Remove a_is_expected from combine relations

9 files changed, 74 insertions, 134 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/relate_tys.rs b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
index fd6c29379a3..5c5274d7d86 100644
--- a/compiler/rustc_borrowck/src/type_check/relate_tys.rs
+++ b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
@@ -120,7 +120,6 @@ impl<'me, 'bccx, 'tcx> NllTypeRelating<'me, 'bccx, 'tcx> {
     fn relate_opaques(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let infcx = self.type_checker.infcx;
         debug_assert!(!infcx.next_trait_solver());
-        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
         // `handle_opaque_type` cannot handle subtyping, so to support subtyping
         // we instead eagerly generalize here. This is a bit of a mess but will go
         // away once we're using the new solver.
diff --git a/compiler/rustc_infer/src/infer/at.rs b/compiler/rustc_infer/src/infer/at.rs
index a086d89b933..94088216c02 100644
--- a/compiler/rustc_infer/src/infer/at.rs
+++ b/compiler/rustc_infer/src/infer/at.rs
@@ -49,7 +49,6 @@ pub struct At<'a, 'tcx> {
 
 pub struct Trace<'a, 'tcx> {
     at: At<'a, 'tcx>,
-    a_is_expected: bool,
     trace: TypeTrace<'tcx>,
 }
 
@@ -105,23 +104,6 @@ pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {
 }
 
 impl<'a, 'tcx> At<'a, 'tcx> {
-    /// Makes `a <: b`, where `a` may or may not be expected.
-    ///
-    /// See [`At::trace_exp`] and [`Trace::sub`] for a version of
-    /// this method that only requires `T: Relate<'tcx>`
-    pub fn sub_exp<T>(
-        self,
-        define_opaque_types: DefineOpaqueTypes,
-        a_is_expected: bool,
-        a: T,
-        b: T,
-    ) -> InferResult<'tcx, ()>
-    where
-        T: ToTrace<'tcx>,
-    {
-        self.trace_exp(a_is_expected, a, b).sub(define_opaque_types, a, b)
-    }
-
     /// Makes `actual <: expected`. For example, if type-checking a
     /// call like `foo(x)`, where `foo: fn(i32)`, you might have
     /// `sup(i32, x)`, since the "expected" type is the type that
@@ -138,7 +120,7 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.sub_exp(define_opaque_types, false, actual, expected)
+        self.trace(expected, actual).sup(define_opaque_types, expected, actual)
     }
 
     /// Makes `expected <: actual`.
@@ -154,24 +136,7 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.sub_exp(define_opaque_types, true, expected, actual)
-    }
-
-    /// Makes `expected <: actual`.
-    ///
-    /// See [`At::trace_exp`] and [`Trace::eq`] for a version of
-    /// this method that only requires `T: Relate<'tcx>`
-    pub fn eq_exp<T>(
-        self,
-        define_opaque_types: DefineOpaqueTypes,
-        a_is_expected: bool,
-        a: T,
-        b: T,
-    ) -> InferResult<'tcx, ()>
-    where
-        T: ToTrace<'tcx>,
-    {
-        self.trace_exp(a_is_expected, a, b).eq(define_opaque_types, a, b)
+        self.trace(expected, actual).sub(define_opaque_types, expected, actual)
     }
 
     /// Makes `expected <: actual`.
@@ -260,48 +225,50 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.trace_exp(true, expected, actual)
+        let trace = ToTrace::to_trace(self.cause, true, expected, actual);
+        Trace { at: self, trace }
     }
+}
 
-    /// Like `trace`, but the expected value is determined by the
-    /// boolean argument (if true, then the first argument `a` is the
-    /// "expected" value).
-    pub fn trace_exp<T>(self, a_is_expected: bool, a: T, b: T) -> Trace<'a, 'tcx>
+impl<'a, 'tcx> Trace<'a, 'tcx> {
+    /// Makes `a <: b`.
+    #[instrument(skip(self), level = "debug")]
+    pub fn sub<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
-        T: ToTrace<'tcx>,
+        T: Relate<'tcx>,
     {
-        let trace = ToTrace::to_trace(self.cause, a_is_expected, a, b);
-        Trace { at: self, trace, a_is_expected }
+        let Trace { at, trace } = self;
+        let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
+        fields
+            .sub()
+            .relate(a, b)
+            .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
-}
 
-impl<'a, 'tcx> Trace<'a, 'tcx> {
-    /// Makes `a <: b` where `a` may or may not be expected (if
-    /// `a_is_expected` is true, then `a` is expected).
+    /// Makes `a :> b`.
     #[instrument(skip(self), level = "debug")]
-    pub fn sub<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
+    pub fn sup<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .sub(a_is_expected)
+            .sup()
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
 
-    /// Makes `a == b`; the expectation is set by the call to
-    /// `trace()`.
+    /// Makes `a == b`.
     #[instrument(skip(self), level = "debug")]
     pub fn eq<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .equate(StructurallyRelateAliases::No, a_is_expected)
+            .equate(StructurallyRelateAliases::No)
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
@@ -313,11 +280,11 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         debug_assert!(at.infcx.next_trait_solver());
         let mut fields = at.infcx.combine_fields(trace, at.param_env, DefineOpaqueTypes::No);
         fields
-            .equate(StructurallyRelateAliases::Yes, a_is_expected)
+            .equate(StructurallyRelateAliases::Yes)
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
@@ -327,10 +294,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .lub(a_is_expected)
+            .lub()
             .relate(a, b)
             .map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
@@ -340,10 +307,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .glb(a_is_expected)
+            .glb()
             .relate(a, b)
             .map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index d381c77ec66..07245643ef5 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -522,13 +522,7 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> InferResult<'tcx, ()> {
         let mut obligations = Vec::new();
 
-        self.insert_hidden_type(
-            opaque_type_key,
-            &cause,
-            param_env,
-            hidden_ty,
-            &mut obligations,
-        )?;
+        self.insert_hidden_type(opaque_type_key, &cause, param_env, hidden_ty, &mut obligations)?;
 
         self.add_item_bounds_for_hidden_type(
             opaque_type_key.def_id.to_def_id(),
diff --git a/compiler/rustc_infer/src/infer/relate/combine.rs b/compiler/rustc_infer/src/infer/relate/combine.rs
index 749c50b57b5..099b7ff7c04 100644
--- a/compiler/rustc_infer/src/infer/relate/combine.rs
+++ b/compiler/rustc_infer/src/infer/relate/combine.rs
@@ -321,21 +321,24 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     pub fn equate<'a>(
         &'a mut self,
         structurally_relate_aliases: StructurallyRelateAliases,
-        a_is_expected: bool,
     ) -> TypeRelating<'a, 'infcx, 'tcx> {
-        TypeRelating::new(self, a_is_expected, structurally_relate_aliases, ty::Invariant)
+        TypeRelating::new(self, structurally_relate_aliases, ty::Invariant)
     }
 
-    pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> TypeRelating<'a, 'infcx, 'tcx> {
-        TypeRelating::new(self, a_is_expected, StructurallyRelateAliases::No, ty::Covariant)
+    pub fn sub<'a>(&'a mut self) -> TypeRelating<'a, 'infcx, 'tcx> {
+        TypeRelating::new(self, StructurallyRelateAliases::No, ty::Covariant)
     }
 
-    pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
-        Lub::new(self, a_is_expected)
+    pub fn sup<'a>(&'a mut self) -> TypeRelating<'a, 'infcx, 'tcx> {
+        TypeRelating::new(self, StructurallyRelateAliases::No, ty::Contravariant)
     }
 
-    pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
-        Glb::new(self, a_is_expected)
+    pub fn lub<'a>(&'a mut self) -> Lub<'a, 'infcx, 'tcx> {
+        Lub::new(self)
+    }
+
+    pub fn glb<'a>(&'a mut self) -> Glb<'a, 'infcx, 'tcx> {
+        Glb::new(self)
     }
 
     pub fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
diff --git a/compiler/rustc_infer/src/infer/relate/glb.rs b/compiler/rustc_infer/src/infer/relate/glb.rs
index 9b77e6888b2..f6796861b12 100644
--- a/compiler/rustc_infer/src/infer/relate/glb.rs
+++ b/compiler/rustc_infer/src/infer/relate/glb.rs
@@ -13,15 +13,11 @@ use crate::traits::{ObligationCause, PredicateObligations};
 /// "Greatest lower bound" (common subtype)
 pub struct Glb<'combine, 'infcx, 'tcx> {
     fields: &'combine mut CombineFields<'infcx, 'tcx>,
-    a_is_expected: bool,
 }
 
 impl<'combine, 'infcx, 'tcx> Glb<'combine, 'infcx, 'tcx> {
-    pub fn new(
-        fields: &'combine mut CombineFields<'infcx, 'tcx>,
-        a_is_expected: bool,
-    ) -> Glb<'combine, 'infcx, 'tcx> {
-        Glb { fields, a_is_expected }
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'tcx>) -> Glb<'combine, 'infcx, 'tcx> {
+        Glb { fields }
     }
 }
 
@@ -35,7 +31,7 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
     }
 
     fn a_is_expected(&self) -> bool {
-        self.a_is_expected
+        true
     }
 
     fn relate_with_variance<T: Relate<'tcx>>(
@@ -46,13 +42,11 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         b: T,
     ) -> RelateResult<'tcx, T> {
         match variance {
-            ty::Invariant => {
-                self.fields.equate(StructurallyRelateAliases::No, self.a_is_expected).relate(a, b)
-            }
+            ty::Invariant => self.fields.equate(StructurallyRelateAliases::No).relate(a, b),
             ty::Covariant => self.relate(a, b),
             // FIXME(#41044) -- not correct, need test
             ty::Bivariant => Ok(a),
-            ty::Contravariant => self.fields.lub(self.a_is_expected).relate(a, b),
+            ty::Contravariant => self.fields.lub().relate(a, b),
         }
     }
 
@@ -126,7 +120,7 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
     }
 
     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);
+        let mut sub = self.fields.sub();
         sub.relate(v, a)?;
         sub.relate(v, b)?;
         Ok(())
diff --git a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
index 90be80f67b4..c94cbb0db03 100644
--- a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
+++ b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
@@ -49,7 +49,13 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             debug!("b_prime={:?}", sup_prime);
 
             // Compare types now that bound regions have been replaced.
-            let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime);
+            // Reorder the inputs so that the expected is passed first.
+            let result = if sub_is_expected {
+                self.sub().relate(sub_prime, sup_prime)
+            } else {
+                self.sup().relate(sup_prime, sub_prime)
+            };
+
             if result.is_ok() {
                 debug!("OK result={result:?}");
             }
diff --git a/compiler/rustc_infer/src/infer/relate/lub.rs b/compiler/rustc_infer/src/infer/relate/lub.rs
index db04e3231d6..3d9cfe7bf05 100644
--- a/compiler/rustc_infer/src/infer/relate/lub.rs
+++ b/compiler/rustc_infer/src/infer/relate/lub.rs
@@ -13,15 +13,11 @@ use rustc_span::Span;
 /// "Least upper bound" (common supertype)
 pub struct Lub<'combine, 'infcx, 'tcx> {
     fields: &'combine mut CombineFields<'infcx, 'tcx>,
-    a_is_expected: bool,
 }
 
 impl<'combine, 'infcx, 'tcx> Lub<'combine, 'infcx, 'tcx> {
-    pub fn new(
-        fields: &'combine mut CombineFields<'infcx, 'tcx>,
-        a_is_expected: bool,
-    ) -> Lub<'combine, 'infcx, 'tcx> {
-        Lub { fields, a_is_expected }
+    pub fn new(fields: &'combine mut CombineFields<'infcx, 'tcx>) -> Lub<'combine, 'infcx, 'tcx> {
+        Lub { fields }
     }
 }
 
@@ -35,7 +31,7 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
     }
 
     fn a_is_expected(&self) -> bool {
-        self.a_is_expected
+        true
     }
 
     fn relate_with_variance<T: Relate<'tcx>>(
@@ -46,13 +42,11 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         b: T,
     ) -> RelateResult<'tcx, T> {
         match variance {
-            ty::Invariant => {
-                self.fields.equate(StructurallyRelateAliases::No, self.a_is_expected).relate(a, b)
-            }
+            ty::Invariant => self.fields.equate(StructurallyRelateAliases::No).relate(a, b),
             ty::Covariant => self.relate(a, b),
             // FIXME(#41044) -- not correct, need test
             ty::Bivariant => Ok(a),
-            ty::Contravariant => self.fields.glb(self.a_is_expected).relate(a, b),
+            ty::Contravariant => self.fields.glb().relate(a, b),
         }
     }
 
@@ -126,7 +120,7 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
     }
 
     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);
+        let mut sub = self.fields.sub();
         sub.relate(a, v)?;
         sub.relate(b, v)?;
         Ok(())
diff --git a/compiler/rustc_infer/src/infer/relate/type_relating.rs b/compiler/rustc_infer/src/infer/relate/type_relating.rs
index ddc4bf9a514..7464b525724 100644
--- a/compiler/rustc_infer/src/infer/relate/type_relating.rs
+++ b/compiler/rustc_infer/src/infer/relate/type_relating.rs
@@ -12,7 +12,6 @@ use rustc_span::Span;
 /// Enforce that `a` is equal to or a subtype of `b`.
 pub struct TypeRelating<'combine, 'a, 'tcx> {
     fields: &'combine mut CombineFields<'a, 'tcx>,
-    a_is_expected: bool,
     structurally_relate_aliases: StructurallyRelateAliases,
     ambient_variance: ty::Variance,
 }
@@ -20,11 +19,10 @@ pub struct TypeRelating<'combine, 'a, 'tcx> {
 impl<'combine, 'infcx, 'tcx> TypeRelating<'combine, 'infcx, 'tcx> {
     pub fn new(
         f: &'combine mut CombineFields<'infcx, 'tcx>,
-        a_is_expected: bool,
         structurally_relate_aliases: StructurallyRelateAliases,
         ambient_variance: ty::Variance,
     ) -> TypeRelating<'combine, 'infcx, 'tcx> {
-        TypeRelating { fields: f, a_is_expected, structurally_relate_aliases, ambient_variance }
+        TypeRelating { fields: f, structurally_relate_aliases, ambient_variance }
     }
 }
 
@@ -38,7 +36,7 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
     }
 
     fn a_is_expected(&self) -> bool {
-        self.a_is_expected
+        true
     }
 
     fn relate_with_variance<T: Relate<'tcx>>(
@@ -79,7 +77,7 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
                             self.fields.trace.cause.clone(),
                             self.fields.param_env,
                             ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
-                                a_is_expected: self.a_is_expected,
+                                a_is_expected: true,
                                 a,
                                 b,
                             })),
@@ -93,7 +91,7 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
                             self.fields.trace.cause.clone(),
                             self.fields.param_env,
                             ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
-                                a_is_expected: !self.a_is_expected,
+                                a_is_expected: false,
                                 a: b,
                                 b: a,
                             })),
@@ -109,18 +107,12 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
             }
 
             (&ty::Infer(TyVar(a_vid)), _) => {
-                infcx.instantiate_ty_var(
-                    self,
-                    self.a_is_expected,
-                    a_vid,
-                    self.ambient_variance,
-                    b,
-                )?;
+                infcx.instantiate_ty_var(self, true, a_vid, self.ambient_variance, b)?;
             }
             (_, &ty::Infer(TyVar(b_vid))) => {
                 infcx.instantiate_ty_var(
                     self,
-                    !self.a_is_expected,
+                    false,
                     b_vid,
                     self.ambient_variance.xform(ty::Contravariant),
                     a,
@@ -147,13 +139,7 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
             {
                 self.fields.obligations.extend(
                     infcx
-                        .handle_opaque_type(
-                            a,
-                            b,
-                            self.a_is_expected,
-                            &self.fields.trace.cause,
-                            self.param_env(),
-                        )?
+                        .handle_opaque_type(a, b, true, &self.fields.trace.cause, self.param_env())?
                         .obligations,
                 );
             }
@@ -239,14 +225,14 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
         } else {
             match self.ambient_variance {
                 ty::Covariant => {
-                    self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
+                    self.fields.higher_ranked_sub(a, b, true)?;
                 }
                 ty::Contravariant => {
-                    self.fields.higher_ranked_sub(b, a, !self.a_is_expected)?;
+                    self.fields.higher_ranked_sub(b, a, false)?;
                 }
                 ty::Invariant => {
-                    self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
-                    self.fields.higher_ranked_sub(b, a, !self.a_is_expected)?;
+                    self.fields.higher_ranked_sub(a, b, true)?;
+                    self.fields.higher_ranked_sub(b, a, false)?;
                 }
                 ty::Bivariant => {
                     unreachable!("Expected bivariance to be handled in relate_with_variance")
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index fa8edd11594..7f7bd867f63 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -1541,12 +1541,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 // since the normalization is just done to improve the error message.
                 let _ = ocx.select_where_possible();
 
-                if let Err(new_err) = ocx.eq(
-                    &obligation.cause,
-                    obligation.param_env,
-                    expected,
-                    actual,
-                ) {
+                if let Err(new_err) =
+                    ocx.eq(&obligation.cause, obligation.param_env, expected, actual)
+                {
                     (Some((data, is_normalized_term_expected, normalized_term, data.term)), new_err)
                 } else {
                     (None, error.err)