Rollup merge of #107102 - compiler-errors:new-solver-new-candidats-4, r=lcnr

Implement some more predicates in the new solver

Implement a few more goals. The subtype goal specifically is important, since it's required for this code to compile:

```
fn main() {
  let mut x = vec![];
  x.push(1i32);
}
```

(I think we emit a subtype goal here because of coercion).

Drive-by: Also implements `--compare-mode=next-solver` -- I've been using this locally a lot to find out what works and what doesn't. I'm also happy to split this out into another PR.

r? `@lcnr`

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -1,7 +1,7 @@
 //! Code shared by trait and projection goals for candidate assembly.
 
 use super::infcx_ext::InferCtxtExt;
-use super::{CanonicalResponse, Certainty, EvalCtxt, Goal, MaybeCause, QueryResult};
+use super::{CanonicalResponse, Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::util::elaborate_predicates;
@@ -148,9 +148,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         if goal.predicate.self_ty().is_ty_var() {
             return vec![Candidate {
                 source: CandidateSource::BuiltinImpl,
-                result: self
-                    .make_canonical_response(Certainty::Maybe(MaybeCause::Ambiguity))
-                    .unwrap(),
+                result: self.make_canonical_response(Certainty::AMBIGUOUS).unwrap(),
             }];
         }
 

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -19,6 +19,7 @@
 
 use std::mem;
 
+use rustc_hir::def_id::DefId;
 use rustc_infer::infer::canonical::{Canonical, CanonicalVarKind, CanonicalVarValues};
 use rustc_infer::infer::canonical::{OriginalQueryValues, QueryRegionConstraints, QueryResponse};
 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
@@ -26,7 +27,9 @@ use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::Obligation;
 use rustc_middle::infer::canonical::Certainty as OldCertainty;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_middle::ty::{RegionOutlivesPredicate, ToPredicate, TypeOutlivesPredicate};
+use rustc_middle::ty::{
+    CoercePredicate, RegionOutlivesPredicate, SubtypePredicate, ToPredicate, TypeOutlivesPredicate,
+};
 use rustc_span::DUMMY_SP;
 
 use crate::traits::ObligationCause;
@@ -87,6 +90,8 @@ pub enum Certainty {
 }
 
 impl Certainty {
+    pub const AMBIGUOUS: Certainty = Certainty::Maybe(MaybeCause::Ambiguity);
+
     /// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
     /// use this function to unify the certainty of these goals
     pub fn unify_and(self, other: Certainty) -> Certainty {
@@ -243,16 +248,28 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => {
                     self.compute_region_outlives_goal(Goal { param_env, predicate })
                 }
+                ty::PredicateKind::Subtype(predicate) => {
+                    self.compute_subtype_goal(Goal { param_env, predicate })
+                }
+                ty::PredicateKind::Coerce(predicate) => {
+                    self.compute_coerce_goal(Goal { param_env, predicate })
+                }
+                ty::PredicateKind::ClosureKind(def_id, substs, kind) => self
+                    .compute_closure_kind_goal(Goal {
+                        param_env,
+                        predicate: (def_id, substs, kind),
+                    }),
+                ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::AMBIGUOUS),
                 // FIXME: implement these predicates :)
                 ty::PredicateKind::WellFormed(_)
                 | ty::PredicateKind::ObjectSafe(_)
-                | ty::PredicateKind::ClosureKind(_, _, _)
-                | ty::PredicateKind::Subtype(_)
-                | ty::PredicateKind::Coerce(_)
                 | ty::PredicateKind::ConstEvaluatable(_)
-                | ty::PredicateKind::ConstEquate(_, _)
-                | ty::PredicateKind::TypeWellFormedFromEnv(_)
-                | ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::Yes),
+                | ty::PredicateKind::ConstEquate(_, _) => {
+                    self.make_canonical_response(Certainty::Yes)
+                }
+                ty::PredicateKind::TypeWellFormedFromEnv(..) => {
+                    bug!("TypeWellFormedFromEnv is only used for Chalk")
+                }
             }
         } else {
             let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
@@ -275,6 +292,58 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
     ) -> QueryResult<'tcx> {
         self.make_canonical_response(Certainty::Yes)
     }
+
+    fn compute_coerce_goal(
+        &mut self,
+        goal: Goal<'tcx, CoercePredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        self.compute_subtype_goal(Goal {
+            param_env: goal.param_env,
+            predicate: SubtypePredicate {
+                a_is_expected: false,
+                a: goal.predicate.a,
+                b: goal.predicate.b,
+            },
+        })
+    }
+
+    fn compute_subtype_goal(
+        &mut self,
+        goal: Goal<'tcx, SubtypePredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
+            // FIXME: Do we want to register a subtype relation between these vars?
+            // That won't actually reflect in the query response, so it seems moot.
+            self.make_canonical_response(Certainty::AMBIGUOUS)
+        } else {
+            self.infcx.probe(|_| {
+                let InferOk { value: (), obligations } = self
+                    .infcx
+                    .at(&ObligationCause::dummy(), goal.param_env)
+                    .sub(goal.predicate.a, goal.predicate.b)?;
+                self.evaluate_all_and_make_canonical_response(
+                    obligations.into_iter().map(|pred| pred.into()).collect(),
+                )
+            })
+        }
+    }
+
+    fn compute_closure_kind_goal(
+        &mut self,
+        goal: Goal<'tcx, (DefId, ty::SubstsRef<'tcx>, ty::ClosureKind)>,
+    ) -> QueryResult<'tcx> {
+        let (_, substs, expected_kind) = goal.predicate;
+        let found_kind = substs.as_closure().kind_ty().to_opt_closure_kind();
+
+        let Some(found_kind) = found_kind else {
+            return self.make_canonical_response(Certainty::AMBIGUOUS);
+        };
+        if found_kind.extends(expected_kind) {
+            self.make_canonical_response(Certainty::Yes)
+        } else {
+            Err(NoSolution)
+        }
+    }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -3,7 +3,7 @@ use crate::traits::{specialization_graph, translate_substs};
 use super::assembly::{self, Candidate, CandidateSource};
 use super::infcx_ext::InferCtxtExt;
 use super::trait_goals::structural_traits;
-use super::{Certainty, EvalCtxt, Goal, MaybeCause, QueryResult};
+use super::{Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
@@ -229,8 +229,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 goal.predicate.def_id(),
                 impl_def_id
             )? else {
-                let certainty = Certainty::Maybe(MaybeCause::Ambiguity);
-                return ecx.make_canonical_response(trait_ref_certainty.unify_and(certainty));
+                return ecx.make_canonical_response(trait_ref_certainty.unify_and(Certainty::AMBIGUOUS));
             };
 
             if !assoc_def.item.defaultness(tcx).has_value() {
@@ -382,7 +381,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 .to_predicate(ecx.tcx());
             Self::consider_assumption(ecx, goal, pred)
         } else {
-            ecx.make_canonical_response(Certainty::Maybe(MaybeCause::Ambiguity))
+            ecx.make_canonical_response(Certainty::AMBIGUOUS)
         }
     }
 

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -4,7 +4,7 @@ use std::iter;
 
 use super::assembly::{self, Candidate, CandidateSource};
 use super::infcx_ext::InferCtxtExt;
-use super::{Certainty, EvalCtxt, Goal, MaybeCause, QueryResult};
+use super::{Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
@@ -133,7 +133,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx> {
         if goal.predicate.self_ty().has_non_region_infer() {
-            return ecx.make_canonical_response(Certainty::Maybe(MaybeCause::Ambiguity));
+            return ecx.make_canonical_response(Certainty::AMBIGUOUS);
         }
 
         let tcx = ecx.tcx();
@@ -171,7 +171,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                 .to_predicate(ecx.tcx());
             Self::consider_assumption(ecx, goal, pred)
         } else {
-            ecx.make_canonical_response(Certainty::Maybe(MaybeCause::Ambiguity))
+            ecx.make_canonical_response(Certainty::AMBIGUOUS)
         }
     }