Move `a_is_expected` out of `CombineFields`

8 files changed, 86 insertions, 82 deletions

diff --git a/src/librustc/infer/bivariate.rs b/src/librustc/infer/bivariate.rs
index 610e25e0f8a..d790391cd4c 100644
--- a/src/librustc/infer/bivariate.rs
+++ b/src/librustc/infer/bivariate.rs
@@ -33,12 +33,13 @@ use ty::TyVar;
 use ty::relate::{Relate, RelateResult, TypeRelation};
 
 pub struct Bivariate<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
-    fields: CombineFields<'infcx, 'gcx, 'tcx>
+    fields: CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
 impl<'infcx, 'gcx, 'tcx> Bivariate<'infcx, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>) -> Bivariate<'infcx, 'gcx, 'tcx> {
-        Bivariate { fields: fields }
+    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool) -> Bivariate<'infcx, 'gcx, 'tcx> {
+        Bivariate { fields: fields, a_is_expected: a_is_expected }
     }
 }
 
@@ -47,7 +48,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Bivariate<'infcx,
 
     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 +87,12 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Bivariate<'infcx,
             }
 
             (&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 36ea228956d..41369299e72 100644
--- a/src/librustc/infer/combine.rs
+++ b/src/librustc/infer/combine.rs
@@ -54,7 +54,6 @@ use syntax_pos::Span;
 #[derive(Clone)]
 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
-    pub a_is_expected: bool,
     pub trace: TypeTrace<'tcx>,
     pub cause: Option<ty::relate::Cause>,
     pub obligations: PredicateObligations<'tcx>,
@@ -155,37 +154,31 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
         self.infcx.tcx
     }
 
-    pub fn switch_expected(&self) -> CombineFields<'infcx, 'gcx, 'tcx> {
-        CombineFields {
-            a_is_expected: !self.a_is_expected,
-            ..(*self).clone()
-        }
-    }
-
-    pub fn equate(&self) -> Equate<'infcx, 'gcx, 'tcx> {
-        Equate::new(self.clone())
+    pub fn equate(&self, a_is_expected: bool) -> Equate<'infcx, 'gcx, 'tcx> {
+        Equate::new(self.clone(), a_is_expected)
     }
 
-    pub fn bivariate(&self) -> Bivariate<'infcx, 'gcx, 'tcx> {
-        Bivariate::new(self.clone())
+    pub fn bivariate(&self, a_is_expected: bool) -> Bivariate<'infcx, 'gcx, 'tcx> {
+        Bivariate::new(self.clone(), a_is_expected)
     }
 
-    pub fn sub(&self) -> Sub<'infcx, 'gcx, 'tcx> {
-        Sub::new(self.clone())
+    pub fn sub(&self, a_is_expected: bool) -> Sub<'infcx, 'gcx, 'tcx> {
+        Sub::new(self.clone(), a_is_expected)
     }
 
-    pub fn lub(&self) -> Lub<'infcx, 'gcx, 'tcx> {
-        Lub::new(self.clone())
+    pub fn lub(&self, a_is_expected: bool) -> Lub<'infcx, 'gcx, 'tcx> {
+        Lub::new(self.clone(), a_is_expected)
     }
 
-    pub fn glb(&self) -> Glb<'infcx, 'gcx, 'tcx> {
-        Glb::new(self.clone())
+    pub fn glb(&self, a_is_expected: bool) -> Glb<'infcx, 'gcx, 'tcx> {
+        Glb::new(self.clone(), a_is_expected)
     }
 
     pub fn instantiate(&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 +248,10 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, '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 f3dc44e8efd..7980ba91c5e 100644
--- a/src/librustc/infer/equate.rs
+++ b/src/librustc/infer/equate.rs
@@ -19,12 +19,13 @@ use traits::PredicateObligations;
 
 /// Ensures `a` is made equal to `b`. Returns `a` on success.
 pub struct Equate<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
-    fields: CombineFields<'infcx, 'gcx, 'tcx>
+    fields: CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
 impl<'infcx, 'gcx, 'tcx> Equate<'infcx, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>) -> Equate<'infcx, 'gcx, 'tcx> {
-        Equate { fields: fields }
+    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool) -> Equate<'infcx, 'gcx, 'tcx> {
+        Equate { fields: fields, a_is_expected: a_is_expected }
     }
 
     pub fn obligations(self) -> PredicateObligations<'tcx> {
@@ -37,7 +38,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Equate<'infcx, 'gc
 
     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 +64,12 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Equate<'infcx, 'gc
             }
 
             (&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 +94,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Equate<'infcx, 'gc
                   -> 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 51386083ab3..7893846b214 100644
--- a/src/librustc/infer/glb.rs
+++ b/src/librustc/infer/glb.rs
@@ -19,12 +19,13 @@ use traits::PredicateObligations;
 
 /// "Greatest lower bound" (common subtype)
 pub struct Glb<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
-    fields: CombineFields<'infcx, 'gcx, 'tcx>
+    fields: CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
 impl<'infcx, 'gcx, 'tcx> Glb<'infcx, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>) -> Glb<'infcx, 'gcx, 'tcx> {
-        Glb { fields: fields }
+    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool) -> Glb<'infcx, 'gcx, 'tcx> {
+        Glb { fields: fields, a_is_expected: a_is_expected }
     }
 
     pub fn obligations(self) -> PredicateObligations<'tcx> {
@@ -37,7 +38,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Glb<'infcx, 'gcx,
 
     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 +47,10 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Glb<'infcx, 'gcx,
                                              -> 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),
         }
     }
 
@@ -71,7 +72,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Glb<'infcx, 'gcx,
                   -> 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)
     }
 }
 
@@ -81,7 +82,7 @@ impl<'infcx, 'gcx, 'tcx> LatticeDir<'infcx, 'gcx, 'tcx> for Glb<'infcx, 'gcx, 't
     }
 
     fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
-        let mut sub = self.fields.sub();
+        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..386c1ba272f 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>(&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.
@@ -109,7 +109,8 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
     pub fn higher_ranked_match<T, U>(&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>(&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>(&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/lub.rs b/src/librustc/infer/lub.rs
index bd87c91dea5..6f3bd004f1f 100644
--- a/src/librustc/infer/lub.rs
+++ b/src/librustc/infer/lub.rs
@@ -19,12 +19,13 @@ use traits::PredicateObligations;
 
 /// "Least upper bound" (common supertype)
 pub struct Lub<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
-    fields: CombineFields<'infcx, 'gcx, 'tcx>
+    fields: CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
 impl<'infcx, 'gcx, 'tcx> Lub<'infcx, 'gcx, 'tcx> {
-    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>) -> Lub<'infcx, 'gcx, 'tcx> {
-        Lub { fields: fields }
+    pub fn new(fields: CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool) -> Lub<'infcx, 'gcx, 'tcx> {
+        Lub { fields: fields, a_is_expected: a_is_expected }
     }
 
     pub fn obligations(self) -> PredicateObligations<'tcx> {
@@ -37,7 +38,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Lub<'infcx, 'gcx,
 
     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 +47,10 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Lub<'infcx, 'gcx,
                                              -> 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),
         }
     }
 
@@ -71,7 +72,7 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Lub<'infcx, 'gcx,
                   -> 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)
     }
 }
 
@@ -81,7 +82,7 @@ impl<'infcx, 'gcx, 'tcx> LatticeDir<'infcx, 'gcx, 'tcx> for Lub<'infcx, 'gcx, 't
     }
 
     fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
-        let mut sub = self.fields.sub();
+        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 4e0c5b58b64..9ee0da4d0ed 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -799,11 +799,10 @@ 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(),
@@ -814,7 +813,7 @@ 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 mut equate = self.combine_fields(trace).equate(a_is_expected);
         let result = equate.relate(a, b);
         result.map(|t| InferOk { value: t, obligations: equate.obligations() })
     }
@@ -823,7 +822,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut sub = self.combine_fields(a_is_expected, trace).sub();
+        let mut sub = self.combine_fields(trace).sub(a_is_expected);
         let result = sub.relate(a, b);
         result.map(|t| InferOk { value: t, obligations: sub.obligations() })
     }
@@ -832,7 +831,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut lub = self.combine_fields(a_is_expected, trace).lub();
+        let mut lub = self.combine_fields(trace).lub(a_is_expected);
         let result = lub.relate(a, b);
         result.map(|t| InferOk { value: t, obligations: lub.obligations() })
     }
@@ -841,7 +840,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         -> InferResult<'tcx, T>
         where T: Relate<'tcx>
     {
-        let mut glb = self.combine_fields(a_is_expected, trace).glb();
+        let mut glb = self.combine_fields(trace).glb(a_is_expected);
         let result = glb.relate(a, b);
         result.map(|t| InferOk { value: t, obligations: glb.obligations() })
     }
@@ -1646,8 +1645,8 @@ 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 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 404763b3bd3..7a25ea9f3c2 100644
--- a/src/librustc/infer/sub.rs
+++ b/src/librustc/infer/sub.rs
@@ -21,22 +21,30 @@ use std::mem;
 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
 pub struct Sub<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
     fields: CombineFields<'infcx, 'gcx, 'tcx>,
+    a_is_expected: bool,
 }
 
 impl<'infcx, 'gcx, 'tcx> Sub<'infcx, 'gcx, 'tcx> {
-    pub fn new(f: CombineFields<'infcx, 'gcx, 'tcx>) -> Sub<'infcx, 'gcx, 'tcx> {
-        Sub { fields: f }
+    pub fn new(f: CombineFields<'infcx, 'gcx, 'tcx>, a_is_expected: bool) -> Sub<'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<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Sub<'infcx, 'gcx, 'tcx> {
     fn tag(&self) -> &'static str { "Sub" }
     fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { self.fields.infcx.tcx }
-    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
+    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 +64,10 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Sub<'infcx, 'gcx,
                                              -> 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 +88,11 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Sub<'infcx, 'gcx,
             }
             (&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 +123,6 @@ impl<'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx> for Sub<'infcx, 'gcx,
                   -> 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)
     }
 }