[DO NOT MERGE] -Znext-solver test Certainty::Yes when only ambiguous goals are "trivial"

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

@@ -14,9 +14,10 @@ use std::iter;
 use rustc_index::IndexVec;
 use rustc_type_ir::data_structures::HashSet;
 use rustc_type_ir::inherent::*;
+use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::relate::solver_relating::RelateExt;
 use rustc_type_ir::{
-    self as ty, Canonical, CanonicalVarValues, InferCtxtLike, Interner, TypeFoldable,
+    self as ty, Canonical, CanonicalVarValues, ClauseKind, InferCtxtLike, Interner, TypeFoldable,
 };
 use tracing::{debug, instrument, trace};
 
@@ -133,6 +134,43 @@ where
                         ),
                     )
                 }
+                (_, Certainty::Yes) => {
+                    let goals = std::mem::take(&mut self.nested_goals);
+                    // As we return all ambiguous nested goals, we can ignore the certainty
+                    // returned by `self.try_evaluate_added_goals()`.
+                    if goals.is_empty() {
+                        assert!(matches!(goals_certainty, Certainty::Yes));
+                    }
+                    if goals.iter().all(|(_, g, _)| {
+                        let Some(clause) = g.predicate.as_clause() else {
+                            return false;
+                        };
+                        match clause.kind().skip_binder() {
+                            ClauseKind::Trait(pred) => {
+                                let def_id = pred.def_id();
+                                (self.cx().is_lang_item(def_id, TraitSolverLangItem::Sized)
+                                    || self
+                                        .cx()
+                                        .is_lang_item(def_id, TraitSolverLangItem::MetaSized))
+                                    && self.resolve_vars_if_possible(pred).self_ty().is_ty_var()
+                            }
+                            ClauseKind::WellFormed(arg) => {
+                                self.resolve_vars_if_possible(arg).is_infer()
+                            }
+                            _ => false,
+                        }
+                    }) {
+                        (
+                            Certainty::Yes,
+                            NestedNormalizationGoals(
+                                goals.into_iter().map(|(s, g, _)| (s, g)).collect(),
+                            ),
+                        )
+                    } else {
+                        let certainty = shallow_certainty.and(goals_certainty);
+                        (certainty, NestedNormalizationGoals::empty())
+                    }
+                }
                 _ => {
                     let certainty = shallow_certainty.and(goals_certainty);
                     (certainty, NestedNormalizationGoals::empty())

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

@@ -171,13 +171,7 @@ pub trait SolverDelegateEvalExt: SolverDelegate {
         &self,
         goal: Goal<Self::Interner, <Self::Interner as Interner>::Predicate>,
         span: <Self::Interner as Interner>::Span,
-    ) -> (
-        Result<
-            (NestedNormalizationGoals<Self::Interner>, GoalEvaluation<Self::Interner>),
-            NoSolution,
-        >,
-        inspect::GoalEvaluation<Self::Interner>,
-    );
+    ) -> (Result<GoalEvaluation<Self::Interner>, NoSolution>, inspect::GoalEvaluation<Self::Interner>);
 }
 
 impl<D, I> SolverDelegateEvalExt for D
@@ -221,16 +215,13 @@ where
         &self,
         goal: Goal<I, I::Predicate>,
         span: I::Span,
-    ) -> (
-        Result<(NestedNormalizationGoals<I>, GoalEvaluation<I>), NoSolution>,
-        inspect::GoalEvaluation<I>,
-    ) {
+    ) -> (Result<GoalEvaluation<I>, NoSolution>, inspect::GoalEvaluation<I>) {
         let (result, proof_tree) = EvalCtxt::enter_root(
             self,
             self.cx().recursion_limit(),
             GenerateProofTree::Yes,
             span,
-            |ecx| ecx.evaluate_goal_raw(GoalEvaluationKind::Root, GoalSource::Misc, goal, None),
+            |ecx| ecx.evaluate_goal(GoalEvaluationKind::Root, GoalSource::Misc, goal, None),
         );
         (result, proof_tree.unwrap())
     }
@@ -411,26 +402,6 @@ where
         goal: Goal<I, I::Predicate>,
         stalled_on: Option<GoalStalledOn<I>>,
     ) -> Result<GoalEvaluation<I>, NoSolution> {
-        let (normalization_nested_goals, goal_evaluation) =
-            self.evaluate_goal_raw(goal_evaluation_kind, source, goal, stalled_on)?;
-        assert!(normalization_nested_goals.is_empty());
-        Ok(goal_evaluation)
-    }
-
-    /// Recursively evaluates `goal`, returning the nested goals in case
-    /// the nested goal is a `NormalizesTo` goal.
-    ///
-    /// As all other goal kinds do not return any nested goals and
-    /// `NormalizesTo` is only used by `AliasRelate`, all other callsites
-    /// should use [`EvalCtxt::evaluate_goal`] which discards that empty
-    /// storage.
-    pub(super) fn evaluate_goal_raw(
-        &mut self,
-        goal_evaluation_kind: GoalEvaluationKind,
-        source: GoalSource,
-        goal: Goal<I, I::Predicate>,
-        stalled_on: Option<GoalStalledOn<I>>,
-    ) -> Result<(NestedNormalizationGoals<I>, GoalEvaluation<I>), NoSolution> {
         // If we have run this goal before, and it was stalled, check that any of the goal's
         // args have changed. Otherwise, we don't need to re-run the goal because it'll remain
         // stalled, since it'll canonicalize the same way and evaluation is pure.
@@ -441,15 +412,13 @@ where
                 .opaque_types_storage_num_entries()
                 .needs_reevaluation(stalled_on.num_opaques)
         {
-            return Ok((
-                NestedNormalizationGoals::empty(),
-                GoalEvaluation {
-                    goal,
-                    certainty: Certainty::Maybe(stalled_on.stalled_cause),
-                    has_changed: HasChanged::No,
-                    stalled_on: Some(stalled_on),
-                },
-            ));
+            return Ok(GoalEvaluation {
+                goal,
+                certainty: Certainty::Maybe(stalled_on.stalled_cause),
+                has_changed: HasChanged::No,
+                stalled_on: Some(stalled_on),
+                nested_goals: NestedNormalizationGoals::empty(),
+            });
         }
 
         // We only care about one entry per `OpaqueTypeKey` here,
@@ -477,7 +446,7 @@ where
         let has_changed =
             if !has_only_region_constraints(response) { HasChanged::Yes } else { HasChanged::No };
 
-        let (normalization_nested_goals, certainty) =
+        let (nested_goals, certainty) =
             self.instantiate_and_apply_query_response(goal.param_env, &orig_values, response);
 
         // FIXME: We previously had an assert here that checked that recomputing
@@ -536,10 +505,7 @@ where
             },
         };
 
-        Ok((
-            normalization_nested_goals,
-            GoalEvaluation { goal, certainty, has_changed, stalled_on },
-        ))
+        Ok(GoalEvaluation { goal, certainty, has_changed, stalled_on, nested_goals })
     }
 
     pub(super) fn compute_goal(&mut self, goal: Goal<I, I::Predicate>) -> QueryResult<I> {
@@ -673,10 +639,13 @@ where
                 let unconstrained_goal =
                     goal.with(cx, ty::NormalizesTo { alias: pred.alias, term: unconstrained_rhs });
 
-                let (
-                    NestedNormalizationGoals(nested_goals),
-                    GoalEvaluation { goal, certainty, stalled_on, has_changed: _ },
-                ) = self.evaluate_goal_raw(
+                let GoalEvaluation {
+                    goal,
+                    certainty,
+                    stalled_on,
+                    has_changed: _,
+                    nested_goals: NestedNormalizationGoals(nested_goals),
+                } = self.evaluate_goal(
                     GoalEvaluationKind::Nested,
                     source,
                     unconstrained_goal,
@@ -733,12 +702,18 @@ where
                     }
                 }
             } else {
-                let GoalEvaluation { goal, certainty, has_changed, stalled_on } =
-                    self.evaluate_goal(GoalEvaluationKind::Nested, source, goal, stalled_on)?;
+                let GoalEvaluation {
+                    goal,
+                    certainty,
+                    has_changed,
+                    stalled_on,
+                    nested_goals: NestedNormalizationGoals(nested_goals),
+                } = self.evaluate_goal(GoalEvaluationKind::Nested, source, goal, stalled_on)?;
+                trace!(?nested_goals);
+                self.nested_goals.extend(nested_goals.into_iter().map(|(s, g)| (s, g, None)));
                 if has_changed == HasChanged::Yes {
                     unchanged_certainty = None;
                 }
-
                 match certainty {
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -413,6 +413,7 @@ pub struct GoalEvaluation<I: Interner> {
     /// then reevaluating this goal now only needs to resolve `?y` while it would otherwise
     /// have to resolve both `?x` and `?y`,
     pub goal: Goal<I, I::Predicate>,
+    pub nested_goals: NestedNormalizationGoals<I>,
     pub certainty: Certainty,
     pub has_changed: HasChanged,
     /// If the [`Certainty`] was `Maybe`, then keep track of whether the goal has changed

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -7,15 +7,16 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::{
-    FromSolverError, PredicateObligation, PredicateObligations, TraitEngine,
+    FromSolverError, Obligation, ObligationCause, PredicateObligation, PredicateObligations,
+    TraitEngine,
 };
 use rustc_middle::ty::{
     self, DelayedSet, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
     TypingMode,
 };
 use rustc_next_trait_solver::delegate::SolverDelegate as _;
 use rustc_next_trait_solver::solve::{
-    GoalEvaluation, GoalStalledOn, HasChanged, SolverDelegateEvalExt as _,
+    GoalEvaluation, GoalStalledOn, HasChanged, NestedNormalizationGoals, SolverDelegateEvalExt as _,
 };
 use rustc_span::Span;
 use thin_vec::ThinVec;
@@ -213,7 +214,13 @@ where
 
                 let result = delegate.evaluate_root_goal(goal, obligation.cause.span, stalled_on);
                 self.inspect_evaluated_obligation(infcx, &obligation, &result);
-                let GoalEvaluation { goal, certainty, has_changed, stalled_on } = match result {
+                let GoalEvaluation {
+                    goal,
+                    certainty,
+                    has_changed,
+                    stalled_on,
+                    nested_goals: NestedNormalizationGoals(nested_goals),
+                } = match result {
                     Ok(result) => result,
                     Err(NoSolution) => {
                         errors.push(E::from_solver_error(
@@ -223,6 +230,18 @@ where
                         continue;
                     }
                 };
+                for (_, nested_goal) in nested_goals {
+                    self.obligations.register(
+                        Obligation::with_depth(
+                            infcx.tcx,
+                            ObligationCause::misc(obligation.cause.span, obligation.cause.body_id),
+                            obligation.recursion_depth + 1,
+                            nested_goal.param_env,
+                            nested_goal.predicate,
+                        ),
+                        None,
+                    );
+                }
 
                 // We've resolved the goal in `evaluate_root_goal`, avoid redoing this work
                 // in the next iteration. This does not resolve the inference variables

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -20,7 +20,7 @@ use rustc_middle::ty::{TyCtxt, VisitorResult, try_visit};
 use rustc_middle::{bug, ty};
 use rustc_next_trait_solver::resolve::eager_resolve_vars;
 use rustc_next_trait_solver::solve::inspect::{self, instantiate_canonical_state};
-use rustc_next_trait_solver::solve::{MaybeCause, SolverDelegateEvalExt as _};
+use rustc_next_trait_solver::solve::{GoalEvaluation, MaybeCause, SolverDelegateEvalExt as _};
 use rustc_span::Span;
 use tracing::instrument;
 
@@ -249,23 +249,26 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
                     // `InspectGoal::new` so that the goal has the right result (and maintains
                     // the impression that we don't do this normalizes-to infer hack at all).
                     let (nested, proof_tree) = infcx.evaluate_root_goal_for_proof_tree(goal, span);
-                    let nested_goals_result = nested.and_then(|(nested, _)| {
-                        normalizes_to_term_hack.constrain_and(
-                            infcx,
-                            span,
-                            proof_tree.uncanonicalized_goal.param_env,
-                            |ocx| {
-                                ocx.register_obligations(nested.0.into_iter().map(|(_, goal)| {
-                                    Obligation::new(
-                                        infcx.tcx,
-                                        ObligationCause::dummy_with_span(span),
-                                        goal.param_env,
-                                        goal.predicate,
-                                    )
-                                }));
-                            },
-                        )
-                    });
+                    let nested_goals_result =
+                        nested.and_then(|GoalEvaluation { nested_goals, .. }| {
+                            normalizes_to_term_hack.constrain_and(
+                                infcx,
+                                span,
+                                proof_tree.uncanonicalized_goal.param_env,
+                                |ocx| {
+                                    ocx.register_obligations(nested_goals.0.into_iter().map(
+                                        |(_, goal)| {
+                                            Obligation::new(
+                                                infcx.tcx,
+                                                ObligationCause::dummy_with_span(span),
+                                                goal.param_env,
+                                                goal.predicate,
+                                            )
+                                        },
+                                    ));
+                                },
+                            )
+                        });
                     (proof_tree, nested_goals_result)
                 });
                 InspectGoal::new(