avoid generalization inside of aliases

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -1,7 +1,8 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::ErrorGuaranteed;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_infer::infer::{NllRegionVariableOrigin, ObligationEmittingRelation};
+use rustc_infer::infer::NllRegionVariableOrigin;
+use rustc_infer::infer::{ObligationEmittingRelation, StructurallyRelateAliases};
 use rustc_infer::traits::{Obligation, PredicateObligations};
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::traits::query::NoSolution;
@@ -548,6 +549,10 @@ impl<'bccx, 'tcx> ObligationEmittingRelation<'tcx> for NllTypeRelating<'_, 'bccx
         self.locations.span(self.type_checker.body)
     }
 
+    fn structurally_relate_aliases(&self) -> StructurallyRelateAliases {
+        StructurallyRelateAliases::No
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.type_checker.param_env
     }

compiler/rustc_hir_typeck/src/coercion.rs

@@ -44,6 +44,8 @@ use rustc_hir::Expr;
 use rustc_hir_analysis::astconv::AstConv;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
+use rustc_infer::traits::TraitEngine;
+use rustc_infer::traits::TraitEngineExt as _;
 use rustc_infer::traits::{Obligation, PredicateObligation};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::traits::BuiltinImplSource;
@@ -61,6 +63,7 @@ use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
+use rustc_trait_selection::traits::TraitEngineExt as _;
 use rustc_trait_selection::traits::{
     self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
 };
@@ -157,17 +160,19 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             // In the new solver, lazy norm may allow us to shallowly equate
             // more types, but we emit possibly impossible-to-satisfy obligations.
             // Filter these cases out to make sure our coercion is more accurate.
-            if self.next_trait_solver() {
-                if let Ok(res) = &res {
-                    for obligation in &res.obligations {
-                        if !self.predicate_may_hold(obligation) {
-                            return Err(TypeError::Mismatch);
-                        }
+            match res {
+                Ok(InferOk { value, obligations }) if self.next_trait_solver() => {
+                    let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(self);
+                    fulfill_cx.register_predicate_obligations(self, obligations);
+                    let errs = fulfill_cx.select_where_possible(self);
+                    if errs.is_empty() {
+                        Ok(InferOk { value, obligations: fulfill_cx.pending_obligations() })
+                    } else {
+                        Err(TypeError::Mismatch)
                     }
                 }
+                res => res,
             }
-
-            res
         })
     }
 
@@ -625,19 +630,18 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         let traits = [coerce_unsized_did, unsize_did];
         while !queue.is_empty() {
             let obligation = queue.remove(0);
-            debug!("coerce_unsized resolve step: {:?}", obligation);
             let trait_pred = match obligation.predicate.kind().no_bound_vars() {
                 Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
                     if traits.contains(&trait_pred.def_id()) =>
                 {
-                    trait_pred
+                    self.resolve_vars_if_possible(trait_pred)
                 }
                 _ => {
                     coercion.obligations.push(obligation);
                     continue;
                 }
             };
-            let trait_pred = self.resolve_vars_if_possible(trait_pred);
+            debug!("coerce_unsized resolve step: {:?}", trait_pred);
             match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
                 // Uncertain or unimplemented.
                 Ok(None) => {

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1516,6 +1516,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// In case there is still ambiguity, the returned type may be an inference
     /// variable. This is different from `structurally_resolve_type` which errors
     /// in this case.
+    #[instrument(level = "debug", skip(self, sp), ret)]
     pub fn try_structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
         let ty = self.resolve_vars_with_obligations(ty);
 

compiler/rustc_infer/src/infer/at.rs

@@ -302,7 +302,23 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
         let Trace { at, trace, a_is_expected } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .equate(a_is_expected)
+            .equate(StructurallyRelateAliases::No, a_is_expected)
+            .relate(a, b)
+            .map(move |_| InferOk { value: (), obligations: fields.obligations })
+    }
+
+    /// Equates `a` and `b` while structurally relating aliases. This should only
+    /// be used inside of the next generation trait solver when relating rigid aliases.
+    #[instrument(skip(self), level = "debug")]
+    pub fn eq_structurally_relating_aliases<T>(self, a: T, b: T) -> InferResult<'tcx, ()>
+    where
+        T: Relate<'tcx>,
+    {
+        let Trace { at, trace, a_is_expected } = 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)
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }

compiler/rustc_infer/src/infer/mod.rs

@@ -54,6 +54,7 @@ use self::region_constraints::{
     RegionConstraintCollector, RegionConstraintStorage, RegionSnapshot,
 };
 pub use self::relate::combine::CombineFields;
+pub use self::relate::StructurallyRelateAliases;
 use self::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 
 pub mod at;

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

@@ -26,6 +26,7 @@ use super::equate::Equate;
 use super::glb::Glb;
 use super::lub::Lub;
 use super::sub::Sub;
+use super::StructurallyRelateAliases;
 use crate::infer::{DefineOpaqueTypes, InferCtxt, TypeTrace};
 use crate::traits::{Obligation, PredicateObligations};
 use rustc_middle::infer::canonical::OriginalQueryValues;
@@ -116,8 +117,15 @@ impl<'tcx> InferCtxt<'tcx> {
             }
 
             (_, ty::Alias(..)) | (ty::Alias(..), _) if self.next_trait_solver() => {
-                relation.register_type_relate_obligation(a, b);
-                Ok(a)
+                match relation.structurally_relate_aliases() {
+                    StructurallyRelateAliases::Yes => {
+                        ty::relate::structurally_relate_tys(relation, a, b)
+                    }
+                    StructurallyRelateAliases::No => {
+                        relation.register_type_relate_obligation(a, b);
+                        Ok(a)
+                    }
+                }
             }
 
             // All other cases of inference are errors
@@ -240,19 +248,26 @@ impl<'tcx> InferCtxt<'tcx> {
             (ty::ConstKind::Unevaluated(..), _) | (_, ty::ConstKind::Unevaluated(..))
                 if self.tcx.features().generic_const_exprs || self.next_trait_solver() =>
             {
-                let (a, b) = if relation.a_is_expected() { (a, b) } else { (b, a) };
-
-                relation.register_predicates([if self.next_trait_solver() {
-                    ty::PredicateKind::AliasRelate(
-                        a.into(),
-                        b.into(),
-                        ty::AliasRelationDirection::Equate,
-                    )
-                } else {
-                    ty::PredicateKind::ConstEquate(a, b)
-                }]);
-
-                Ok(b)
+                match relation.structurally_relate_aliases() {
+                    StructurallyRelateAliases::No => {
+                        let (a, b) = if relation.a_is_expected() { (a, b) } else { (b, a) };
+
+                        relation.register_predicates([if self.next_trait_solver() {
+                            ty::PredicateKind::AliasRelate(
+                                a.into(),
+                                b.into(),
+                                ty::AliasRelationDirection::Equate,
+                            )
+                        } else {
+                            ty::PredicateKind::ConstEquate(a, b)
+                        }]);
+
+                        Ok(b)
+                    }
+                    StructurallyRelateAliases::Yes => {
+                        ty::relate::structurally_relate_consts(relation, a, b)
+                    }
+                }
             }
             _ => ty::relate::structurally_relate_consts(relation, a, b),
         }
@@ -303,8 +318,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         self.infcx.tcx
     }
 
-    pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
-        Equate::new(self, a_is_expected)
+    pub fn equate<'a>(
+        &'a mut self,
+        structurally_relate_aliases: StructurallyRelateAliases,
+        a_is_expected: bool,
+    ) -> Equate<'a, 'infcx, 'tcx> {
+        Equate::new(self, structurally_relate_aliases, a_is_expected)
     }
 
     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
@@ -335,6 +354,11 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
 
     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 
+    /// Whether aliases should be related structurally. This is pretty much
+    /// always `No` unless you're equating in some specific locations of the
+    /// new solver. See the comments in these use-cases for more details.
+    fn structurally_relate_aliases(&self) -> StructurallyRelateAliases;
+
     /// Register obligations that must hold in order for this relation to hold
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);
 

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

@@ -1,4 +1,5 @@
 use super::combine::{CombineFields, ObligationEmittingRelation};
+use super::StructurallyRelateAliases;
 use crate::infer::{DefineOpaqueTypes, SubregionOrigin};
 use crate::traits::PredicateObligations;
 
@@ -13,15 +14,17 @@ use rustc_span::Span;
 /// Ensures `a` is made equal to `b`. Returns `a` on success.
 pub struct Equate<'combine, 'infcx, 'tcx> {
     fields: &'combine mut CombineFields<'infcx, 'tcx>,
+    structurally_relate_aliases: StructurallyRelateAliases,
     a_is_expected: bool,
 }
 
 impl<'combine, 'infcx, 'tcx> Equate<'combine, 'infcx, 'tcx> {
     pub fn new(
         fields: &'combine mut CombineFields<'infcx, 'tcx>,
+        structurally_relate_aliases: StructurallyRelateAliases,
         a_is_expected: bool,
     ) -> Equate<'combine, 'infcx, 'tcx> {
-        Equate { fields, a_is_expected }
+        Equate { fields, structurally_relate_aliases, a_is_expected }
     }
 }
 
@@ -99,7 +102,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: a_def_id, .. }),
                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }),
             ) if a_def_id == b_def_id => {
-                self.fields.infcx.super_combine_tys(self, a, b)?;
+                infcx.super_combine_tys(self, a, b)?;
             }
             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
@@ -120,7 +123,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
                 );
             }
             _ => {
-                self.fields.infcx.super_combine_tys(self, a, b)?;
+                infcx.super_combine_tys(self, a, b)?;
             }
         }
 
@@ -180,6 +183,10 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.fields.trace.span()
     }
 
+    fn structurally_relate_aliases(&self) -> StructurallyRelateAliases {
+        self.structurally_relate_aliases
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }

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

@@ -1,5 +1,6 @@
 use std::mem;
 
+use super::StructurallyRelateAliases;
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind, TypeVariableValue};
 use crate::infer::{InferCtxt, ObligationEmittingRelation, RegionVariableOrigin};
 use rustc_data_structures::sso::SsoHashMap;
@@ -45,8 +46,14 @@ impl<'tcx> InferCtxt<'tcx> {
         // 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 } =
-            self.generalize(relation.span(), target_vid, instantiation_variance, source_ty)?;
+        let Generalization { value_may_be_infer: generalized_ty, has_unconstrained_ty_var } = self
+            .generalize(
+                relation.span(),
+                relation.structurally_relate_aliases(),
+                target_vid,
+                instantiation_variance,
+                source_ty,
+            )?;
 
         // Constrain `b_vid` to the generalized type `generalized_ty`.
         if let &ty::Infer(ty::TyVar(generalized_vid)) = generalized_ty.kind() {
@@ -178,8 +185,14 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> RelateResult<'tcx, ()> {
         // 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 } =
-            self.generalize(relation.span(), target_vid, ty::Variance::Invariant, source_ct)?;
+        let Generalization { value_may_be_infer: generalized_ct, has_unconstrained_ty_var } = self
+            .generalize(
+                relation.span(),
+                relation.structurally_relate_aliases(),
+                target_vid,
+                ty::Variance::Invariant,
+                source_ct,
+            )?;
 
         debug_assert!(!generalized_ct.is_ct_infer());
         if has_unconstrained_ty_var {
@@ -217,6 +230,7 @@ impl<'tcx> InferCtxt<'tcx> {
     fn generalize<T: Into<Term<'tcx>> + Relate<'tcx>>(
         &self,
         span: Span,
+        structurally_relate_aliases: StructurallyRelateAliases,
         target_vid: impl Into<ty::TermVid>,
         ambient_variance: ty::Variance,
         source_term: T,
@@ -237,6 +251,7 @@ impl<'tcx> InferCtxt<'tcx> {
         let mut generalizer = Generalizer {
             infcx: self,
             span,
+            structurally_relate_aliases,
             root_vid,
             for_universe,
             ambient_variance,
@@ -270,6 +285,10 @@ struct Generalizer<'me, 'tcx> {
 
     span: Span,
 
+    /// Whether aliases should be related structurally. If not, we have to
+    /// be careful when generalizing aliases.
+    structurally_relate_aliases: StructurallyRelateAliases,
+
     /// The vid of the type variable that is in the process of being
     /// instantiated. If we find this within the value we are folding,
     /// that means we would have created a cyclic value.
@@ -314,13 +333,30 @@ impl<'tcx> Generalizer<'_, 'tcx> {
     /// to normalize the alias after all.
     ///
     /// We handle this by lazily equating the alias and generalizing
-    /// it to an inference variable.
+    /// it to an inference variable. In the new solver, we always
+    /// generalize to an infer var unless the alias contains escaping
+    /// bound variables.
     ///
-    /// This is incomplete and will hopefully soon get fixed by #119106.
+    /// Correctly handling aliases with escaping bound variables is
+    /// difficult and currently incomplete in two opposite ways:
+    /// - if we get an occurs check failure in the alias, replace it with a new infer var.
+    ///   This causes us to later emit an alias-relate goal and is incomplete in case the
+    ///   alias normalizes to type containing one of the bound variables.
+    /// - if the alias contains an inference variable not nameable by `for_universe`, we
+    ///   continue generalizing the alias. This ends up pulling down the universe of the
+    ///   inference variable and is incomplete in case the alias would normalize to a type
+    ///   which does not mention that inference variable.
     fn generalize_alias_ty(
         &mut self,
         alias: ty::AliasTy<'tcx>,
     ) -> Result<Ty<'tcx>, TypeError<'tcx>> {
+        if self.infcx.next_trait_solver() && !alias.has_escaping_bound_vars() {
+            return Ok(self.infcx.next_ty_var_in_universe(
+                TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span: self.span },
+                self.for_universe,
+            ));
+        }
+
         let is_nested_alias = mem::replace(&mut self.in_alias, true);
         let result = match self.relate(alias, alias) {
             Ok(alias) => Ok(alias.to_ty(self.tcx())),
@@ -490,7 +526,10 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 }
             }
 
-            ty::Alias(_, data) => self.generalize_alias_ty(data),
+            ty::Alias(_, data) => match self.structurally_relate_aliases {
+                StructurallyRelateAliases::No => self.generalize_alias_ty(data),
+                StructurallyRelateAliases::Yes => relate::structurally_relate_tys(self, t, t),
+            },
 
             _ => relate::structurally_relate_tys(self, t, t),
         }?;