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_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