxtUndoLogs, MiscVariable, RegionVariableOrigin, Rollback, Snapshot, SubregionOrigin,
+};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::undo_log::UndoLogs;
 use rustc_data_structures::unify as ut;
+use rustc_data_structures::unify::UnifyKey;
 use rustc_hir::def_id::DefId;
 use rustc_index::vec::IndexVec;
 use rustc_middle::ty::ReStatic;
@@ -26,7 +30,7 @@ mod leak_check;
 pub use rustc_middle::infer::MemberConstraint;
 
 #[derive(Default)]
-pub struct RegionConstraintCollector<'tcx> {
+pub struct RegionConstraintStorage<'tcx> {
     /// For each `RegionVid`, the corresponding `RegionVariableOrigin`.
     var_infos: IndexVec<RegionVid, RegionVariableInfo>,
 
@@ -42,20 +46,6 @@ pub struct RegionConstraintCollector<'tcx> {
     /// exist). This prevents us from making many such regions.
     glbs: CombineMap<'tcx>,
 
-    /// The undo log records actions that might later be undone.
-    ///
-    /// Note: `num_open_snapshots` is used to track if we are actively
-    /// snapshotting. When the `start_snapshot()` method is called, we
-    /// increment `num_open_snapshots` to indicate that we are now actively
-    /// snapshotting. The reason for this is that otherwise we end up adding
-    /// entries for things like the lower bound on a variable and so forth,
-    /// which can never be rolled back.
-    undo_log: Vec<UndoLog<'tcx>>,
-
-    /// The number of open snapshots, i.e., those that haven't been committed or
-    /// rolled back.
-    num_open_snapshots: usize,
-
     /// When we add a R1 == R2 constriant, we currently add (a) edges
     /// R1 <= R2 and R2 <= R1 and (b) we unify the two regions in this
     /// table. You can then call `opportunistic_resolve_var` early
@@ -64,13 +54,31 @@ pub struct RegionConstraintCollector<'tcx> {
     /// is iterating to a fixed point, because otherwise we sometimes
     /// would wind up with a fresh stream of region variables that
     /// have been equated but appear distinct.
-    unification_table: ut::UnificationTable<ut::InPlace<ty::RegionVid>>,
+    pub(super) unification_table: ut::UnificationTableStorage<ty::RegionVid>,
 
     /// a flag set to true when we perform any unifications; this is used
     /// to micro-optimize `take_and_reset_data`
     any_unifications: bool,
 }
 
+pub struct RegionConstraintCollector<'a, 'tcx> {
+    storage: &'a mut RegionConstraintStorage<'tcx>,
+    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+}
+
+impl std::ops::Deref for RegionConstraintCollector<'_, 'tcx> {
+    type Target = RegionConstraintStorage<'tcx>;
+    fn deref(&self) -> &RegionConstraintStorage<'tcx> {
+        self.storage
+    }
+}
+
+impl std::ops::DerefMut for RegionConstraintCollector<'_, 'tcx> {
+    fn deref_mut(&mut self) -> &mut RegionConstraintStorage<'tcx> {
+        self.storage
+    }
+}
+
 pub type VarInfos = IndexVec<RegionVid, RegionVariableInfo>;
 
 /// The full set of region constraints gathered up by the collector.
@@ -258,13 +266,13 @@ pub enum VerifyBound<'tcx> {
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
-struct TwoRegions<'tcx> {
+pub(crate) struct TwoRegions<'tcx> {
     a: Region<'tcx>,
     b: Region<'tcx>,
 }
 
 #[derive(Copy, Clone, PartialEq)]
-enum UndoLog<'tcx> {
+pub(crate) enum UndoLog<'tcx> {
     /// We added `RegionVid`.
     AddVar(RegionVid),
 
@@ -290,7 +298,7 @@ enum UndoLog<'tcx> {
 }
 
 #[derive(Copy, Clone, PartialEq)]
-enum CombineMapType {
+pub(crate) enum CombineMapType {
     Lub,
     Glb,
 }
@@ -304,8 +312,6 @@ pub struct RegionVariableInfo {
 }
 
 pub struct RegionSnapshot {
-    length: usize,
-    region_snapshot: ut::Snapshot<ut::InPlace<ty::RegionVid>>,
     any_unifications: bool,
 }
 
@@ -334,129 +340,16 @@ impl TaintDirections {
     }
 }
 
-impl<'tcx> RegionConstraintCollector<'tcx> {
+impl<'tcx> RegionConstraintStorage<'tcx> {
     pub fn new() -> Self {
         Self::default()
     }
 
-    pub fn num_region_vars(&self) -> usize {
-        self.var_infos.len()
-    }
-
-    pub fn region_constraint_data(&self) -> &RegionConstraintData<'tcx> {
-        &self.data
-    }
-
-    /// Once all the constraints have been gathered, extract out the final data.
-    ///
-    /// Not legal during a snapshot.
-    pub fn into_infos_and_data(self) -> (VarInfos, RegionConstraintData<'tcx>) {
-        assert!(!self.in_snapshot());
-        (self.var_infos, self.data)
-    }
-
-    /// Takes (and clears) the current set of constraints. Note that
-    /// the set of variables remains intact, but all relationships
-    /// between them are reset. This is used during NLL checking to
-    /// grab the set of constraints that arose from a particular
-    /// operation.
-    ///
-    /// We don't want to leak relationships between variables between
-    /// points because just because (say) `r1 == r2` was true at some
-    /// point P in the graph doesn't imply that it will be true at
-    /// some other point Q, in NLL.
-    ///
-    /// Not legal during a snapshot.
-    pub fn take_and_reset_data(&mut self) -> RegionConstraintData<'tcx> {
-        assert!(!self.in_snapshot());
-
-        // If you add a new field to `RegionConstraintCollector`, you
-        // should think carefully about whether it needs to be cleared
-        // or updated in some way.
-        let RegionConstraintCollector {
-            var_infos: _,
-            data,
-            lubs,
-            glbs,
-            undo_log: _,
-            num_open_snapshots: _,
-            unification_table,
-            any_unifications,
-        } = self;
-
-        // Clear the tables of (lubs, glbs), so that we will create
-        // fresh regions if we do a LUB operation. As it happens,
-        // LUB/GLB are not performed by the MIR type-checker, which is
-        // the one that uses this method, but it's good to be correct.
-        lubs.clear();
-        glbs.clear();
-
-        // Clear all unifications and recreate the variables a "now
-        // un-unified" state. Note that when we unify `a` and `b`, we
-        // also insert `a <= b` and a `b <= a` edges, so the
-        // `RegionConstraintData` contains the relationship here.
-        if *any_unifications {
-            unification_table.reset_unifications(|vid| unify_key::RegionVidKey { min_vid: vid });
-            *any_unifications = false;
-        }
-
-        mem::take(data)
-    }
-
-    pub fn data(&self) -> &RegionConstraintData<'tcx> {
-        &self.data
-    }
-
-    fn in_snapshot(&self) -> bool {
-        self.num_open_snapshots > 0
-    }
-
-    pub fn start_snapshot(&mut self) -> RegionSnapshot {
-        let length = self.undo_log.len();
-        debug!("RegionConstraintCollector: start_snapshot({})", length);
-        self.num_open_snapshots += 1;
-        RegionSnapshot {
-            length,
-            region_snapshot: self.unification_table.snapshot(),
-            any_unifications: self.any_unifications,
-        }
-    }
-
-    fn assert_open_snapshot(&self, snapshot: &RegionSnapshot) {
-        assert!(self.undo_log.len() >= snapshot.length);
-        assert!(self.num_open_snapshots > 0);
-    }
-
-    pub fn commit(&mut self, snapshot: RegionSnapshot) {
-        debug!("RegionConstraintCollector: commit({})", snapshot.length);
-        self.assert_open_snapshot(&snapshot);
-
-        if self.num_open_snapshots == 1 {
-            // The root snapshot. It's safe to clear the undo log because
-            // there's no snapshot further out that we might need to roll back
-            // to.
-            assert!(snapshot.length == 0);
-            self.undo_log.clear();
-        }
-
-        self.num_open_snapshots -= 1;
-
-        self.unification_table.commit(snapshot.region_snapshot);
-    }
-
-    pub fn rollback_to(&mut self, snapshot: RegionSnapshot) {
-        debug!("RegionConstraintCollector: rollback_to({:?})", snapshot);
-        self.assert_open_snapshot(&snapshot);
-
-        while self.undo_log.len() > snapshot.length {
-            let undo_entry = self.undo_log.pop().unwrap();
-            self.rollback_undo_entry(undo_entry);
-        }
-
-        self.num_open_snapshots -= 1;
-
-        self.unification_table.rollback_to(snapshot.region_snapshot);
-        self.any_unifications = snapshot.any_unifications;
+    pub(crate) fn with_log<'a>(
+        &'a mut self,
+        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+    ) -> RegionConstraintCollector<'a, 'tcx> {
+        RegionConstraintCollector { storage: self, undo_log }
     }
 
     fn rollback_undo_entry(&mut self, undo_entry: UndoLog<'tcx>) {
@@ -486,6 +379,87 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             }
         }
     }
+}
+
+impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
+    pub fn num_region_vars(&self) -> usize {
+        self.var_infos.len()
+    }
+
+    pub fn region_constraint_data(&self) -> &RegionConstraintData<'tcx> {
+        &self.data
+    }
+
+    /// Once all the constraints have been gathered, extract out the final data.
+    ///
+    /// Not legal during a snapshot.
+    pub fn into_infos_and_data(self) -> (VarInfos, RegionConstraintData<'tcx>) {
+        assert!(!UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
+        (mem::take(&mut self.storage.var_infos), mem::take(&mut self.storage.data))
+    }
+
+    /// Takes (and clears) the current set of constraints. Note that
+    /// the set of variables remains intact, but all relationships
+    /// between them are reset. This is used during NLL checking to
+    /// grab the set of constraints that arose from a particular
+    /// operation.
+    ///
+    /// We don't want to leak relationships between variables between
+    /// points because just because (say) `r1 == r2` was true at some
+    /// point P in the graph doesn't imply that it will be true at
+    /// some other point Q, in NLL.
+    ///
+    /// Not legal during a snapshot.
+    pub fn take_and_reset_data(&mut self) -> RegionConstraintData<'tcx> {
+        assert!(!UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
+
+        // If you add a new field to `RegionConstraintCollector`, you
+        // should think carefully about whether it needs to be cleared
+        // or updated in some way.
+        let RegionConstraintStorage {
+            var_infos: _,
+            data,
+            lubs,
+            glbs,
+            unification_table: _,
+            any_unifications,
+        } = self.storage;
+
+        // Clear the tables of (lubs, glbs), so that we will create
+        // fresh regions if we do a LUB operation. As it happens,
+        // LUB/GLB are not performed by the MIR type-checker, which is
+        // the one that uses this method, but it's good to be correct.
+        lubs.clear();
+        glbs.clear();
+
+        let data = mem::take(data);
+
+        // Clear all unifications and recreate the variables a "now
+        // un-unified" state. Note that when we unify `a` and `b`, we
+        // also insert `a <= b` and a `b <= a` edges, so the
+        // `RegionConstraintData` contains the relationship here.
+        if *any_unifications {
+            *any_unifications = false;
+            self.unification_table()
+                .reset_unifications(|vid| unify_key::RegionVidKey { min_vid: vid });
+        }
+
+        data
+    }
+
+    pub fn data(&self) -> &RegionConstraintData<'tcx> {
+        &self.data
+    }
+
+    pub fn start_snapshot(&mut self) -> RegionSnapshot {
+        debug!("RegionConstraintCollector: start_snapshot");
+        RegionSnapshot { any_unifications: self.any_unifications }
+    }
+
+    pub fn rollback_to(&mut self, snapshot: RegionSnapshot) {
+        debug!("RegionConstraintCollector: rollback_to({:?})", snapshot);
+        self.any_unifications = snapshot.any_unifications;
+    }
 
     pub fn new_region_var(
         &mut self,
@@ -494,11 +468,9 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     ) -> RegionVid {
         let vid = self.var_infos.push(RegionVariableInfo { origin, universe });
 
-        let u_vid = self.unification_table.new_key(unify_key::RegionVidKey { min_vid: vid });
+        let u_vid = self.unification_table().new_key(unify_key::RegionVidKey { min_vid: vid });
         assert_eq!(vid, u_vid);
-        if self.in_snapshot() {
-            self.undo_log.push(AddVar(vid));
-        }
+        self.undo_log.push(AddVar(vid));
         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
         vid
     }
@@ -520,19 +492,29 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     pub fn pop_placeholders(&mut self, placeholders: &FxHashSet<ty::Region<'tcx>>) {
         debug!("pop_placeholders(placeholders={:?})", placeholders);
 
-        assert!(self.in_snapshot());
+        assert!(UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
 
         let constraints_to_kill: Vec<usize> = self
             .undo_log
             .iter()
             .enumerate()
             .rev()
-            .filter(|&(_, undo_entry)| kill_constraint(placeholders, undo_entry))
+            .filter(|&(_, undo_entry)| match undo_entry {
+                super::UndoLog::RegionConstraintCollector(undo_entry) => {
+                    kill_constraint(placeholders, undo_entry)
+                }
+                _ => false,
+            })
             .map(|(index, _)| index)
             .collect();
 
         for index in constraints_to_kill {
-            let undo_entry = mem::replace(&mut self.undo_log[index], Purged);
+            let undo_entry = match &mut self.undo_log[index] {
+                super::UndoLog::RegionConstraintCollector(undo_entry) => {
+                    mem::replace(undo_entry, Purged)
+                }
+                _ => unreachable!(),
+            };
             self.rollback_undo_entry(undo_entry);
         }
 
@@ -566,12 +548,9 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         // never overwrite an existing (constraint, origin) - only insert one if it isn't
         // present in the map yet. This prevents origins from outside the snapshot being
         // replaced with "less informative" origins e.g., during calls to `can_eq`
-        let in_snapshot = self.in_snapshot();
         let undo_log = &mut self.undo_log;
-        self.data.constraints.entry(constraint).or_insert_with(|| {
-            if in_snapshot {
-                undo_log.push(AddConstraint(constraint));
-            }
+        self.storage.data.constraints.entry(constraint).or_insert_with(|| {
+            undo_log.push(AddConstraint(constraint));
             origin
         });
     }
@@ -589,9 +568,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
 
         let index = self.data.verifys.len();
         self.data.verifys.push(verify);
-        if self.in_snapshot() {
-            self.undo_log.push(AddVerify(index));
-        }
+        self.undo_log.push(AddVerify(index));
     }
 
     pub fn add_given(&mut self, sub: Region<'tcx>, sup: ty::RegionVid) {
@@ -599,9 +576,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         if self.data.givens.insert((sub, sup)) {
             debug!("add_given({:?} <= {:?})", sub, sup);
 
-            if self.in_snapshot() {
-                self.undo_log.push(AddGiven(sub, sup));
-            }
+            self.undo_log.push(AddGiven(sub, sup));
         }
     }
 
@@ -619,7 +594,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
 
             if let (ty::ReVar(sub), ty::ReVar(sup)) = (*sub, *sup) {
                 debug!("make_eqregion: uniying {:?} with {:?}", sub, sup);
-                self.unification_table.union(sub, sup);
+                self.unification_table().union(sub, sup);
                 self.any_unifications = true;
             }
         }
@@ -741,7 +716,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         tcx: TyCtxt<'tcx>,
         rid: RegionVid,
     ) -> ty::Region<'tcx> {
-        let vid = self.unification_table.probe_value(rid).min_vid;
+        let vid = self.unification_table().probe_value(rid).min_vid;
         tcx.mk_region(ty::ReVar(vid))
     }
 
@@ -769,9 +744,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         let c_universe = cmp::max(a_universe, b_universe);
         let c = self.new_region_var(c_universe, MiscVariable(origin.span()));
         self.combine_map(t).insert(vars, c);
-        if self.in_snapshot() {
-            self.undo_log.push(AddCombination(t, vars));
-        }
+        self.undo_log.push(AddCombination(t, vars));
         let new_r = tcx.mk_region(ReVar(c));
         for &old_r in &[a, b] {
             match t {
@@ -799,9 +772,10 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
 
     pub fn vars_since_snapshot(
         &self,
-        mark: &RegionSnapshot,
+        value_count: usize,
     ) -> (Range<RegionVid>, Vec<RegionVariableOrigin>) {
-        let range = self.unification_table.vars_since_snapshot(&mark.region_snapshot);
+        let range = RegionVid::from_index(value_count as u32)
+            ..RegionVid::from_index(self.unification_table.len() as u32);
         (
             range.clone(),
             (range.start.index()..range.end.index())
@@ -810,10 +784,10 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         )
     }
 
-    /// See `InferCtxt::region_constraints_added_in_snapshot`.
-    pub fn region_constraints_added_in_snapshot(&self, mark: &RegionSnapshot) -> Option<bool> {
-        self.undo_log[mark.length..]
-            .iter()
+    /// See [`RegionInference::region_constraints_added_in_snapshot`].
+    pub fn region_constraints_added_in_snapshot(&self, mark: &Snapshot<'tcx>) -> Option<bool> {
+        self.undo_log
+            .region_constraints_in_snapshot(mark)
             .map(|&elt| match elt {
                 AddConstraint(constraint) => Some(constraint.involves_placeholders()),
                 _ => None,
@@ -821,11 +795,15 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             .max()
             .unwrap_or(None)
     }
+
+    fn unification_table(&mut self) -> super::UnificationTable<'_, 'tcx, ty::RegionVid> {
+        ut::UnificationTable::with_log(&mut self.storage.unification_table, self.undo_log)
+    }
 }
 
 impl fmt::Debug for RegionSnapshot {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "RegionSnapshot(length={})", self.length)
+        write!(f, "RegionSnapshot")
     }
 }
 
@@ -910,3 +888,9 @@ impl<'tcx> RegionConstraintData<'tcx> {
             && givens.is_empty()
     }
 }
+
+impl<'tcx> Rollback<UndoLog<'tcx>> for RegionConstraintStorage<'tcx> {
+    fn reverse(&mut self, undo: UndoLog<'tcx>) {
+        self.rollback_undo_entry(undo)
+    }
+}
diff --git a/src/librustc_infer/infer/resolve.rs b/src/librustc_infer/infer/resolve.rs
index bd9d108cfe8..e28cf49c7f2 100644
--- a/src/librustc_infer/infer/resolve.rs
+++ b/src/librustc_infer/infer/resolve.rs
@@ -123,7 +123,8 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for UnresolvedTypeFinder<'a, 'tcx> {
                 // Since we called `shallow_resolve` above, this must
                 // be an (as yet...) unresolved inference variable.
                 let ty_var_span = if let ty::TyVar(ty_vid) = infer_ty {
-                    let ty_vars = &self.infcx.inner.borrow().type_variables;
+                    let mut inner = self.infcx.inner.borrow_mut();
+                    let ty_vars = &inner.type_variables();
                     if let TypeVariableOrigin {
                         kind: TypeVariableOriginKind::TypeParameterDefinition(_, _),
                         span,
diff --git a/src/librustc_infer/infer/sub.rs b/src/librustc_infer/infer/sub.rs
index 080af37492d..0abcc15d6fc 100644
--- a/src/librustc_infer/infer/sub.rs
+++ b/src/librustc_infer/infer/sub.rs
@@ -80,8 +80,8 @@ impl TypeRelation<'tcx> for Sub<'combine, 'infcx, 'tcx> {
         }
 
         let infcx = self.fields.infcx;
-        let a = infcx.inner.borrow_mut().type_variables.replace_if_possible(a);
-        let b = infcx.inner.borrow_mut().type_variables.replace_if_possible(b);
+        let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
+        let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
         match (&a.kind, &b.kind) {
             (&ty::Infer(TyVar(a_vid)), &ty::Infer(TyVar(b_vid))) => {
                 // Shouldn't have any LBR here, so we can safely put
@@ -95,7 +95,7 @@ impl TypeRelation<'tcx> for Sub<'combine, 'infcx, 'tcx> {
                 // have to record in the `type_variables` tracker that
                 // the two variables are equal modulo subtyping, which
                 // is important to the occurs check later on.
-                infcx.inner.borrow_mut().type_variables.sub(a_vid, b_vid);
+                infcx.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
                 self.fields.obligations.push(Obligation::new(
                     self.fields.trace.cause.clone(),
                     self.fields.param_env,
diff --git a/src/librustc_infer/infer/type_variable.rs b/src/librustc_infer/infer/type_variable.rs
index 1de820cdb62..f68692391a2 100644
--- a/src/librustc_infer/infer/type_variable.rs
+++ b/src/librustc_infer/infer/type_variable.rs
@@ -3,19 +3,68 @@ use rustc_middle::ty::{self, Ty, TyVid};
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 
+use crate::infer::InferCtxtUndoLogs;
+
 use rustc_data_structures::snapshot_vec as sv;
 use rustc_data_structures::unify as ut;
 use std::cmp;
 use std::marker::PhantomData;
 use std::ops::Range;
 
-pub struct TypeVariableTable<'tcx> {
-    values: sv::SnapshotVec<Delegate>,
+use rustc_data_structures::undo_log::{Rollback, UndoLogs};
+
+/// Represents a single undo-able action that affects a type inference variable.
+pub(crate) enum UndoLog<'tcx> {
+    EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
+    SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
+    Values(sv::UndoLog<Delegate>),
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for UndoLog<'tcx> {
+    fn from(l: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) -> Self {
+        UndoLog::EqRelation(l)
+    }
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<sv::UndoLog<ut::Delegate<ty::TyVid>>> for UndoLog<'tcx> {
+    fn from(l: sv::UndoLog<ut::Delegate<ty::TyVid>>) -> Self {
+        UndoLog::SubRelation(l)
+    }
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<sv::UndoLog<Delegate>> for UndoLog<'tcx> {
+    fn from(l: sv::UndoLog<Delegate>) -> Self {
+        UndoLog::Values(l)
+    }
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<Instantiate> for UndoLog<'tcx> {
+    fn from(l: Instantiate) -> Self {
+        UndoLog::Values(sv::UndoLog::Other(l))
+    }
+}
+
+impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
+    fn reverse(&mut self, undo: UndoLog<'tcx>) {
+        match undo {
+            UndoLog::EqRelation(undo) => self.eq_relations.reverse(undo),
+            UndoLog::SubRelation(undo) => self.sub_relations.reverse(undo),
+            UndoLog::Values(undo) => self.values.reverse(undo),
+        }
+    }
+}
+
+pub struct TypeVariableStorage<'tcx> {
+    values: sv::SnapshotVecStorage<Delegate>,
 
     /// Two variables are unified in `eq_relations` when we have a
     /// constraint `?X == ?Y`. This table also stores, for each key,
     /// the known value.
-    eq_relations: ut::UnificationTable<ut::InPlace<TyVidEqKey<'tcx>>>,
+    eq_relations: ut::UnificationTableStorage<TyVidEqKey<'tcx>>,
 
     /// Two variables are unified in `sub_relations` when we have a
     /// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
@@ -34,7 +83,17 @@ pub struct TypeVariableTable<'tcx> {
     /// This is reasonable because, in Rust, subtypes have the same
     /// "skeleton" and hence there is no possible type such that
     /// (e.g.)  `Box<?3> <: ?3` for any `?3`.
-    sub_relations: ut::UnificationTable<ut::InPlace<ty::TyVid>>,
+    sub_relations: ut::UnificationTableStorage<ty::TyVid>,
+}
+
+pub struct TypeVariableTable<'a, 'tcx> {
+    values: &'a mut sv::SnapshotVecStorage<Delegate>,
+
+    eq_relations: &'a mut ut::UnificationTableStorage<TyVidEqKey<'tcx>>,
+
+    sub_relations: &'a mut ut::UnificationTableStorage<ty::TyVid>,
+
+    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -62,7 +121,7 @@ pub enum TypeVariableOriginKind {
     LatticeVariable,
 }
 
-struct TypeVariableData {
+pub(crate) struct TypeVariableData {
     origin: TypeVariableOrigin,
     diverging: bool,
 }
@@ -91,27 +150,31 @@ impl<'tcx> TypeVariableValue<'tcx> {
     }
 }
 
-pub struct Snapshot<'tcx> {
-    snapshot: sv::Snapshot,
-    eq_snapshot: ut::Snapshot<ut::InPlace<TyVidEqKey<'tcx>>>,
-    sub_snapshot: ut::Snapshot<ut::InPlace<ty::TyVid>>,
-}
-
-struct Instantiate {
+pub(crate) struct Instantiate {
     vid: ty::TyVid,
 }
 
-struct Delegate;
+pub(crate) struct Delegate;
 
-impl<'tcx> TypeVariableTable<'tcx> {
-    pub fn new() -> TypeVariableTable<'tcx> {
-        TypeVariableTable {
-            values: sv::SnapshotVec::new(),
-            eq_relations: ut::UnificationTable::new(),
-            sub_relations: ut::UnificationTable::new(),
+impl<'tcx> TypeVariableStorage<'tcx> {
+    pub fn new() -> TypeVariableStorage<'tcx> {
+        TypeVariableStorage {
+            values: sv::SnapshotVecStorage::new(),
+            eq_relations: ut::UnificationTableStorage::new(),
+            sub_relations: ut::UnificationTableStorage::new(),
         }
     }
 
+    pub(crate) fn with_log<'a>(
+        &'a mut self,
+        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+    ) -> TypeVariableTable<'a, 'tcx> {
+        let TypeVariableStorage { values, eq_relations, sub_relations } = self;
+        TypeVariableTable { values, eq_relations, sub_relations, undo_log }
+    }
+}
+
+impl<'tcx> TypeVariableTable<'_, 'tcx> {
     /// Returns the diverges flag given when `vid` was created.
     ///
     /// Note that this function does not return care whether
@@ -134,8 +197,8 @@ impl<'tcx> TypeVariableTable<'tcx> {
     pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
         debug_assert!(self.probe(a).is_unknown());
         debug_assert!(self.probe(b).is_unknown());
-        self.eq_relations.union(a, b);
-        self.sub_relations.union(a, b);
+        self.eq_relations().union(a, b);
+        self.sub_relations().union(a, b);
     }
 
     /// Records that `a <: b`, depending on `dir`.
@@ -144,7 +207,7 @@ impl<'tcx> TypeVariableTable<'tcx> {
     pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
         debug_assert!(self.probe(a).is_unknown());
         debug_assert!(self.probe(b).is_unknown());
-        self.sub_relations.union(a, b);
+        self.sub_relations().union(a, b);
     }
 
     /// Instantiates `vid` with the type `ty`.
@@ -154,18 +217,18 @@ impl<'tcx> TypeVariableTable<'tcx> {
         let vid = self.root_var(vid);
         debug_assert!(self.probe(vid).is_unknown());
         debug_assert!(
-            self.eq_relations.probe_value(vid).is_unknown(),
+            self.eq_relations().probe_value(vid).is_unknown(),
             "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
             vid,
             ty,
-            self.eq_relations.probe_value(vid)
+            self.eq_relations().probe_value(vid)
         );
-        self.eq_relations.union_value(vid, TypeVariableValue::Known { value: ty });
+        self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });
 
         // Hack: we only need this so that `types_escaping_snapshot`
         // can see what has been unified; see the Delegate impl for
         // more details.
-        self.values.record(Instantiate { vid });
+        self.undo_log.push(Instantiate { vid });
     }
 
     /// Creates a new type variable.
@@ -184,12 +247,12 @@ impl<'tcx> TypeVariableTable<'tcx> {
         diverging: bool,
         origin: TypeVariableOrigin,
     ) -> ty::TyVid {
-        let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
+        let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });
 
-        let sub_key = self.sub_relations.new_key(());
+        let sub_key = self.sub_relations().new_key(());
         assert_eq!(eq_key.vid, sub_key);
 
-        let index = self.values.push(TypeVariableData { origin, diverging });
+        let index = self.values().push(TypeVariableData { origin, diverging });
         assert_eq!(eq_key.vid.index, index as u32);
 
         debug!(
@@ -211,7 +274,7 @@ impl<'tcx> TypeVariableTable<'tcx> {
     /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
     /// b` (transitively).
     pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
-        self.eq_relations.find(vid).vid
+        self.eq_relations().find(vid).vid
     }
 
     /// Returns the "root" variable of `vid` in the `sub_relations`
@@ -222,7 +285,7 @@ impl<'tcx> TypeVariableTable<'tcx> {
     ///
     ///     exists X. (a <: X || X <: a) && (b <: X || X <: b)
     pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
-        self.sub_relations.find(vid)
+        self.sub_relations().find(vid)
     }
 
     /// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
@@ -240,7 +303,7 @@ impl<'tcx> TypeVariableTable<'tcx> {
     /// An always-inlined variant of `probe`, for very hot call sites.
     #[inline(always)]
     pub fn inlined_probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
-        self.eq_relations.inlined_probe_value(vid)
+        self.eq_relations().inlined_probe_value(vid)
     }
 
     /// If `t` is a type-inference variable, and it has been
@@ -256,56 +319,29 @@ impl<'tcx> TypeVariableTable<'tcx> {
         }
     }
 
-    /// Creates a snapshot of the type variable state. This snapshot
-    /// must later be committed (`commit()`) or rolled back
-    /// (`rollback_to()`). Nested snapshots are permitted, but must
-    /// be processed in a stack-like fashion.
-    pub fn snapshot(&mut self) -> Snapshot<'tcx> {
-        Snapshot {
-            snapshot: self.values.start_snapshot(),
-            eq_snapshot: self.eq_relations.snapshot(),
-            sub_snapshot: self.sub_relations.snapshot(),
-        }
+    fn values(
+        &mut self,
+    ) -> sv::SnapshotVec<Delegate, &mut Vec<TypeVariableData>, &mut InferCtxtUndoLogs<'tcx>> {
+        self.values.with_log(self.undo_log)
     }
 
-    /// Undoes all changes since the snapshot was created. Any
-    /// snapshots created since that point must already have been
-    /// committed or rolled back.
-    pub fn rollback_to(&mut self, s: Snapshot<'tcx>) {
-        debug!("rollback_to{:?}", {
-            for action in self.values.actions_since_snapshot(&s.snapshot) {
-                if let sv::UndoLog::NewElem(index) = *action {
-                    debug!("inference variable _#{}t popped", index)
-                }
-            }
-        });
-
-        let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
-        self.values.rollback_to(snapshot);
-        self.eq_relations.rollback_to(eq_snapshot);
-        self.sub_relations.rollback_to(sub_snapshot);
+    fn eq_relations(&mut self) -> super::UnificationTable<'_, 'tcx, TyVidEqKey<'tcx>> {
+        self.eq_relations.with_log(self.undo_log)
     }
 
-    /// Commits all changes since the snapshot was created, making
-    /// them permanent (unless this snapshot was created within
-    /// another snapshot). Any snapshots created since that point
-    /// must already have been committed or rolled back.
-    pub fn commit(&mut self, s: Snapshot<'tcx>) {
-        let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
-        self.values.commit(snapshot);
-        self.eq_relations.commit(eq_snapshot);
-        self.sub_relations.commit(sub_snapshot);
+    fn sub_relations(&mut self) -> super::UnificationTable<'_, 'tcx, ty::TyVid> {
+        self.sub_relations.with_log(self.undo_log)
     }
 
     /// Returns a range of the type variables created during the snapshot.
     pub fn vars_since_snapshot(
         &mut self,
-        s: &Snapshot<'tcx>,
+        value_count: usize,
     ) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
-        let range = self.eq_relations.vars_since_snapshot(&s.eq_snapshot);
+        let range = TyVid { index: value_count as u32 }..TyVid { index: self.num_vars() as u32 };
         (
-            range.start.vid..range.end.vid,
-            (range.start.vid.index..range.end.vid.index)
+            range.start..range.end,
+            (range.start.index..range.end.index)
                 .map(|index| self.values.get(index as usize).origin)
                 .collect(),
         )
@@ -317,14 +353,15 @@ impl<'tcx> TypeVariableTable<'tcx> {
     /// a type variable `V0`, then we started the snapshot, then we
     /// created a type variable `V1`, unified `V0` with `T0`, and
     /// unified `V1` with `T1`, this function would return `{T0}`.
-    pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec<Ty<'tcx>> {
+    pub fn types_escaping_snapshot(&mut self, s: &super::Snapshot<'tcx>) -> Vec<Ty<'tcx>> {
         let mut new_elem_threshold = u32::MAX;
         let mut escaping_types = Vec::new();
-        let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
+        let actions_since_snapshot = self.undo_log.actions_since_snapshot(s);
         debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len());
-        for action in actions_since_snapshot {
-            match *action {
-                sv::UndoLog::NewElem(index) => {
+        for i in 0..actions_since_snapshot.len() {
+            let actions_since_snapshot = self.undo_log.actions_since_snapshot(s);
+            match actions_since_snapshot[i] {
+                super::UndoLog::TypeVariables(UndoLog::Values(sv::UndoLog::NewElem(index))) => {
                     // if any new variables were created during the
                     // snapshot, remember the lower index (which will
                     // always be the first one we see). Note that this
@@ -334,11 +371,17 @@ impl<'tcx> TypeVariableTable<'tcx> {
                     debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold);
                 }
 
-                sv::UndoLog::Other(Instantiate { vid, .. }) => {
+                super::UndoLog::TypeVariables(UndoLog::Values(sv::UndoLog::Other(
+                    Instantiate { vid, .. },
+                ))) => {
                     if vid.index < new_elem_threshold {
                         // quick check to see if this variable was
                         // created since the snapshot started or not.
-                        let escaping_type = match self.eq_relations.probe_value(vid) {
+                        let mut eq_relations = ut::UnificationTable::with_log(
+                            &mut *self.eq_relations,
+                            &mut *self.undo_log,
+                        );
+                        let escaping_type = match eq_relations.probe_value(vid) {
                             TypeVariableValue::Unknown { .. } => bug!(),
                             TypeVariableValue::Known { value } => value,
                         };
@@ -395,7 +438,7 @@ impl sv::SnapshotVecDelegate for Delegate {
 /// for the `eq_relations`; they carry a `TypeVariableValue` along
 /// with them.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
-struct TyVidEqKey<'tcx> {
+pub(crate) struct TyVidEqKey<'tcx> {
     vid: ty::TyVid,
 
     // in the table, we map each ty-vid to one of these:
diff --git a/src/librustc_infer/infer/undo_log.rs b/src/librustc_infer/infer/undo_log.rs
new file mode 100644
index 00000000000..56cb182dbf0
--- /dev/null
+++ b/src/librustc_infer/infer/undo_log.rs
@@ -0,0 +1,217 @@
+use std::marker::PhantomData;
+
+use rustc_data_structures::snapshot_vec as sv;
+use rustc_data_structures::undo_log::{Rollback, UndoLogs};
+use rustc_data_structures::unify as ut;
+use rustc_middle::ty;
+
+use crate::{
+    infer::{region_constraints, type_variable, InferCtxtInner},
+    traits,
+};
+
+pub struct Snapshot<'tcx> {
+    pub(crate) undo_len: usize,
+    _marker: PhantomData<&'tcx ()>,
+}
+
+/// Records the 'undo' data fora single operation that affects some form of inference variable.
+pub(crate) enum UndoLog<'tcx> {
+    TypeVariables(type_variable::UndoLog<'tcx>),
+    ConstUnificationTable(sv::UndoLog<ut::Delegate<ty::ConstVid<'tcx>>>),
+    IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
+    FloatUnificationTable(sv::UndoLog<ut::Delegate<ty::FloatVid>>),
+    RegionConstraintCollector(region_constraints::UndoLog<'tcx>),
+    RegionUnificationTable(sv::UndoLog<ut::Delegate<ty::RegionVid>>),
+    ProjectionCache(traits::UndoLog<'tcx>),
+    PushRegionObligation,
+}
+
+macro_rules! impl_from {
+    ($($ctor: ident ($ty: ty),)*) => {
+        $(
+        impl<'tcx> From<$ty> for UndoLog<'tcx> {
+            fn from(x: $ty) -> Self {
+                UndoLog::$ctor(x.into())
+            }
+        }
+        )*
+    }
+}
+
+// Upcast from a single kind of "undoable action" to the general enum
+impl_from! {
+    RegionConstraintCollector(region_constraints::UndoLog<'tcx>),
+    TypeVariables(type_variable::UndoLog<'tcx>),
+
+    TypeVariables(sv::UndoLog<ut::Delegate<type_variable::TyVidEqKey<'tcx>>>),
+    TypeVariables(sv::UndoLog<ut::Delegate<ty::TyVid>>),
+    TypeVariables(sv::UndoLog<type_variable::Delegate>),
+    TypeVariables(type_variable::Instantiate),
+
+    IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
+
+    FloatUnificationTable(sv::UndoLog<ut::Delegate<ty::FloatVid>>),
+
+    ConstUnificationTable(sv::UndoLog<ut::Delegate<ty::ConstVid<'tcx>>>),
+
+    RegionUnificationTable(sv::UndoLog<ut::Delegate<ty::RegionVid>>),
+    ProjectionCache(traits::UndoLog<'tcx>),
+}
+
+/// The Rollback trait defines how to rollback a particular action.
+impl<'tcx> Rollback<UndoLog<'tcx>> for InferCtxtInner<'tcx> {
+    fn reverse(&mut self, undo: UndoLog<'tcx>) {
+        match undo {
+            UndoLog::TypeVariables(undo) => self.type_variable_storage.reverse(undo),
+            UndoLog::ConstUnificationTable(undo) => self.const_unification_storage.reverse(undo),
+            UndoLog::IntUnificationTable(undo) => self.int_unification_storage.reverse(undo),
+            UndoLog::FloatUnificationTable(undo) => self.float_unification_storage.reverse(undo),
+            UndoLog::RegionConstraintCollector(undo) => {
+                self.region_constraint_storage.as_mut().unwrap().reverse(undo)
+            }
+            UndoLog::RegionUnificationTable(undo) => {
+                self.region_constraint_storage.as_mut().unwrap().unification_table.reverse(undo)
+            }
+            UndoLog::ProjectionCache(undo) => self.projection_cache.reverse(undo),
+            UndoLog::PushRegionObligation => {
+                self.region_obligations.pop();
+            }
+        }
+    }
+}
+
+/// The combined undo log for all the various unification tables. For each change to the storage
+/// for any kind of inference variable, we record an UndoLog entry in the vector here.
+pub(crate) struct InferCtxtUndoLogs<'tcx> {
+    logs: Vec<UndoLog<'tcx>>,
+    num_open_snapshots: usize,
+}
+
+impl Default for InferCtxtUndoLogs<'_> {
+    fn default() -> Self {
+        Self { logs: Default::default(), num_open_snapshots: Default::default() }
+    }
+}
+
+/// The UndoLogs trait defines how we undo a particular kind of action (of type T). We can undo any
+/// action that is convertable into a UndoLog (per the From impls above).
+impl<'tcx, T> UndoLogs<T> for InferCtxtUndoLogs<'tcx>
+where
+    UndoLog<'tcx>: From<T>,
+{
+    fn num_open_snapshots(&self) -> usize {
+        self.num_open_snapshots
+    }
+
+    fn push(&mut self, undo: T) {
+        if self.in_snapshot() {
+            self.logs.push(undo.into())
+        }
+    }
+
+    fn clear(&mut self) {
+        self.logs.clear();
+        self.num_open_snapshots = 0;
+    }
+
+    fn extend<J>(&mut self, undos: J)
+    where
+        Self: Sized,
+        J: IntoIterator<Item = T>,
+    {
+        if self.in_snapshot() {
+            self.logs.extend(undos.into_iter().map(UndoLog::from))
+        }
+    }
+}
+
+impl<'tcx> InferCtxtInner<'tcx> {
+    pub fn rollback_to(&mut self, snapshot: Snapshot<'tcx>) {
+        debug!("rollback_to({})", snapshot.undo_len);
+        self.undo_log.assert_open_snapshot(&snapshot);
+
+        while self.undo_log.logs.len() > snapshot.undo_len {
+            let undo = self.undo_log.logs.pop().unwrap();
+            self.reverse(undo);
+        }
+
+        if self.undo_log.num_open_snapshots == 1 {
+            // The root snapshot. It's safe to clear the undo log because
+            // there's no snapshot further out that we might need to roll back
+            // to.
+            assert!(snapshot.undo_len == 0);
+            self.undo_log.logs.clear();
+        }
+
+        self.undo_log.num_open_snapshots -= 1;
+    }
+
+    pub fn commit(&mut self, snapshot: Snapshot<'tcx>) {
+        debug!("commit({})", snapshot.undo_len);
+
+        if self.undo_log.num_open_snapshots == 1 {
+            // The root snapshot. It's safe to clear the undo log because
+            // there's no snapshot further out that we might need to roll back
+            // to.
+            assert!(snapshot.undo_len == 0);
+            self.undo_log.logs.clear();
+        }
+
+        self.undo_log.num_open_snapshots -= 1;
+    }
+}
+
+impl<'tcx> InferCtxtUndoLogs<'tcx> {
+    pub fn actions_since_snapshot(&self, snapshot: &Snapshot<'tcx>) -> &[UndoLog<'tcx>] {
+        &self.logs[snapshot.undo_len..]
+    }
+
+    pub fn start_snapshot(&mut self) -> Snapshot<'tcx> {
+        self.num_open_snapshots += 1;
+        Snapshot { undo_len: self.logs.len(), _marker: PhantomData }
+    }
+
+    pub(crate) fn region_constraints_in_snapshot(
+        &self,
+        s: &Snapshot<'tcx>,
+    ) -> impl Iterator<Item = &'_ region_constraints::UndoLog<'tcx>> + Clone {
+        self.logs[s.undo_len..].iter().filter_map(|log| match log {
+            UndoLog::RegionConstraintCollector(log) => Some(log),
+            _ => None,
+        })
+    }
+
+    pub(crate) fn region_constraints(
+        &self,
+    ) -> impl Iterator<Item = &'_ region_constraints::UndoLog<'tcx>> + Clone {
+        self.logs.iter().filter_map(|log| match log {
+            UndoLog::RegionConstraintCollector(log) => Some(log),
+            _ => None,
+        })
+    }
+
+    fn assert_open_snapshot(&self, snapshot: &Snapshot<'tcx>) {
+        // Failures here may indicate a failure to follow a stack discipline.
+        assert!(self.logs.len() >= snapshot.undo_len);
+        assert!(self.num_open_snapshots > 0);
+    }
+
+    pub(crate) fn iter(&self) -> std::slice::Iter<'_, UndoLog<'tcx>> {
+        self.logs.iter()
+    }
+}
+
+impl<'tcx> std::ops::Index<usize> for InferCtxtUndoLogs<'tcx> {
+    type Output = UndoLog<'tcx>;
+
+    fn index(&self, key: usize) -> &Self::Output {
+        &self.logs[key]
+    }
+}
+
+impl<'tcx> std::ops::IndexMut<usize> for InferCtxtUndoLogs<'tcx> {
+    fn index_mut(&mut self, key: usize) -> &mut Self::Output {
+        &mut self.logs[key]
+    }
+}
diff --git a/src/librustc_infer/traits/mod.rs b/src/librustc_infer/traits/mod.rs
index 2210c663d14..a8585fd2935 100644
--- a/src/librustc_infer/traits/mod.rs
+++ b/src/librustc_infer/traits/mod.rs
@@ -20,9 +20,10 @@ pub use self::Vtable::*;
 
 pub use self::engine::{TraitEngine, TraitEngineExt};
 pub use self::project::MismatchedProjectionTypes;
+pub(crate) use self::project::UndoLog;
 pub use self::project::{
     Normalized, NormalizedTy, ProjectionCache, ProjectionCacheEntry, ProjectionCacheKey,
-    ProjectionCacheSnapshot, Reveal,
+    ProjectionCacheStorage, Reveal,
 };
 crate use self::util::elaborate_predicates;
 
diff --git a/src/librustc_infer/traits/project.rs b/src/librustc_infer/traits/project.rs
index 48375a9ddf4..f0d21a7d022 100644
--- a/src/librustc_infer/traits/project.rs
+++ b/src/librustc_infer/traits/project.rs
@@ -2,12 +2,19 @@
 
 use super::PredicateObligation;
 
-use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
-use rustc_middle::ty::fold::TypeFoldable;
+use crate::infer::InferCtxtUndoLogs;
+
+use rustc_data_structures::{
+    snapshot_map::{self, SnapshotMapRef, SnapshotMapStorage},
+    undo_log::Rollback,
+};
 use rustc_middle::ty::{self, Ty};
 
 pub use rustc_middle::traits::Reveal;
 
+pub(crate) type UndoLog<'tcx> =
+    snapshot_map::UndoLog<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>;
+
 #[derive(Clone)]
 pub struct MismatchedProjectionTypes<'tcx> {
     pub err: ty::error::TypeError<'tcx>,
@@ -58,9 +65,14 @@ impl<'tcx, T> Normalized<'tcx, T> {
 //
 // FIXME: we probably also want some sort of cross-infcx cache here to
 // reduce the amount of duplication. Let's see what we get with the Chalk reforms.
+pub struct ProjectionCache<'a, 'tcx> {
+    map: &'a mut SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
+    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+}
+
 #[derive(Default)]
-pub struct ProjectionCache<'tcx> {
-    map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
+pub struct ProjectionCacheStorage<'tcx> {
+    map: SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
 }
 
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
@@ -82,30 +94,29 @@ pub enum ProjectionCacheEntry<'tcx> {
     NormalizedTy(NormalizedTy<'tcx>),
 }
 
-// N.B., intentionally not Clone
-pub struct ProjectionCacheSnapshot {
-    snapshot: Snapshot,
+impl<'tcx> ProjectionCacheStorage<'tcx> {
+    pub(crate) fn with_log<'a>(
+        &'a mut self,
+        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+    ) -> ProjectionCache<'a, 'tcx> {
+        ProjectionCache { map: &mut self.map, undo_log }
+    }
 }
 
-impl<'tcx> ProjectionCache<'tcx> {
+impl<'tcx> ProjectionCache<'_, 'tcx> {
+    fn map(
+        &mut self,
+    ) -> SnapshotMapRef<
+        '_,
+        ProjectionCacheKey<'tcx>,
+        ProjectionCacheEntry<'tcx>,
+        InferCtxtUndoLogs<'tcx>,
+    > {
+        self.map.with_log(self.undo_log)
+    }
+
     pub fn clear(&mut self) {
-        self.map.clear();
-    }
-
-    pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
-        ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
-    }
-
-    pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
-        self.map.rollback_to(snapshot.snapshot);
-    }
-
-    pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
-        self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
-    }
-
-    pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
-        self.map.commit(snapshot.snapshot);
+        self.map().clear();
     }
 
     /// Try to start normalize `key`; returns an error if
@@ -115,11 +126,12 @@ impl<'tcx> ProjectionCache<'tcx> {
         &mut self,
         key: ProjectionCacheKey<'tcx>,
     ) -> Result<(), ProjectionCacheEntry<'tcx>> {
-        if let Some(entry) = self.map.get(&key) {
+        let mut map = self.map();
+        if let Some(entry) = map.get(&key) {
             return Err(entry.clone());
         }
 
-        self.map.insert(key, ProjectionCacheEntry::InProgress);
+        map.insert(key, ProjectionCacheEntry::InProgress);
         Ok(())
     }
 
@@ -129,7 +141,7 @@ impl<'tcx> ProjectionCache<'tcx> {
             "ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
             key, value
         );
-        let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
+        let fresh_key = self.map().insert(key, ProjectionCacheEntry::NormalizedTy(value));
         assert!(!fresh_key, "never started projecting `{:?}`", key);
     }
 
@@ -138,7 +150,8 @@ impl<'tcx> ProjectionCache<'tcx> {
     /// snapshot - if the snapshot is rolled back, the obligations will be
     /// marked as incomplete again).
     pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
-        let ty = match self.map.get(&key) {
+        let mut map = self.map();
+        let ty = match map.get(&key) {
             Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
                 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
                 ty.value
@@ -151,7 +164,7 @@ impl<'tcx> ProjectionCache<'tcx> {
             }
         };
 
-        self.map.insert(
+        map.insert(
             key,
             ProjectionCacheEntry::NormalizedTy(Normalized { value: ty, obligations: vec![] }),
         );
@@ -163,7 +176,7 @@ impl<'tcx> ProjectionCache<'tcx> {
         // We want to insert `ty` with no obligations. If the existing value
         // already has no obligations (as is common) we don't insert anything.
         if !ty.obligations.is_empty() {
-            self.map.insert(
+            self.map().insert(
                 key,
                 ProjectionCacheEntry::NormalizedTy(Normalized {
                     value: ty.value,
@@ -178,14 +191,20 @@ impl<'tcx> ProjectionCache<'tcx> {
     /// type information (in which case, the "fully resolved" key will
     /// be different).
     pub fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
-        let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
+        let fresh = self.map().insert(key, ProjectionCacheEntry::Ambiguous);
         assert!(!fresh, "never started projecting `{:?}`", key);
     }
 
     /// Indicates that trying to normalize `key` resulted in
     /// error.
     pub fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
-        let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
+        let fresh = self.map().insert(key, ProjectionCacheEntry::Error);
         assert!(!fresh, "never started projecting `{:?}`", key);
     }
 }
+
+impl<'tcx> Rollback<UndoLog<'tcx>> for ProjectionCacheStorage<'tcx> {
+    fn reverse(&mut self, undo: UndoLog<'tcx>) {
+        self.map.reverse(undo);
+    }
+}
diff --git a/src/librustc_middle/infer/unify_key.rs b/src/librustc_middle/infer/unify_key.rs
index e205453a48c..2580ac6bebd 100644
--- a/src/librustc_middle/infer/unify_key.rs
+++ b/src/librustc_middle/infer/unify_key.rs
@@ -1,6 +1,9 @@
 use crate::ty::{self, FloatVarValue, InferConst, IntVarValue, Ty, TyCtxt};
-use rustc_data_structures::unify::InPlace;
-use rustc_data_structures::unify::{EqUnifyValue, NoError, UnificationTable, UnifyKey, UnifyValue};
+use rustc_data_structures::snapshot_vec;
+use rustc_data_structures::undo_log::UndoLogs;
+use rustc_data_structures::unify::{
+    self, EqUnifyValue, InPlace, NoError, UnificationTable, UnifyKey, UnifyValue,
+};
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
 
@@ -212,10 +215,14 @@ impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
 
 impl<'tcx> EqUnifyValue for &'tcx ty::Const<'tcx> {}
 
-pub fn replace_if_possible(
-    table: &mut UnificationTable<InPlace<ty::ConstVid<'tcx>>>,
+pub fn replace_if_possible<V, L>(
+    table: &mut UnificationTable<InPlace<ty::ConstVid<'tcx>, V, L>>,
     c: &'tcx ty::Const<'tcx>,
-) -> &'tcx ty::Const<'tcx> {
+) -> &'tcx ty::Const<'tcx>
+where
+    V: snapshot_vec::VecLike<unify::Delegate<ty::ConstVid<'tcx>>>,
+    L: UndoLogs<snapshot_vec::UndoLog<unify::Delegate<ty::ConstVid<'tcx>>>>,
+{
     if let ty::Const { val: ty::ConstKind::Infer(InferConst::Var(vid)), .. } = c {
         match table.probe_value(*vid).val.known() {
             Some(c) => c,
diff --git a/src/librustc_trait_selection/traits/auto_trait.rs b/src/librustc_trait_selection/traits/auto_trait.rs
index 6326a87c5ed..e19ddcd9e5e 100644
--- a/src/librustc_trait_selection/traits/auto_trait.rs
+++ b/src/librustc_trait_selection/traits/auto_trait.rs
@@ -195,7 +195,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             let body_id_map: FxHashMap<_, _> = infcx
                 .inner
                 .borrow()
-                .region_obligations
+                .region_obligations()
                 .iter()
                 .map(|&(id, _)| (id, vec![]))
                 .collect();
diff --git a/src/librustc_trait_selection/traits/fulfill.rs b/src/librustc_trait_selection/traits/fulfill.rs
index 300acf95c99..1e056c96acd 100644
--- a/src/librustc_trait_selection/traits/fulfill.rs
+++ b/src/librustc_trait_selection/traits/fulfill.rs
@@ -240,9 +240,15 @@ struct FulfillProcessor<'a, 'b, 'tcx> {
     register_region_obligations: bool,
 }
 
-fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
+fn mk_pending(
+    infcx: &InferCtxt<'_, 'tcx>,
+    os: Vec<PredicateObligation<'tcx>>,
+) -> Vec<PendingPredicateObligation<'tcx>> {
     os.into_iter()
-        .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
+        .map(|mut o| {
+            o.predicate = infcx.resolve_vars_if_possible(&o.predicate);
+            PendingPredicateObligation { obligation: o, stalled_on: vec![] }
+        })
         .collect()
 }
 
@@ -312,6 +318,8 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 
         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 
+        let infcx = self.selcx.infcx();
+
         match obligation.predicate {
             ty::Predicate::Trait(ref data, _) => {
                 let trait_obligation = obligation.with(*data);
@@ -319,7 +327,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                 if data.is_global() {
                     // no type variables present, can use evaluation for better caching.
                     // FIXME: consider caching errors too.
-                    if self.selcx.infcx().predicate_must_hold_considering_regions(&obligation) {
+                    if infcx.predicate_must_hold_considering_regions(&obligation) {
                         debug!(
                             "selecting trait `{:?}` at depth {} evaluated to holds",
                             data, obligation.recursion_depth
@@ -334,7 +342,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                             "selecting trait `{:?}` at depth {} yielded Ok(Some)",
                             data, obligation.recursion_depth
                         );
-                        ProcessResult::Changed(mk_pending(vtable.nested_obligations()))
+                        ProcessResult::Changed(mk_pending(infcx, vtable.nested_obligations()))
                     }
                     Ok(None) => {
                         debug!(
@@ -351,7 +359,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 
                         debug!(
                             "process_predicate: pending obligation {:?} now stalled on {:?}",
-                            self.selcx.infcx().resolve_vars_if_possible(obligation),
+                            infcx.resolve_vars_if_possible(obligation),
                             pending_obligation.stalled_on
                         );
 
@@ -369,7 +377,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
             }
 
             ty::Predicate::RegionOutlives(ref binder) => {
-                match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
+                match infcx.region_outlives_predicate(&obligation.cause, binder) {
                     Ok(()) => ProcessResult::Changed(vec![]),
                     Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
                 }
@@ -428,7 +436,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                             trait_ref_type_vars(self.selcx, data.to_poly_trait_ref(tcx));
                         ProcessResult::Unchanged
                     }
-                    Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
+                    Ok(Some(os)) => ProcessResult::Changed(mk_pending(infcx, os)),
                     Err(e) => ProcessResult::Error(CodeProjectionError(e)),
                 }
             }
@@ -467,7 +475,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                             vec![TyOrConstInferVar::maybe_from_ty(ty).unwrap()];
                         ProcessResult::Unchanged
                     }
-                    Some(os) => ProcessResult::Changed(mk_pending(os)),
+                    Some(os) => ProcessResult::Changed(mk_pending(infcx, os)),
                 }
             }
 
@@ -485,7 +493,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         ];
                         ProcessResult::Unchanged
                     }
-                    Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
+                    Some(Ok(ok)) => ProcessResult::Changed(mk_pending(infcx, ok.obligations)),
                     Some(Err(err)) => {
                         let expected_found = ExpectedFound::new(
                             subtype.skip_binder().a_is_expected,
diff --git a/src/librustc_trait_selection/traits/project.rs b/src/librustc_trait_selection/traits/project.rs
index 2b4a0409fd1..7536b444351 100644
--- a/src/librustc_trait_selection/traits/project.rs
+++ b/src/librustc_trait_selection/traits/project.rs
@@ -471,7 +471,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
     // bounds. It might be the case that we want two distinct caches,
     // or else another kind of cache entry.
 
-    let cache_result = infcx.inner.borrow_mut().projection_cache.try_start(cache_key);
+    let cache_result = infcx.inner.borrow_mut().projection_cache().try_start(cache_key);
     match cache_result {
         Ok(()) => {}
         Err(ProjectionCacheEntry::Ambiguous) => {
@@ -537,7 +537,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             // Once we have inferred everything we need to know, we
             // can ignore the `obligations` from that point on.
             if infcx.unresolved_type_vars(&ty.value).is_none() {
-                infcx.inner.borrow_mut().projection_cache.complete_normalized(cache_key, &ty);
+                infcx.inner.borrow_mut().projection_cache().complete_normalized(cache_key, &ty);
             // No need to extend `obligations`.
             } else {
                 obligations.extend(ty.obligations);
@@ -604,7 +604,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             };
 
             let cache_value = prune_cache_value_obligations(infcx, &result);
-            infcx.inner.borrow_mut().projection_cache.insert_ty(cache_key, cache_value);
+            infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
             obligations.extend(result.obligations);
             Some(result.value)
         }
@@ -615,7 +615,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
                 projected_ty
             );
             let result = Normalized { value: projected_ty, obligations: vec![] };
-            infcx.inner.borrow_mut().projection_cache.insert_ty(cache_key, result.clone());
+            infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
             // No need to extend `obligations`.
             Some(result.value)
         }
@@ -624,7 +624,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
                 "opt_normalize_projection_type: \
                  too many candidates"
             );
-            infcx.inner.borrow_mut().projection_cache.ambiguous(cache_key);
+            infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
             None
         }
         Err(ProjectionTyError::TraitSelectionError(_)) => {
@@ -634,7 +634,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             // Trait`, which when processed will cause the error to be
             // reported later
 
-            infcx.inner.borrow_mut().projection_cache.error(cache_key);
+            infcx.inner.borrow_mut().projection_cache().error(cache_key);
             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
             obligations.extend(result.obligations);
             Some(result.value)
diff --git a/src/librustc_trait_selection/traits/select.rs b/src/librustc_trait_selection/traits/select.rs
index dfbb0742448..38590abf1f7 100644
--- a/src/librustc_trait_selection/traits/select.rs
+++ b/src/librustc_trait_selection/traits/select.rs
@@ -471,7 +471,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         if let Some(key) =
                             ProjectionCacheKey::from_poly_projection_predicate(self, data)
                         {
-                            self.infcx.inner.borrow_mut().projection_cache.complete(key);
+                            self.infcx.inner.borrow_mut().projection_cache().complete(key);
                         }
                         result
                     }