Rollup merge of #112122 - compiler-errors:next-coherence, r=lcnr

Add `-Ztrait-solver=next-coherence`

Flag that conditionally uses the trait solver *only* during coherence, for more testing and/or eventual partial-migration onto the trait solver (in the medium- to long-term).

* This still uses the selection context in some of the coherence methods I think, so it's not "complete". Putting this up for review and/or for further work in-tree.
* I probably need to spend a bit more time making sure that we don't sneakily create any other infcx's during coherence that also need the new solver enabled.

r? `@lcnr`

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -285,7 +285,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
         let infcx = self
             .tcx
             .infer_ctxt()
-            .with_opaque_type_inference(if self.tcx.trait_solver_next() {
+            .with_opaque_type_inference(if self.next_trait_solver() {
                 DefiningAnchor::Bind(def_id)
             } else {
                 DefiningAnchor::Bubble

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -188,7 +188,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
 
     // FIXME(-Ztrait-solver=next): A bit dubious that we're only registering
     // predefined opaques in the typeck root.
-    if infcx.tcx.trait_solver_next() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
+    if infcx.next_trait_solver() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
         checker.register_predefined_opaques_in_new_solver();
     }
 

compiler/rustc_hir_analysis/src/autoderef.rs

@@ -73,7 +73,7 @@ impl<'a, 'tcx> Iterator for Autoderef<'a, 'tcx> {
             // NOTE: we may still need to normalize the built-in deref in case
             // we have some type like `&<Ty as Trait>::Assoc`, since users of
             // autoderef expect this type to have been structurally normalized.
-            if self.infcx.tcx.trait_solver_next()
+            if self.infcx.next_trait_solver()
                 && let ty::Alias(ty::Projection, _) = ty.kind()
             {
                 let (normalized_ty, obligations) = self.structurally_normalize(ty)?;
@@ -161,8 +161,7 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
         &self,
         ty: Ty<'tcx>,
     ) -> Option<(Ty<'tcx>, Vec<traits::PredicateObligation<'tcx>>)> {
-        let tcx = self.infcx.tcx;
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new_in_snapshot(tcx);
+        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new_in_snapshot(self.infcx);
 
         let cause = traits::ObligationCause::misc(self.span, self.body_id);
         let normalized_ty = match self

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1549,7 +1549,7 @@ pub(super) fn check_generator_obligations(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         .with_opaque_type_inference(DefiningAnchor::Bind(def_id))
         .build();
 
-    let mut fulfillment_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
+    let mut fulfillment_cx = <dyn TraitEngine<'_>>::new(&infcx);
     for (predicate, cause) in generator_interior_predicates {
         let obligation = Obligation::new(tcx, cause.clone(), param_env, *predicate);
         fulfillment_cx.register_predicate_obligation(&infcx, obligation);

compiler/rustc_hir_typeck/src/coercion.rs

@@ -156,7 +156,7 @@ 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.tcx.trait_solver_next() {
+            if self.next_trait_solver() {
                 if let Ok(res) = &res {
                     for obligation in &res.obligations {
                         if !self.predicate_may_hold(&obligation) {

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1476,7 +1476,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn structurally_resolved_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
         let mut ty = self.resolve_vars_with_obligations(ty);
 
-        if self.tcx.trait_solver_next()
+        if self.next_trait_solver()
             && let ty::Alias(ty::Projection, _) = ty.kind()
         {
             match self

compiler/rustc_hir_typeck/src/inherited.rs

@@ -86,8 +86,8 @@ impl<'tcx> Inherited<'tcx> {
 
         Inherited {
             typeck_results,
+            fulfillment_cx: RefCell::new(<dyn TraitEngine<'_>>::new(&infcx)),
             infcx,
-            fulfillment_cx: RefCell::new(<dyn TraitEngine<'_>>::new(tcx)),
             locals: RefCell::new(Default::default()),
             deferred_sized_obligations: RefCell::new(Vec::new()),
             deferred_call_resolutions: RefCell::new(Default::default()),

compiler/rustc_hir_typeck/src/writeback.rs

@@ -591,7 +591,7 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
                 .insert(opaque_type_key, hidden_type)
                 && last_opaque_ty.ty != hidden_type.ty
             {
-                assert!(!self.tcx().trait_solver_next());
+                assert!(!self.fcx.next_trait_solver());
                 hidden_type
                     .report_mismatch(&last_opaque_ty, opaque_type_key.def_id, self.tcx())
                     .stash(
@@ -812,7 +812,7 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Resolver<'cx, 'tcx> {
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         match self.fcx.fully_resolve(t) {
-            Ok(t) if self.fcx.tcx.trait_solver_next() => {
+            Ok(t) if self.fcx.next_trait_solver() => {
                 // We must normalize erasing regions here, since later lints
                 // expect that types that show up in the typeck are fully
                 // normalized.

compiler/rustc_infer/src/infer/at.rs

@@ -82,6 +82,7 @@ impl<'tcx> InferCtxt<'tcx> {
             in_snapshot: self.in_snapshot.clone(),
             universe: self.universe.clone(),
             intercrate: self.intercrate,
+            next_trait_solver: self.next_trait_solver,
         }
     }
 }

compiler/rustc_infer/src/infer/combine.rs

@@ -109,11 +109,11 @@ impl<'tcx> InferCtxt<'tcx> {
             | (
                 ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)),
                 ty::Alias(AliasKind::Projection, _),
-            ) if self.tcx.trait_solver_next() => {
+            ) if self.next_trait_solver() => {
                 bug!()
             }
 
-            (_, ty::Alias(..)) | (ty::Alias(..), _) if self.tcx.trait_solver_next() => {
+            (_, ty::Alias(..)) | (ty::Alias(..), _) if self.next_trait_solver() => {
                 relation.register_type_relate_obligation(a, b);
                 Ok(a)
             }
@@ -227,9 +227,20 @@ impl<'tcx> InferCtxt<'tcx> {
                 return self.unify_const_variable(vid, a, relation.param_env());
             }
             (ty::ConstKind::Unevaluated(..), _) | (_, ty::ConstKind::Unevaluated(..))
-                if self.tcx.features().generic_const_exprs || self.tcx.trait_solver_next() =>
+                if self.tcx.features().generic_const_exprs || self.next_trait_solver() =>
             {
-                relation.register_const_equate_obligation(a, b);
+                let (a, b) = if relation.a_is_expected() { (a, b) } else { (b, a) };
+
+                relation.register_predicates([ty::Binder::dummy(if self.next_trait_solver() {
+                    ty::PredicateKind::AliasRelate(
+                        a.into(),
+                        b.into(),
+                        ty::AliasRelationDirection::Equate,
+                    )
+                } else {
+                    ty::PredicateKind::ConstEquate(a, b)
+                })]);
+
                 return Ok(b);
             }
             _ => {}
@@ -453,19 +464,6 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     /// be used if control over the obligation causes is required.
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ToPredicate<'tcx>>);
 
-    /// Register an obligation that both constants must be equal to each other.
-    ///
-    /// If they aren't equal then the relation doesn't hold.
-    fn register_const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
-
-        self.register_predicates([ty::Binder::dummy(if self.tcx().trait_solver_next() {
-            ty::PredicateKind::AliasRelate(a.into(), b.into(), ty::AliasRelationDirection::Equate)
-        } else {
-            ty::PredicateKind::ConstEquate(a, b)
-        })]);
-    }
-
     /// Register an obligation that both types must be related to each other according to
     /// the [`ty::AliasRelationDirection`] given by [`ObligationEmittingRelation::alias_relate_direction`]
     fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {

compiler/rustc_infer/src/infer/equate.rs

@@ -105,7 +105,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
                 if self.fields.define_opaque_types == DefineOpaqueTypes::Yes
                     && def_id.is_local()
-                    && !self.tcx().trait_solver_next() =>
+                    && !self.fields.infcx.next_trait_solver() =>
             {
                 self.fields.obligations.extend(
                     infcx

compiler/rustc_infer/src/infer/lattice.rs

@@ -113,7 +113,7 @@ where
         | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
             if this.define_opaque_types() == DefineOpaqueTypes::Yes
                 && def_id.is_local()
-                && !this.tcx().trait_solver_next() =>
+                && !this.infcx().next_trait_solver() =>
         {
             this.register_obligations(
                 infcx

compiler/rustc_infer/src/infer/mod.rs

@@ -330,6 +330,8 @@ pub struct InferCtxt<'tcx> {
     /// there is no type that the user could *actually name* that
     /// would satisfy it. This avoids crippling inference, basically.
     pub intercrate: bool,
+
+    next_trait_solver: bool,
 }
 
 /// See the `error_reporting` module for more details.
@@ -545,6 +547,9 @@ pub struct InferCtxtBuilder<'tcx> {
     skip_leak_check: bool,
     /// Whether we are in coherence mode.
     intercrate: bool,
+    /// Whether we should use the new trait solver in the local inference context,
+    /// which affects things like which solver is used in `predicate_may_hold`.
+    next_trait_solver: bool,
 }
 
 pub trait TyCtxtInferExt<'tcx> {
@@ -559,6 +564,7 @@ impl<'tcx> TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
             considering_regions: true,
             skip_leak_check: false,
             intercrate: false,
+            next_trait_solver: self.next_trait_solver_globally(),
         }
     }
 }
@@ -575,6 +581,11 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
+    pub fn with_next_trait_solver(mut self, next_trait_solver: bool) -> Self {
+        self.next_trait_solver = next_trait_solver;
+        self
+    }
+
     pub fn intercrate(mut self, intercrate: bool) -> Self {
         self.intercrate = intercrate;
         self
@@ -617,6 +628,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             considering_regions,
             skip_leak_check,
             intercrate,
+            next_trait_solver,
         } = *self;
         InferCtxt {
             tcx,
@@ -634,6 +646,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             in_snapshot: Cell::new(false),
             universe: Cell::new(ty::UniverseIndex::ROOT),
             intercrate,
+            next_trait_solver,
         }
     }
 }
@@ -670,6 +683,10 @@ pub struct CombinedSnapshot<'tcx> {
 }
 
 impl<'tcx> InferCtxt<'tcx> {
+    pub fn next_trait_solver(&self) -> bool {
+        self.next_trait_solver
+    }
+
     /// Creates a `TypeErrCtxt` for emitting various inference errors.
     /// During typeck, use `FnCtxt::err_ctxt` instead.
     pub fn err_ctxt(&self) -> TypeErrCtxt<'_, 'tcx> {

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -491,12 +491,12 @@ where
             (
                 &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 || infcx.tcx.trait_solver_next() => {
+            ) if a_def_id == b_def_id || infcx.next_trait_solver() => {
                 infcx.super_combine_tys(self, a, b).or_else(|err| {
                     // This behavior is only there for the old solver, the new solver
                     // shouldn't ever fail. Instead, it unconditionally emits an
                     // alias-relate goal.
-                    assert!(!self.tcx().trait_solver_next());
+                    assert!(!self.infcx.next_trait_solver());
                     self.tcx().sess.delay_span_bug(
                         self.delegate.span(),
                         "failure to relate an opaque to itself should result in an error later on",
@@ -506,7 +506,7 @@ where
             }
             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
-                if def_id.is_local() && !self.tcx().trait_solver_next() =>
+                if def_id.is_local() && !self.infcx.next_trait_solver() =>
             {
                 self.relate_opaques(a, b)
             }

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -49,7 +49,7 @@ impl<'tcx> InferCtxt<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
     ) -> InferOk<'tcx, T> {
         // We handle opaque types differently in the new solver.
-        if self.tcx.trait_solver_next() {
+        if self.next_trait_solver() {
             return InferOk { value, obligations: vec![] };
         }
 
@@ -578,7 +578,7 @@ impl<'tcx> InferCtxt<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         hidden_ty: Ty<'tcx>,
     ) -> InferResult<'tcx, ()> {
-        assert!(self.tcx.trait_solver_next());
+        assert!(self.next_trait_solver());
         let origin = self
             .opaque_type_origin(opaque_type_key.def_id)
             .expect("should be called for defining usages only");
@@ -614,7 +614,7 @@ impl<'tcx> InferCtxt<'tcx> {
                     ty::Alias(ty::Projection, projection_ty)
                         if !projection_ty.has_escaping_bound_vars()
                             && !tcx.is_impl_trait_in_trait(projection_ty.def_id)
-                            && !tcx.trait_solver_next() =>
+                            && !self.next_trait_solver() =>
                     {
                         self.infer_projection(
                             param_env,

compiler/rustc_infer/src/infer/projection.rs

@@ -21,7 +21,7 @@ impl<'tcx> InferCtxt<'tcx> {
         recursion_depth: usize,
         obligations: &mut Vec<PredicateObligation<'tcx>>,
     ) -> Ty<'tcx> {
-        if self.tcx.trait_solver_next() {
+        if self.next_trait_solver() {
             // FIXME(-Ztrait-solver=next): Instead of branching here,
             // completely change the normalization routine with the new solver.
             //

compiler/rustc_infer/src/infer/sub.rs

@@ -132,7 +132,7 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
                 if self.fields.define_opaque_types == DefineOpaqueTypes::Yes
                     && def_id.is_local()
-                    && !self.tcx().trait_solver_next() =>
+                    && !self.fields.infcx.next_trait_solver() =>
             {
                 self.fields.obligations.extend(
                     infcx

compiler/rustc_middle/src/ty/context.rs

@@ -2333,10 +2333,18 @@ impl<'tcx> TyCtxt<'tcx> {
         self.opt_local_def_id_to_hir_id(local_def_id).unwrap()
     }
 
-    pub fn trait_solver_next(self) -> bool {
+    pub fn next_trait_solver_globally(self) -> bool {
         self.sess.opts.unstable_opts.trait_solver == rustc_session::config::TraitSolver::Next
     }
 
+    pub fn next_trait_solver_in_coherence(self) -> bool {
+        matches!(
+            self.sess.opts.unstable_opts.trait_solver,
+            rustc_session::config::TraitSolver::Next
+                | rustc_session::config::TraitSolver::NextCoherence
+        )
+    }
+
     pub fn lower_impl_trait_in_trait_to_assoc_ty(self) -> bool {
         self.sess.opts.unstable_opts.lower_impl_trait_in_trait_to_assoc_ty
     }

compiler/rustc_session/src/config.rs

@@ -610,6 +610,8 @@ pub enum TraitSolver {
     Chalk,
     /// Experimental trait solver in `rustc_trait_selection::solve`
     Next,
+    /// Use the new trait solver during coherence
+    NextCoherence,
 }
 
 pub enum Input {

compiler/rustc_session/src/options.rs

@@ -986,6 +986,7 @@ mod parse {
             Some("classic") => *slot = TraitSolver::Classic,
             Some("chalk") => *slot = TraitSolver::Chalk,
             Some("next") => *slot = TraitSolver::Next,
+            Some("next-coherence") => *slot = TraitSolver::NextCoherence,
             // default trait solver is subject to change..
             Some("default") => *slot = TraitSolver::Classic,
             _ => return false,

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -187,6 +187,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             let (ref infcx, input, var_values) = tcx
                 .infer_ctxt()
                 .intercrate(intercrate)
+                .with_next_trait_solver(true)
                 .with_opaque_type_inference(canonical_input.value.anchor)
                 .build_with_canonical(DUMMY_SP, &canonical_input);
 

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -182,6 +182,7 @@ fn overlap<'tcx>(
         .with_opaque_type_inference(DefiningAnchor::Bubble)
         .skip_leak_check(skip_leak_check.is_yes())
         .intercrate(true)
+        .with_next_trait_solver(tcx.next_trait_solver_in_coherence())
         .build();
     let selcx = &mut SelectionContext::new(&infcx);
     if track_ambiguity_causes.is_yes() {