move ty var instantiation into the generalize module

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -23,19 +23,18 @@
 //! this should be correctly updated.
 
 use super::equate::Equate;
-use super::generalize::{self, CombineDelegate, Generalization};
 use super::glb::Glb;
 use super::lub::Lub;
 use super::sub::Sub;
 use crate::infer::{DefineOpaqueTypes, InferCtxt, TypeTrace};
 use crate::traits::{Obligation, PredicateObligations};
 use rustc_middle::infer::canonical::OriginalQueryValues;
-use rustc_middle::infer::unify_key::{ConstVariableValue, EffectVarValue};
+use rustc_middle::infer::unify_key::EffectVarValue;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{RelateResult, TypeRelation};
 use rustc_middle::ty::{self, InferConst, ToPredicate, Ty, TyCtxt, TypeVisitableExt};
-use rustc_middle::ty::{AliasRelationDirection, TyVar};
 use rustc_middle::ty::{IntType, UintType};
+use rustc_span::Span;
 
 #[derive(Clone)]
 pub struct CombineFields<'infcx, 'tcx> {
@@ -221,11 +220,11 @@ impl<'tcx> InferCtxt<'tcx> {
             }
 
             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
-                return self.unify_const_variable(vid, b);
+                return self.instantiate_const_var(vid, b);
             }
 
             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
-                return self.unify_const_variable(vid, a);
+                return self.instantiate_const_var(vid, a);
             }
 
             (ty::ConstKind::Infer(InferConst::EffectVar(vid)), _) => {
@@ -259,73 +258,6 @@ impl<'tcx> InferCtxt<'tcx> {
         ty::relate::structurally_relate_consts(relation, a, b)
     }
 
-    /// Unifies the const variable `target_vid` with the given constant.
-    ///
-    /// This also tests if the given const `ct` contains an inference variable which was previously
-    /// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
-    /// would result in an infinite type as we continuously replace an inference variable
-    /// in `ct` with `ct` itself.
-    ///
-    /// This is especially important as unevaluated consts use their parents generics.
-    /// They therefore often contain unused args, making these errors far more likely.
-    ///
-    /// A good example of this is the following:
-    ///
-    /// ```compile_fail,E0308
-    /// #![feature(generic_const_exprs)]
-    ///
-    /// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
-    ///     todo!()
-    /// }
-    ///
-    /// fn main() {
-    ///     let mut arr = Default::default();
-    ///     arr = bind(arr);
-    /// }
-    /// ```
-    ///
-    /// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
-    /// of `fn bind` (meaning that its args contain `N`).
-    ///
-    /// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
-    /// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
-    ///
-    /// As `3 + 4` contains `N` in its args, this must not succeed.
-    ///
-    /// See `tests/ui/const-generics/occurs-check/` for more examples where this is relevant.
-    #[instrument(level = "debug", skip(self))]
-    fn unify_const_variable(
-        &self,
-        target_vid: ty::ConstVid,
-        ct: ty::Const<'tcx>,
-    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        let span = match self.inner.borrow_mut().const_unification_table().probe_value(target_vid) {
-            ConstVariableValue::Known { value } => {
-                bug!("instantiating a known const var: {target_vid:?} {value} {ct}")
-            }
-            ConstVariableValue::Unknown { origin, universe: _ } => origin.span,
-        };
-        // FIXME(generic_const_exprs): Occurs check failures for unevaluated
-        // constants and generic expressions are not yet handled correctly.
-        let Generalization { value_may_be_infer: value, has_unconstrained_ty_var } =
-            generalize::generalize(
-                self,
-                &mut CombineDelegate { infcx: self, span },
-                ct,
-                target_vid,
-                ty::Variance::Invariant,
-            )?;
-
-        if has_unconstrained_ty_var {
-            span_bug!(span, "unconstrained ty var when generalizing `{ct:?}`");
-        }
-        self.inner
-            .borrow_mut()
-            .const_unification_table()
-            .union_value(target_vid, ConstVariableValue::Known { value });
-        Ok(value)
-    }
-
     fn unify_integral_variable(
         &self,
         vid_is_expected: bool,
@@ -387,132 +319,6 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         Glb::new(self, a_is_expected)
     }
 
-    /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
-    /// The idea is that we should ensure that the type `a_ty` is equal
-    /// to, a subtype of, or a supertype of (respectively) the type
-    /// to which `b_vid` is bound.
-    ///
-    /// Since `b_vid` has not yet been instantiated with a type, we
-    /// will first instantiate `b_vid` with a *generalized* version
-    /// of `a_ty`. Generalization introduces other inference
-    /// variables wherever subtyping could occur.
-    #[instrument(skip(self), level = "debug")]
-    pub fn instantiate(
-        &mut self,
-        a_ty: Ty<'tcx>,
-        ambient_variance: ty::Variance,
-        b_vid: ty::TyVid,
-        a_is_expected: bool,
-    ) -> RelateResult<'tcx, ()> {
-        // 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());
-
-        // Generalize type of `a_ty` appropriately depending on the
-        // direction. As an example, assume:
-        //
-        // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
-        //   inference variable,
-        // - and `dir` == `SubtypeOf`.
-        //
-        // Then the generalized form `b_ty` would be `&'?2 ?3`, where
-        // `'?2` and `?3` are fresh region/type inference
-        // variables. (Down below, we will relate `a_ty <: b_ty`,
-        // adding constraints like `'x: '?2` and `?1 <: ?3`.)
-        let Generalization { value_may_be_infer: b_ty, has_unconstrained_ty_var } =
-            generalize::generalize(
-                self.infcx,
-                &mut CombineDelegate { infcx: self.infcx, span: self.trace.span() },
-                a_ty,
-                b_vid,
-                ambient_variance,
-            )?;
-
-        // Constrain `b_vid` to the generalized type `b_ty`.
-        if let &ty::Infer(TyVar(b_ty_vid)) = b_ty.kind() {
-            self.infcx.inner.borrow_mut().type_variables().equate(b_vid, b_ty_vid);
-        } else {
-            self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
-        }
-
-        if has_unconstrained_ty_var {
-            self.obligations.push(Obligation::new(
-                self.tcx(),
-                self.trace.cause.clone(),
-                self.param_env,
-                ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
-                    b_ty.into(),
-                ))),
-            ));
-        }
-
-        // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
-        //
-        // FIXME(#16847): This code is non-ideal because all these subtype
-        // relations wind up attributed to the same spans. We need
-        // to associate causes/spans with each of the relations in
-        // the stack to get this right.
-        if b_ty.is_ty_var() {
-            // This happens for cases like `<?0 as Trait>::Assoc == ?0`.
-            // We can't instantiate `?0` here as that would result in a
-            // cyclic type. We instead delay the unification in case
-            // the alias can be normalized to something which does not
-            // mention `?0`.
-            if self.infcx.next_trait_solver() {
-                let (lhs, rhs, direction) = match ambient_variance {
-                    ty::Variance::Invariant => {
-                        (a_ty.into(), b_ty.into(), AliasRelationDirection::Equate)
-                    }
-                    ty::Variance::Covariant => {
-                        (a_ty.into(), b_ty.into(), AliasRelationDirection::Subtype)
-                    }
-                    ty::Variance::Contravariant => {
-                        (b_ty.into(), a_ty.into(), AliasRelationDirection::Subtype)
-                    }
-                    ty::Variance::Bivariant => unreachable!("bivariant generalization"),
-                };
-                self.obligations.push(Obligation::new(
-                    self.tcx(),
-                    self.trace.cause.clone(),
-                    self.param_env,
-                    ty::PredicateKind::AliasRelate(lhs, rhs, direction),
-                ));
-            } else {
-                match a_ty.kind() {
-                    &ty::Alias(ty::Projection, data) => {
-                        // FIXME: This does not handle subtyping correctly, we could
-                        // instead create a new inference variable for `a_ty`, emitting
-                        // `Projection(a_ty, a_infer)` and `a_infer <: b_ty`.
-                        self.obligations.push(Obligation::new(
-                            self.tcx(),
-                            self.trace.cause.clone(),
-                            self.param_env,
-                            ty::ProjectionPredicate { projection_ty: data, term: b_ty.into() },
-                        ))
-                    }
-                    // The old solver only accepts projection predicates for associated types.
-                    ty::Alias(ty::Inherent | ty::Weak | ty::Opaque, _) => {
-                        return Err(TypeError::CyclicTy(a_ty));
-                    }
-                    _ => bug!("generalizated `{a_ty:?} to infer, not an alias"),
-                }
-            }
-        } else {
-            match ambient_variance {
-                ty::Variance::Invariant => self.equate(a_is_expected).relate(a_ty, b_ty),
-                ty::Variance::Covariant => self.sub(a_is_expected).relate(a_ty, b_ty),
-                ty::Variance::Contravariant => self.sub(a_is_expected).relate_with_variance(
-                    ty::Contravariant,
-                    ty::VarianceDiagInfo::default(),
-                    a_ty,
-                    b_ty,
-                ),
-                ty::Variance::Bivariant => unreachable!("bivariant generalization"),
-            }?;
-        }
-
-        Ok(())
-    }
-
     pub fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.obligations.extend(obligations);
     }
@@ -525,6 +331,8 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 }
 
 pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
+    fn span(&self) -> Span;
+
     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 
     /// Register obligations that must hold in order for this relation to hold

compiler/rustc_infer/src/infer/relate/equate.rs

@@ -8,6 +8,7 @@ use rustc_middle::ty::TyVar;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 
 use rustc_hir::def_id::DefId;
+use rustc_span::Span;
 
 /// Ensures `a` is made equal to `b`. Returns `a` on success.
 pub struct Equate<'combine, 'infcx, 'tcx> {
@@ -81,12 +82,12 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
                 infcx.inner.borrow_mut().type_variables().equate(a_id, b_id);
             }
 
-            (&ty::Infer(TyVar(a_id)), _) => {
+            (&ty::Infer(TyVar(a_vid)), _) => {
-                self.fields.instantiate(b, ty::Invariant, a_id, self.a_is_expected)?;
+                infcx.instantiate_ty_var(self, self.a_is_expected, a_vid, ty::Invariant, b)?;
             }
 
-            (_, &ty::Infer(TyVar(b_id))) => {
+            (_, &ty::Infer(TyVar(b_vid))) => {
-                self.fields.instantiate(a, ty::Invariant, b_id, self.a_is_expected)?;
+                infcx.instantiate_ty_var(self, !self.a_is_expected, b_vid, ty::Invariant, a)?;
             }
 
             (
@@ -170,6 +171,10 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Equate<'_, '_, 'tcx> {
+    fn span(&self) -> Span {
+        self.fields.trace.span()
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -1,5 +1,8 @@
 use std::mem;
 
+use crate::infer::nll_relate::TypeRelatingDelegate;
+use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind, TypeVariableValue};
+use crate::infer::{InferCtxt, ObligationEmittingRelation, RegionVariableOrigin};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::def_id::DefId;
@@ -7,17 +10,204 @@ use rustc_middle::infer::unify_key::ConstVariableValue;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::visit::MaxUniverse;
-use rustc_middle::ty::{self, InferConst, Term, Ty, TyCtxt, TypeVisitable, TypeVisitableExt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{AliasRelationDirection, InferConst, Term, TypeVisitable, TypeVisitableExt};
 use rustc_span::Span;
 
-use crate::infer::nll_relate::TypeRelatingDelegate;
+impl<'tcx> InferCtxt<'tcx> {
-use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind, TypeVariableValue};
+    /// The idea is that we should ensure that the type variable `target_vid`
-use crate::infer::{InferCtxt, RegionVariableOrigin};
+    /// is equal to, a subtype of, or a supertype of `source_ty`.
+    ///
+    /// For this, we will instantiate `target_vid` with a *generalized* version
+    /// of `source_ty`. Generalization introduces other inference variables wherever
+    /// subtyping could occur. This also does the occurs checks, detecting whether
+    /// instantiating `target_vid` would result in a cyclic type. We eagerly error
+    /// in this case.
+    #[instrument(skip(self, relation, target_is_expected), level = "debug")]
+    pub(super) fn instantiate_ty_var<R: ObligationEmittingRelation<'tcx>>(
+        &self,
+        relation: &mut R,
+        target_is_expected: bool,
+        target_vid: ty::TyVid,
+        ambient_variance: ty::Variance,
+        source_ty: Ty<'tcx>,
+    ) -> RelateResult<'tcx, ()> {
+        debug_assert!(self.inner.borrow_mut().type_variables().probe(target_vid).is_unknown());
+
+        // Generalize `source_ty` depending on the current variance. As an example, assume
+        // `?target <: &'x ?1`, where `'x` is some free region and `?1` is an inference
+        // variable.
+        //
+        // Then the `generalized_ty` would be `&'?2 ?3`, where `'?2` and `?3` are fresh
+        // region/type inference variables.
+        //
+        // We then relate `generalized_ty <: source_ty`,adding constraints like `'x: '?2` and `?1 <: ?3`.
+        let Generalization { value_may_be_infer: generalized_ty, has_unconstrained_ty_var } =
+            generalize(
+                self,
+                &mut CombineDelegate { infcx: self, span: relation.span() },
+                source_ty,
+                target_vid,
+                ambient_variance,
+            )?;
+
+        // Constrain `b_vid` to the generalized type `generalized_ty`.
+        if let &ty::Infer(ty::TyVar(generalized_vid)) = generalized_ty.kind() {
+            self.inner.borrow_mut().type_variables().equate(target_vid, generalized_vid);
+        } else {
+            self.inner.borrow_mut().type_variables().instantiate(target_vid, generalized_ty);
+        }
+
+        // See the comment on `Generalization::has_unconstrained_ty_var`.
+        if has_unconstrained_ty_var {
+            relation.register_predicates([ty::ClauseKind::WellFormed(generalized_ty.into())]);
+        }
+
+        // Finally, relate `generalized_ty` to `source_ty`, as described in previous comment.
+        //
+        // FIXME(#16847): This code is non-ideal because all these subtype
+        // relations wind up attributed to the same spans. We need
+        // to associate causes/spans with each of the relations in
+        // the stack to get this right.
+        if generalized_ty.is_ty_var() {
+            // This happens for cases like `<?0 as Trait>::Assoc == ?0`.
+            // We can't instantiate `?0` here as that would result in a
+            // cyclic type. We instead delay the unification in case
+            // the alias can be normalized to something which does not
+            // mention `?0`.
+            if self.next_trait_solver() {
+                let (lhs, rhs, direction) = match ambient_variance {
+                    ty::Variance::Invariant => {
+                        (generalized_ty.into(), source_ty.into(), AliasRelationDirection::Equate)
+                    }
+                    ty::Variance::Covariant => {
+                        (generalized_ty.into(), source_ty.into(), AliasRelationDirection::Subtype)
+                    }
+                    ty::Variance::Contravariant => {
+                        (source_ty.into(), generalized_ty.into(), AliasRelationDirection::Subtype)
+                    }
+                    ty::Variance::Bivariant => unreachable!("bivariant generalization"),
+                };
+
+                relation.register_predicates([ty::PredicateKind::AliasRelate(lhs, rhs, direction)]);
+            } else {
+                match source_ty.kind() {
+                    &ty::Alias(ty::Projection, data) => {
+                        // FIXME: This does not handle subtyping correctly, we could
+                        // instead create a new inference variable `?normalized_source`, emitting
+                        // `Projection(normalized_source, ?ty_normalized)` and `?normalized_source <: generalized_ty`.
+                        relation.register_predicates([ty::ProjectionPredicate {
+                            projection_ty: data,
+                            term: generalized_ty.into(),
+                        }]);
+                    }
+                    // The old solver only accepts projection predicates for associated types.
+                    ty::Alias(ty::Inherent | ty::Weak | ty::Opaque, _) => {
+                        return Err(TypeError::CyclicTy(source_ty));
+                    }
+                    _ => bug!("generalized `{source_ty:?} to infer, not an alias"),
+                }
+            }
+        } else {
+            // HACK: make sure that we `a_is_expected` continues to be
+            // correct when relating the generalized type with the source.
+            if target_is_expected == relation.a_is_expected() {
+                relation.relate_with_variance(
+                    ambient_variance,
+                    ty::VarianceDiagInfo::default(),
+                    generalized_ty,
+                    source_ty,
+                )?;
+            } else {
+                relation.relate_with_variance(
+                    ambient_variance.xform(ty::Contravariant),
+                    ty::VarianceDiagInfo::default(),
+                    source_ty,
+                    generalized_ty,
+                )?;
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Instantiates the const variable `target_vid` with the given constant.
+    ///
+    /// This also tests if the given const `ct` contains an inference variable which was previously
+    /// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
+    /// would result in an infinite type as we continuously replace an inference variable
+    /// in `ct` with `ct` itself.
+    ///
+    /// This is especially important as unevaluated consts use their parents generics.
+    /// They therefore often contain unused args, making these errors far more likely.
+    ///
+    /// A good example of this is the following:
+    ///
+    /// ```compile_fail,E0308
+    /// #![feature(generic_const_exprs)]
+    ///
+    /// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
+    ///     todo!()
+    /// }
+    ///
+    /// fn main() {
+    ///     let mut arr = Default::default();
+    ///     arr = bind(arr);
+    /// }
+    /// ```
+    ///
+    /// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
+    /// of `fn bind` (meaning that its args contain `N`).
+    ///
+    /// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
+    /// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
+    ///
+    /// As `3 + 4` contains `N` in its args, this must not succeed.
+    ///
+    /// See `tests/ui/const-generics/occurs-check/` for more examples where this is relevant.
+    #[instrument(level = "debug", skip(self))]
+    pub(super) fn instantiate_const_var(
+        &self,
+        target_vid: ty::ConstVid,
+        source_ct: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        let span = match self.inner.borrow_mut().const_unification_table().probe_value(target_vid) {
+            ConstVariableValue::Known { value } => {
+                bug!("instantiating a known const var: {target_vid:?} {value} {source_ct}")
+            }
+            ConstVariableValue::Unknown { origin, universe: _ } => origin.span,
+        };
+        // FIXME(generic_const_exprs): Occurs check failures for unevaluated
+        // constants and generic expressions are not yet handled correctly.
+        let Generalization { value_may_be_infer: generalized_ct, has_unconstrained_ty_var } =
+            generalize(
+                self,
+                &mut CombineDelegate { infcx: self, span },
+                source_ct,
+                target_vid,
+                ty::Variance::Invariant,
+            )?;
+
+        debug_assert!(!generalized_ct.is_ct_infer());
+        if has_unconstrained_ty_var {
+            span_bug!(span, "unconstrained ty var when generalizing `{source_ct:?}`");
+        }
+
+        self.inner
+            .borrow_mut()
+            .const_unification_table()
+            .union_value(target_vid, ConstVariableValue::Known { value: generalized_ct });
+
+        // FIXME(generic_const_exprs): We have to make sure we actually equate
+        // `generalized_ct` and `source_ct` here.`
+        Ok(generalized_ct)
+    }
+}
 
 /// Attempts to generalize `term` for the type variable `for_vid`.
 /// This checks for cycles -- that is, whether the type `term`
 /// references `for_vid`.
-pub fn generalize<'tcx, D: GeneralizerDelegate<'tcx>, T: Into<Term<'tcx>> + Relate<'tcx>>(
+pub(super) fn generalize<'tcx, D: GeneralizerDelegate<'tcx>, T: Into<Term<'tcx>> + Relate<'tcx>>(
     infcx: &InferCtxt<'tcx>,
     delegate: &mut D,
     term: T,

compiler/rustc_infer/src/infer/relate/glb.rs

@@ -2,6 +2,7 @@
 
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_span::Span;
 
 use super::combine::{CombineFields, ObligationEmittingRelation};
 use super::lattice::{self, LatticeDir};
@@ -134,6 +135,10 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, 'tcx> {
+    fn span(&self) -> Span {
+        self.fields.trace.span()
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/relate/lub.rs

@@ -7,6 +7,7 @@ use crate::traits::{ObligationCause, PredicateObligations};
 
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_span::Span;
 
 /// "Least upper bound" (common supertype)
 pub struct Lub<'combine, 'infcx, 'tcx> {
@@ -134,6 +135,10 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Lub<'_, '_, 'tcx> {
+    fn span(&self) -> Span {
+        self.fields.trace.span()
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

compiler/rustc_infer/src/infer/relate/nll.rs

@@ -670,6 +670,10 @@ impl<'tcx, D> ObligationEmittingRelation<'tcx> for TypeRelating<'_, 'tcx, D>
 where
     D: TypeRelatingDelegate<'tcx>,
 {
+    fn span(&self) -> Span {
+        self.delegate.span()
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.delegate.param_env()
     }

compiler/rustc_infer/src/infer/relate/sub.rs

@@ -6,6 +6,7 @@ use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::TyVar;
 use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::Span;
 use std::mem;
 
 /// Ensures `a` is made a subtype of `b`. Returns `a` on success.
@@ -103,12 +104,12 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
 
                 Ok(a)
             }
-            (&ty::Infer(TyVar(a_id)), _) => {
+            (&ty::Infer(TyVar(a_vid)), _) => {
-                self.fields.instantiate(b, ty::Contravariant, a_id, !self.a_is_expected)?;
+                infcx.instantiate_ty_var(self, self.a_is_expected, a_vid, ty::Covariant, b)?;
                 Ok(a)
             }
-            (_, &ty::Infer(TyVar(b_id))) => {
+            (_, &ty::Infer(TyVar(b_vid))) => {
-                self.fields.instantiate(a, ty::Covariant, b_id, self.a_is_expected)?;
+                infcx.instantiate_ty_var(self, !self.a_is_expected, b_vid, ty::Contravariant, a)?;
                 Ok(a)
             }
 
@@ -199,6 +200,10 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for Sub<'_, '_, 'tcx> {
+    fn span(&self) -> Span {
+        self.fields.trace.span()
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

tests/ui/coherence/occurs-check/associated-type.next.stderr

@@ -1,11 +1,11 @@
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 error[E0119]: conflicting implementations of trait `Overlap<for<'a> fn(&'a (), ())>` for type `for<'a> fn(&'a (), ())`
   --> $DIR/associated-type.rs:31:1
    |

tests/ui/coherence/occurs-check/associated-type.old.stderr

@@ -1,11 +1,11 @@
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!3_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!3_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!3_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
-WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!3_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
+WARN rustc_infer::infer::relate::generalize may incompletely handle alias type: Alias(Projection, AliasTy { args: [*const ?1t, RePlaceholder(!2_BoundRegion { var: 0, kind: BrNamed(DefId(0:27 ~ associated_type[f554]::{impl#3}::'a#1), 'a) })], def_id: DefId(0:5 ~ associated_type[f554]::ToUnit::Unit) })
 error[E0119]: conflicting implementations of trait `Overlap<for<'a> fn(&'a (), _)>` for type `for<'a> fn(&'a (), _)`
   --> $DIR/associated-type.rs:31:1
    |

tests/ui/traits/next-solver/generalize/occurs-check-nested-alias.next.stderr

@@ -1,8 +1,8 @@
-error[E0284]: type annotations needed: cannot satisfy `<<T as Id<_>>::Id as Unnormalizable>::Assoc == _`
+error[E0284]: type annotations needed: cannot satisfy `_ == <<T as Id<_>>::Id as Unnormalizable>::Assoc`
   --> $DIR/occurs-check-nested-alias.rs:36:9
    |
 LL |     x = y;
-   |         ^ cannot satisfy `<<T as Id<_>>::Id as Unnormalizable>::Assoc == _`
+   |         ^ cannot satisfy `_ == <<T as Id<_>>::Id as Unnormalizable>::Assoc`
 
 error: aborting due to 1 previous error