Rollup merge of rust-lang#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`

Author: Dylan-DPC

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>) {