Fast path for subtype and coercion goals

compiler/rustc_next_trait_solver/src/delegate.rs

@@ -3,8 +3,6 @@ use std::ops::Deref;
 use rustc_type_ir::solve::{Certainty, Goal, NoSolution};
 use rustc_type_ir::{self as ty, InferCtxtLike, Interner, TypeFoldable};
 
-use crate::solve::HasChanged;
-
 pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
     type Infcx: InferCtxtLike<Interner = Self::Interner>;
     type Interner: Interner;
@@ -23,7 +21,7 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         &self,
         goal: Goal<Self::Interner, <Self::Interner as Interner>::Predicate>,
         span: <Self::Interner as Interner>::Span,
-    ) -> Option<HasChanged>;
+    ) -> Option<Certainty>;
 
     fn fresh_var_for_kind_with_span(
         &self,

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -671,10 +671,13 @@ where
         // If this loop did not result in any progress, what's our final certainty.
         let mut unchanged_certainty = Some(Certainty::Yes);
         for (source, goal, stalled_on) in mem::take(&mut self.nested_goals) {
-            if let Some(has_changed) = self.delegate.compute_goal_fast_path(goal, self.origin_span)
-            {
-                if matches!(has_changed, HasChanged::Yes) {
-                    unchanged_certainty = None;
+            if let Some(certainty) = self.delegate.compute_goal_fast_path(goal, self.origin_span) {
+                match certainty {
+                    Certainty::Yes => {}
+                    Certainty::Maybe(_) => {
+                        self.nested_goals.push((source, goal, None));
+                        unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
+                    }
                 }
                 continue;
             }

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -12,7 +12,6 @@ use rustc_infer::traits::solve::Goal;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::Certainty;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitableExt as _, TypingMode};
-use rustc_next_trait_solver::solve::HasChanged;
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
 
 use crate::traits::{EvaluateConstErr, ObligationCause, specialization_graph};
@@ -61,19 +60,19 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
         &self,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
         span: Span,
-    ) -> Option<HasChanged> {
+    ) -> Option<Certainty> {
         let pred = goal.predicate.kind();
         match pred.no_bound_vars()? {
             ty::PredicateKind::DynCompatible(def_id) if self.0.tcx.is_dyn_compatible(def_id) => {
-                Some(HasChanged::No)
+                Some(Certainty::Yes)
             }
             ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(outlives)) => {
                 self.0.sub_regions(
                     SubregionOrigin::RelateRegionParamBound(span, None),
                     outlives.1,
                     outlives.0,
                 );
-                Some(HasChanged::No)
+                Some(Certainty::Yes)
             }
             ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(outlives)) => {
                 self.0.register_type_outlives_constraint(
@@ -82,23 +81,34 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
                     &ObligationCause::dummy_with_span(span),
                 );
 
-                Some(HasChanged::No)
+                Some(Certainty::Yes)
             }
             ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) => {
                 match self.0.tcx.as_lang_item(trait_pred.def_id()) {
                     Some(LangItem::Sized)
                         if trait_pred.self_ty().is_trivially_sized(self.0.tcx) =>
                     {
-                        Some(HasChanged::No)
+                        Some(Certainty::Yes)
                     }
                     Some(LangItem::Copy | LangItem::Clone)
                         if trait_pred.self_ty().is_trivially_pure_clone_copy() =>
                     {
-                        Some(HasChanged::No)
+                        Some(Certainty::Yes)
                     }
                     _ => None,
                 }
             }
+            ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, .. })
+            | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                if self.shallow_resolve(a).is_ty_var() && self.shallow_resolve(b).is_ty_var() {
+                    // FIXME: We also need to register a subtype relation between these vars
+                    // when those are added, and if they aren't in the same sub root then
+                    // we should mark this goal as `has_changed`.
+                    Some(Certainty::AMBIGUOUS)
+                } else {
+                    None
+                }
+            }
             _ => None,
         }
     }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -195,10 +195,15 @@ where
 
                 let goal = obligation.as_goal();
                 let delegate = <&SolverDelegate<'tcx>>::from(infcx);
-                if let Some(fast_path_has_changed) =
+                if let Some(certainty) =
                     delegate.compute_goal_fast_path(goal, obligation.cause.span)
                 {
-                    any_changed |= matches!(fast_path_has_changed, HasChanged::Yes);
+                    match certainty {
+                        Certainty::Yes => {}
+                        Certainty::Maybe(_) => {
+                            self.obligations.register(obligation, None);
+                        }
+                    }
                     continue;
                 }