Rollup merge of #35040 - soltanmm:couple-is-a-pun, r=nikomatsakis

Use &mut for `CombineFields` in inference relations

Because dropping obligations on the floor makes a mess.

r? @nikomatsakis

9 files changed, 159 insertions, 150 deletions

diff --git a/src/librustc/infer/bivariate.rs b/src/librustc/infer/bivariate.rs
index 96b14a6c321..125f815feda 100644
--- a/src/librustc/infer/bivariate.rs
+++ b/src/librustc/infer/bivariate.rs
@@ -32,22 +32,27 @@ use ty::{self, Ty, TyCtxt};
 use ty::TyVar;
 use ty::relate::{Relate, RelateResult, TypeRelation};
 
-pub struct Bivariate<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    fields: CombineFields<'a, 'gcx, 'tcx>
+pub struct Bivariate<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
-impl<'a, 'gcx, 'tcx> Bivariate<'a, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'a, 'gcx, 'tcx>) -> Bivariate<'a, 'gcx, 'tcx> {
-        Bivariate { fields: fields }
+impl<'combine, 'infcx, 'gcx, 'tcx> Bivariate<'combine, 'infcx, 'gcx, 'tcx> {
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
+        -> Bivariate<'combine, 'infcx, 'gcx, 'tcx>
+    {
+        Bivariate { fields: fields, a_is_expected: a_is_expected }
     }
 }
 
-impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Bivariate<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
+    for Bivariate<'combine, 'infcx, 'gcx, 'tcx>
+{
     fn tag(&self) -> &'static str { "Bivariate" }
 
-    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> { self.fields.tcx() }
+    fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
 
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    fn a_is_expected(&self) -> bool { self.a_is_expected }
 
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              variance: ty::Variance,
@@ -86,12 +91,12 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Bivariate<'a, 'gcx, 'tcx>
             }
 
             (&ty::TyInfer(TyVar(a_id)), _) => {
-                self.fields.instantiate(b, BiTo, a_id)?;
+                self.fields.instantiate(b, BiTo, a_id, self.a_is_expected)?;
                 Ok(a)
             }
 
             (_, &ty::TyInfer(TyVar(b_id))) => {
-                self.fields.instantiate(a, BiTo, b_id)?;
+                self.fields.instantiate(a, BiTo, b_id, self.a_is_expected)?;
                 Ok(a)
             }
 
diff --git a/src/librustc/infer/combine.rs b/src/librustc/infer/combine.rs
index c9235d063cb..b4818f963b3 100644
--- a/src/librustc/infer/combine.rs
+++ b/src/librustc/infer/combine.rs
@@ -52,21 +52,20 @@ use syntax::ast;
 use syntax_pos::Span;
 
 #[derive(Clone)]
-pub struct CombineFields<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    pub infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
-    pub a_is_expected: bool,
+pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
     pub trace: TypeTrace<'tcx>,
     pub cause: Option<ty::relate::Cause>,
     pub obligations: PredicateObligations<'tcx>,
 }
 
-impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
+impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
     pub fn super_combine_tys<R>(&self,
                                 relation: &mut R,
                                 a: Ty<'tcx>,
                                 b: Ty<'tcx>)
                                 -> RelateResult<'tcx, Ty<'tcx>>
-        where R: TypeRelation<'a, 'gcx, 'tcx>
+        where R: TypeRelation<'infcx, 'gcx, 'tcx>
     {
         let a_is_expected = relation.a_is_expected();
 
@@ -150,42 +149,36 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     }
 }
 
-impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
-    pub fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> {
+impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
+    pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
         self.infcx.tcx
     }
 
-    pub fn switch_expected(&self) -> CombineFields<'a, 'gcx, 'tcx> {
-        CombineFields {
-            a_is_expected: !self.a_is_expected,
-            ..(*self).clone()
-        }
-    }
-
-    pub fn equate(&self) -> Equate<'a, 'gcx, 'tcx> {
-        Equate::new(self.clone())
+    pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
+        Equate::new(self, a_is_expected)
     }
 
-    pub fn bivariate(&self) -> Bivariate<'a, 'gcx, 'tcx> {
-        Bivariate::new(self.clone())
+    pub fn bivariate<'a>(&'a mut self, a_is_expected: bool) -> Bivariate<'a, 'infcx, 'gcx, 'tcx> {
+        Bivariate::new(self, a_is_expected)
     }
 
-    pub fn sub(&self) -> Sub<'a, 'gcx, 'tcx> {
-        Sub::new(self.clone())
+    pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
+        Sub::new(self, a_is_expected)
     }
 
-    pub fn lub(&self) -> Lub<'a, 'gcx, 'tcx> {
-        Lub::new(self.clone())
+    pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
+        Lub::new(self, a_is_expected)
     }
 
-    pub fn glb(&self) -> Glb<'a, 'gcx, 'tcx> {
-        Glb::new(self.clone())
+    pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
+        Glb::new(self, a_is_expected)
     }
 
-    pub fn instantiate(&self,
+    pub fn instantiate(&mut self,
                        a_ty: Ty<'tcx>,
                        dir: RelationDir,
-                       b_vid: ty::TyVid)
+                       b_vid: ty::TyVid,
+                       a_is_expected: bool)
                        -> RelateResult<'tcx, ()>
     {
         let mut stack = Vec::new();
@@ -255,10 +248,11 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
             // to associate causes/spans with each of the relations in
             // the stack to get this right.
             match dir {
-                BiTo => self.bivariate().relate(&a_ty, &b_ty),
-                EqTo => self.equate().relate(&a_ty, &b_ty),
-                SubtypeOf => self.sub().relate(&a_ty, &b_ty),
-                SupertypeOf => self.sub().relate_with_variance(ty::Contravariant, &a_ty, &b_ty),
+                BiTo => self.bivariate(a_is_expected).relate(&a_ty, &b_ty),
+                EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
+                SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
+                SupertypeOf => self.sub(a_is_expected).relate_with_variance(
+                    ty::Contravariant, &a_ty, &b_ty),
             }?;
         }
 
diff --git a/src/librustc/infer/equate.rs b/src/librustc/infer/equate.rs
index 408f22cf15c..e06f7303acb 100644
--- a/src/librustc/infer/equate.rs
+++ b/src/librustc/infer/equate.rs
@@ -15,29 +15,29 @@ use super::type_variable::{EqTo};
 use ty::{self, Ty, TyCtxt};
 use ty::TyVar;
 use ty::relate::{Relate, RelateResult, TypeRelation};
-use traits::PredicateObligations;
 
 /// Ensures `a` is made equal to `b`. Returns `a` on success.
-pub struct Equate<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    fields: CombineFields<'a, 'gcx, 'tcx>
+pub struct Equate<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
-impl<'a, 'gcx, 'tcx> Equate<'a, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'a, 'gcx, 'tcx>) -> Equate<'a, 'gcx, 'tcx> {
-        Equate { fields: fields }
-    }
-
-    pub fn obligations(self) -> PredicateObligations<'tcx> {
-        self.fields.obligations
+impl<'combine, 'infcx, 'gcx, 'tcx> Equate<'combine, 'infcx, 'gcx, 'tcx> {
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
+        -> Equate<'combine, 'infcx, 'gcx, 'tcx>
+    {
+        Equate { fields: fields, a_is_expected: a_is_expected }
     }
 }
 
-impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Equate<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
+    for Equate<'combine, 'infcx, 'gcx, 'tcx>
+{
     fn tag(&self) -> &'static str { "Equate" }
 
-    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> { self.fields.tcx() }
+    fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
 
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    fn a_is_expected(&self) -> bool { self.a_is_expected }
 
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              _: ty::Variance,
@@ -63,12 +63,12 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Equate<'a, 'gcx, 'tcx> {
             }
 
             (&ty::TyInfer(TyVar(a_id)), _) => {
-                self.fields.instantiate(b, EqTo, a_id)?;
+                self.fields.instantiate(b, EqTo, a_id, self.a_is_expected)?;
                 Ok(a)
             }
 
             (_, &ty::TyInfer(TyVar(b_id))) => {
-                self.fields.instantiate(a, EqTo, b_id)?;
+                self.fields.instantiate(a, EqTo, b_id, self.a_is_expected)?;
                 Ok(a)
             }
 
@@ -93,7 +93,7 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Equate<'a, 'gcx, 'tcx> {
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
-        self.fields.higher_ranked_sub(a, b)?;
-        self.fields.higher_ranked_sub(b, a)
+        self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
+        self.fields.higher_ranked_sub(b, a, self.a_is_expected)
     }
 }
diff --git a/src/librustc/infer/glb.rs b/src/librustc/infer/glb.rs
index b7085f0829f..5dd85a31a9a 100644
--- a/src/librustc/infer/glb.rs
+++ b/src/librustc/infer/glb.rs
@@ -15,29 +15,29 @@ use super::Subtype;
 
 use ty::{self, Ty, TyCtxt};
 use ty::relate::{Relate, RelateResult, TypeRelation};
-use traits::PredicateObligations;
 
 /// "Greatest lower bound" (common subtype)
-pub struct Glb<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    fields: CombineFields<'a, 'gcx, 'tcx>
+pub struct Glb<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
-impl<'a, 'gcx, 'tcx> Glb<'a, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'a, 'gcx, 'tcx>) -> Glb<'a, 'gcx, 'tcx> {
-        Glb { fields: fields }
-    }
-
-    pub fn obligations(self) -> PredicateObligations<'tcx> {
-        self.fields.obligations
+impl<'combine, 'infcx, 'gcx, 'tcx> Glb<'combine, 'infcx, 'gcx, 'tcx> {
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
+        -> Glb<'combine, 'infcx, 'gcx, 'tcx>
+    {
+        Glb { fields: fields, a_is_expected: a_is_expected }
     }
 }
 
-impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Glb<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
+    for Glb<'combine, 'infcx, 'gcx, 'tcx>
+{
     fn tag(&self) -> &'static str { "Glb" }
 
-    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> { self.fields.tcx() }
+    fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
 
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    fn a_is_expected(&self) -> bool { self.a_is_expected }
 
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              variance: ty::Variance,
@@ -46,10 +46,10 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Glb<'a, 'gcx, 'tcx> {
                                              -> RelateResult<'tcx, T>
     {
         match variance {
-            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
             ty::Covariant => self.relate(a, b),
-            ty::Bivariant => self.fields.bivariate().relate(a, b),
-            ty::Contravariant => self.fields.lub().relate(a, b),
+            ty::Bivariant => self.fields.bivariate(self.a_is_expected).relate(a, b),
+            ty::Contravariant => self.fields.lub(self.a_is_expected).relate(a, b),
         }
     }
 
@@ -71,17 +71,19 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Glb<'a, 'gcx, 'tcx> {
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
-        self.fields.higher_ranked_glb(a, b)
+        self.fields.higher_ranked_glb(a, b, self.a_is_expected)
     }
 }
 
-impl<'a, 'gcx, 'tcx> LatticeDir<'a, 'gcx, 'tcx> for Glb<'a, 'gcx, 'tcx> {
-    fn infcx(&self) -> &'a InferCtxt<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> LatticeDir<'infcx, 'gcx, 'tcx>
+    for Glb<'combine, 'infcx, 'gcx, 'tcx>
+{
+    fn infcx(&self) -> &'infcx InferCtxt<'infcx, 'gcx, 'tcx> {
         self.fields.infcx
     }
 
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
-        let mut sub = self.fields.sub();
+    fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
+        let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(&v, &a)?;
         sub.relate(&v, &b)?;
         Ok(())
diff --git a/src/librustc/infer/higher_ranked/mod.rs b/src/librustc/infer/higher_ranked/mod.rs
index 03a09917c53..743d6135fbb 100644
--- a/src/librustc/infer/higher_ranked/mod.rs
+++ b/src/librustc/infer/higher_ranked/mod.rs
@@ -40,7 +40,7 @@ pub struct HrMatchResult<U> {
 }
 
 impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
-    pub fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>)
+    pub fn higher_ranked_sub<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
                                 -> RelateResult<'tcx, Binder<T>>
         where T: Relate<'tcx>
     {
@@ -77,11 +77,11 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
             debug!("b_prime={:?}", b_prime);
 
             // Compare types now that bound regions have been replaced.
-            let result = self.sub().relate(&a_prime, &b_prime)?;
+            let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
 
             // Presuming type comparison succeeds, we need to check
             // that the skolemized regions do not "leak".
-            self.infcx.leak_check(!self.a_is_expected, span, &skol_map, snapshot)?;
+            self.infcx.leak_check(!a_is_expected, span, &skol_map, snapshot)?;
 
             // We are finished with the skolemized regions now so pop
             // them off.
@@ -106,10 +106,11 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
     /// NB. It should not happen that there are LBR appearing in `U`
     /// that do not appear in `T`. If that happens, those regions are
     /// unconstrained, and this routine replaces them with `'static`.
-    pub fn higher_ranked_match<T, U>(&self,
+    pub fn higher_ranked_match<T, U>(&mut self,
                                      span: Span,
                                      a_pair: &Binder<(T, U)>,
-                                     b_match: &T)
+                                     b_match: &T,
+                                     a_is_expected: bool)
                                      -> RelateResult<'tcx, HrMatchResult<U>>
         where T: Relate<'tcx>,
               U: TypeFoldable<'tcx>
@@ -129,7 +130,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
             debug!("higher_ranked_match: skol_map={:?}", skol_map);
 
             // Equate types now that bound regions have been replaced.
-            try!(self.equate().relate(&a_match, &b_match));
+            try!(self.equate(a_is_expected).relate(&a_match, &b_match));
 
             // Map each skolemized region to a vector of other regions that it
             // must be equated with. (Note that this vector may include other
@@ -221,7 +222,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
         });
     }
 
-    pub fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>)
+    pub fn higher_ranked_lub<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
                                 -> RelateResult<'tcx, Binder<T>>
         where T: Relate<'tcx>
     {
@@ -239,7 +240,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
 
             // Collect constraints.
             let result0 =
-                self.lub().relate(&a_with_fresh, &b_with_fresh)?;
+                self.lub(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
             let result0 =
                 self.infcx.resolve_type_vars_if_possible(&result0);
             debug!("lub result0 = {:?}", result0);
@@ -311,7 +312,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
         }
     }
 
-    pub fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>)
+    pub fn higher_ranked_glb<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
                                 -> RelateResult<'tcx, Binder<T>>
         where T: Relate<'tcx>
     {
@@ -333,7 +334,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
 
             // Collect constraints.
             let result0 =
-                self.glb().relate(&a_with_fresh, &b_with_fresh)?;
+                self.glb(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
             let result0 =
                 self.infcx.resolve_type_vars_if_possible(&result0);
             debug!("glb result0 = {:?}", result0);
diff --git a/src/librustc/infer/lattice.rs b/src/librustc/infer/lattice.rs
index 1a2bc4b5cf2..eda78428e61 100644
--- a/src/librustc/infer/lattice.rs
+++ b/src/librustc/infer/lattice.rs
@@ -40,7 +40,7 @@ pub trait LatticeDir<'f, 'gcx: 'f+'tcx, 'tcx: 'f> : TypeRelation<'f, 'gcx, 'tcx>
 
     // Relates the type `v` to `a` and `b` such that `v` represents
     // the LUB/GLB of `a` and `b` as appropriate.
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>;
+    fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>;
 }
 
 pub fn super_lattice_tys<'a, 'gcx, 'tcx, L>(this: &mut L,
diff --git a/src/librustc/infer/lub.rs b/src/librustc/infer/lub.rs
index bd46f3a26a2..ad1b32ffaeb 100644
--- a/src/librustc/infer/lub.rs
+++ b/src/librustc/infer/lub.rs
@@ -15,29 +15,29 @@ use super::Subtype;
 
 use ty::{self, Ty, TyCtxt};
 use ty::relate::{Relate, RelateResult, TypeRelation};
-use traits::PredicateObligations;
 
 /// "Least upper bound" (common supertype)
-pub struct Lub<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    fields: CombineFields<'a, 'gcx, 'tcx>
+pub struct Lub<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
-impl<'a, 'gcx, 'tcx> Lub<'a, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'a, 'gcx, 'tcx>) -> Lub<'a, 'gcx, 'tcx> {
-        Lub { fields: fields }
-    }
-
-    pub fn obligations(self) -> PredicateObligations<'tcx> {
-        self.fields.obligations
+impl<'combine, 'infcx, 'gcx, 'tcx> Lub<'combine, 'infcx, 'gcx, 'tcx> {
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
+        -> Lub<'combine, 'infcx, 'gcx, 'tcx>
+    {
+        Lub { fields: fields, a_is_expected: a_is_expected }
     }
 }
 
-impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Lub<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
+    for Lub<'combine, 'infcx, 'gcx, 'tcx>
+{
     fn tag(&self) -> &'static str { "Lub" }
 
-    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> { self.fields.tcx() }
+    fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.tcx() }
 
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    fn a_is_expected(&self) -> bool { self.a_is_expected }
 
     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
                                              variance: ty::Variance,
@@ -46,10 +46,10 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Lub<'a, 'gcx, 'tcx> {
                                              -> RelateResult<'tcx, T>
     {
         match variance {
-            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
             ty::Covariant => self.relate(a, b),
-            ty::Bivariant => self.fields.bivariate().relate(a, b),
-            ty::Contravariant => self.fields.glb().relate(a, b),
+            ty::Bivariant => self.fields.bivariate(self.a_is_expected).relate(a, b),
+            ty::Contravariant => self.fields.glb(self.a_is_expected).relate(a, b),
         }
     }
 
@@ -71,17 +71,19 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Lub<'a, 'gcx, 'tcx> {
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
-        self.fields.higher_ranked_lub(a, b)
+        self.fields.higher_ranked_lub(a, b, self.a_is_expected)
     }
 }
 
-impl<'a, 'gcx, 'tcx> LatticeDir<'a, 'gcx, 'tcx> for Lub<'a, 'gcx, 'tcx> {
-    fn infcx(&self) -> &'a InferCtxt<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> LatticeDir<'infcx, 'gcx, 'tcx>
+    for Lub<'combine, 'infcx, 'gcx, 'tcx>
+{
+    fn infcx(&self) -> &'infcx InferCtxt<'infcx, 'gcx, 'tcx> {
         self.fields.infcx
     }
 
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
-        let mut sub = self.fields.sub();
+    fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
+        let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(&a, &v)?;
         sub.relate(&b, &v)?;
         Ok(())
diff --git a/src/librustc/infer/mod.rs b/src/librustc/infer/mod.rs
index 87882c5528e..be9adf7085c 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -48,18 +48,18 @@ use self::higher_ranked::HrMatchResult;
 use self::region_inference::{RegionVarBindings, RegionSnapshot};
 use self::unify_key::ToType;
 
-pub mod bivariate;
-pub mod combine;
-pub mod equate;
+mod bivariate;
+mod combine;
+mod equate;
 pub mod error_reporting;
-pub mod glb;
+mod glb;
 mod higher_ranked;
 pub mod lattice;
-pub mod lub;
+mod lub;
 pub mod region_inference;
 pub mod resolve;
 mod freshen;
-pub mod sub;
+mod sub;
 pub mod type_variable;
 pub mod unify_key;
 
@@ -821,11 +821,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         return variables;
     }
 
-    fn combine_fields(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+    fn combine_fields(&'a self, trace: TypeTrace<'tcx>)
                       -> CombineFields<'a, 'gcx, 'tcx> {
         CombineFields {
             infcx: self,
-            a_is_expected: a_is_expected,
             trace: trace,
             cause: None,
             obligations: PredicateObligations::new(),
@@ -836,36 +835,36 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut equate = self.combine_fields(a_is_expected, trace).equate();
-        let result = equate.relate(a, b);
-        result.map(|t| InferOk { value: t, obligations: equate.obligations() })
+        let mut fields = self.combine_fields(trace);
+        let result = fields.equate(a_is_expected).relate(a, b);
+        result.map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
 
     pub fn sub<T>(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>, a: &T, b: &T)
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut sub = self.combine_fields(a_is_expected, trace).sub();
-        let result = sub.relate(a, b);
-        result.map(|t| InferOk { value: t, obligations: sub.obligations() })
+        let mut fields = self.combine_fields(trace);
+        let result = fields.sub(a_is_expected).relate(a, b);
+        result.map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
 
     pub fn lub<T>(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>, a: &T, b: &T)
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut lub = self.combine_fields(a_is_expected, trace).lub();
-        let result = lub.relate(a, b);
-        result.map(|t| InferOk { value: t, obligations: lub.obligations() })
+        let mut fields = self.combine_fields(trace);
+        let result = fields.lub(a_is_expected).relate(a, b);
+        result.map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
 
     pub fn glb<T>(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>, a: &T, b: &T)
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut glb = self.combine_fields(a_is_expected, trace).glb();
-        let result = glb.relate(a, b);
-        result.map(|t| InferOk { value: t, obligations: glb.obligations() })
+        let mut fields = self.combine_fields(trace);
+        let result = fields.glb(a_is_expected).relate(a, b);
+        result.map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
 
     fn start_snapshot(&self) -> CombinedSnapshot {
@@ -1614,8 +1613,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         };
 
         let match_pair = match_a.map_bound(|p| (p.projection_ty.trait_ref, p.ty));
-        let combine = self.combine_fields(true, trace);
-        let result = combine.higher_ranked_match(span, &match_pair, &match_b)?;
+        let mut combine = self.combine_fields(trace);
+        let result = combine.higher_ranked_match(span, &match_pair, &match_b, true)?;
         Ok(InferOk { value: result, obligations: combine.obligations })
     }
 
diff --git a/src/librustc/infer/sub.rs b/src/librustc/infer/sub.rs
index 680dd0d6355..2f7f5254727 100644
--- a/src/librustc/infer/sub.rs
+++ b/src/librustc/infer/sub.rs
@@ -15,28 +15,35 @@ use super::type_variable::{SubtypeOf, SupertypeOf};
 use ty::{self, Ty, TyCtxt};
 use ty::TyVar;
 use ty::relate::{Cause, Relate, RelateResult, TypeRelation};
-use traits::PredicateObligations;
 use std::mem;
 
 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
-pub struct Sub<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
-    fields: CombineFields<'a, 'gcx, 'tcx>,
+pub struct Sub<'combine, 'infcx: 'combine, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
+    fields: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
-impl<'a, 'gcx, 'tcx> Sub<'a, 'gcx, 'tcx> {
-    pub fn new(f: CombineFields<'a, 'gcx, 'tcx>) -> Sub<'a, 'gcx, 'tcx> {
-        Sub { fields: f }
+impl<'combine, 'infcx, 'gcx, 'tcx> Sub<'combine, 'infcx, 'gcx, 'tcx> {
+    pub fn new(f: &'combine mut CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool)
+        -> Sub<'combine, 'infcx, 'gcx, 'tcx>
+    {
+        Sub { fields: f, a_is_expected: a_is_expected }
     }
 
-    pub fn obligations(self) -> PredicateObligations<'tcx> {
-        self.fields.obligations
+    fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
+        self.a_is_expected = !self.a_is_expected;
+        let result = f(self);
+        self.a_is_expected = !self.a_is_expected;
+        result
     }
 }
 
-impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Sub<'a, 'gcx, 'tcx> {
+impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
+    for Sub<'combine, 'infcx, 'gcx, 'tcx>
+{
     fn tag(&self) -> &'static str { "Sub" }
-    fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> { self.fields.infcx.tcx }
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.infcx.tcx }
+    fn a_is_expected(&self) -> bool { self.a_is_expected }
 
     fn with_cause<F,R>(&mut self, cause: Cause, f: F) -> R
         where F: FnOnce(&mut Self) -> R
@@ -56,10 +63,10 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Sub<'a, 'gcx, 'tcx> {
                                              -> RelateResult<'tcx, T>
     {
         match variance {
-            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
             ty::Covariant => self.relate(a, b),
-            ty::Bivariant => self.fields.bivariate().relate(a, b),
-            ty::Contravariant => self.fields.switch_expected().sub().relate(b, a),
+            ty::Bivariant => self.fields.bivariate(self.a_is_expected).relate(a, b),
+            ty::Contravariant => self.with_expected_switched(|this| { this.relate(b, a) }),
         }
     }
 
@@ -80,12 +87,11 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Sub<'a, 'gcx, 'tcx> {
             }
             (&ty::TyInfer(TyVar(a_id)), _) => {
                 self.fields
-                    .switch_expected()
-                    .instantiate(b, SupertypeOf, a_id)?;
+                    .instantiate(b, SupertypeOf, a_id, !self.a_is_expected)?;
                 Ok(a)
             }
             (_, &ty::TyInfer(TyVar(b_id))) => {
-                self.fields.instantiate(a, SubtypeOf, b_id)?;
+                self.fields.instantiate(a, SubtypeOf, b_id, self.a_is_expected)?;
                 Ok(a)
             }
 
@@ -116,6 +122,6 @@ impl<'a, 'gcx, 'tcx> TypeRelation<'a, 'gcx, 'tcx> for Sub<'a, 'gcx, 'tcx> {
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
-        self.fields.higher_ranked_sub(a, b)
+        self.fields.higher_ranked_sub(a, b, self.a_is_expected)
     }
 }