NormalizesTo: return nested goals to caller

compiler/rustc_middle/src/traits/solve.rs

@@ -164,6 +164,19 @@ pub struct ExternalConstraintsData<'tcx> {
     // FIXME: implement this.
     pub region_constraints: QueryRegionConstraints<'tcx>,
     pub opaque_types: Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)>,
+    pub normalization_nested_goals: NestedNormalizationGoals<'tcx>,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Hash, HashStable, Default, TypeVisitable, TypeFoldable)]
+pub struct NestedNormalizationGoals<'tcx>(pub Vec<(GoalSource, Goal<'tcx, ty::Predicate<'tcx>>)>);
+impl<'tcx> NestedNormalizationGoals<'tcx> {
+    pub fn empty() -> Self {
+        NestedNormalizationGoals(vec![])
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 // FIXME: Having to clone `region_constraints` for folding feels bad and
@@ -183,21 +196,27 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
                 .iter()
                 .map(|opaque| opaque.try_fold_with(folder))
                 .collect::<Result<_, F::Error>>()?,
+            normalization_nested_goals: self
+                .normalization_nested_goals
+                .clone()
+                .try_fold_with(folder)?,
         }))
     }
 
     fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
         TypeFolder::interner(folder).mk_external_constraints(ExternalConstraintsData {
             region_constraints: self.region_constraints.clone().fold_with(folder),
             opaque_types: self.opaque_types.iter().map(|opaque| opaque.fold_with(folder)).collect(),
+            normalization_nested_goals: self.normalization_nested_goals.clone().fold_with(folder),
         })
     }
 }
 
 impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
     fn visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, visitor: &mut V) -> V::Result {
         try_visit!(self.region_constraints.visit_with(visitor));
-        self.opaque_types.visit_with(visitor)
+        try_visit!(self.opaque_types.visit_with(visitor));
+        self.normalization_nested_goals.visit_with(visitor)
     }
 }
 
@@ -239,7 +258,7 @@ impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for PredefinedOpaques<'tcx> {
 ///
 /// This is necessary as we treat nested goals different depending on
 /// their source.
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TypeVisitable, TypeFoldable)]
 pub enum GoalSource {
     Misc,
     /// We're proving a where-bound of an impl.
@@ -256,12 +275,6 @@ pub enum GoalSource {
     ImplWhereBound,
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
-pub enum IsNormalizesToHack {
-    Yes,
-    No,
-}
-
 /// Possible ways the given goal can be proven.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum CandidateSource {

compiler/rustc_middle/src/traits/solve/inspect.rs

@@ -19,8 +19,8 @@
 //! [canonicalized]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html
 
 use super::{
-    CandidateSource, Canonical, CanonicalInput, Certainty, Goal, GoalSource, IsNormalizesToHack,
-    NoSolution, QueryInput, QueryResult,
+    CandidateSource, Canonical, CanonicalInput, Certainty, Goal, GoalSource, NoSolution,
+    QueryInput, QueryResult,
 };
 use crate::{infer::canonical::CanonicalVarValues, ty};
 use format::ProofTreeFormatter;
@@ -50,7 +50,7 @@ pub type CanonicalState<'tcx, T> = Canonical<'tcx, State<'tcx, T>>;
 #[derive(Eq, PartialEq)]
 pub enum GoalEvaluationKind<'tcx> {
     Root { orig_values: Vec<ty::GenericArg<'tcx>> },
-    Nested { is_normalizes_to_hack: IsNormalizesToHack },
+    Nested,
 }
 
 #[derive(Eq, PartialEq)]

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -55,10 +55,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
     pub(super) fn format_goal_evaluation(&mut self, eval: &GoalEvaluation<'_>) -> std::fmt::Result {
         let goal_text = match eval.kind {
             GoalEvaluationKind::Root { orig_values: _ } => "ROOT GOAL",
-            GoalEvaluationKind::Nested { is_normalizes_to_hack } => match is_normalizes_to_hack {
-                IsNormalizesToHack::No => "GOAL",
-                IsNormalizesToHack::Yes => "NORMALIZES-TO HACK GOAL",
-            },
+            GoalEvaluationKind::Nested => "GOAL",
         };
         write!(self.f, "{}: {:?}", goal_text, eval.uncanonicalized_goal)?;
         self.nested(|this| this.format_canonical_goal_evaluation(&eval.evaluation))

compiler/rustc_trait_selection/src/solve/alias_relate.rs

@@ -21,8 +21,9 @@
 //! However, if `?fresh_var` ends up geteting equated to another type, we retry the
 //! `NormalizesTo` goal, at which point the opaque is actually defined.
 
-use super::{EvalCtxt, GoalSource};
+use super::EvalCtxt;
 use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::solve::GoalSource;
 use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
 use rustc_middle::ty::{self, Ty};
 
@@ -121,10 +122,11 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             ty::TermKind::Const(_) => {
                 if let Some(alias) = term.to_alias_ty(self.tcx()) {
                     let term = self.next_term_infer_of_kind(term);
-                    self.add_goal(
-                        GoalSource::Misc,
-                        Goal::new(self.tcx(), param_env, ty::NormalizesTo { alias, term }),
-                    );
+                    self.add_normalizes_to_goal(Goal::new(
+                        self.tcx(),
+                        param_env,
+                        ty::NormalizesTo { alias, term },
+                    ));
                     self.try_evaluate_added_goals()?;
                     Ok(Some(self.resolve_vars_if_possible(term)))
                 } else {
@@ -145,18 +147,25 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             return None;
         }
 
-        let ty::Alias(_, alias) = *ty.kind() else {
+        let ty::Alias(kind, alias) = *ty.kind() else {
             return Some(ty);
         };
 
         match self.commit_if_ok(|this| {
+            let tcx = this.tcx();
             let normalized_ty = this.next_ty_infer();
-            let normalizes_to_goal = Goal::new(
-                this.tcx(),
-                param_env,
-                ty::NormalizesTo { alias, term: normalized_ty.into() },
-            );
-            this.add_goal(GoalSource::Misc, normalizes_to_goal);
+            let normalizes_to = ty::NormalizesTo { alias, term: normalized_ty.into() };
+            match kind {
+                ty::AliasKind::Opaque => {
+                    // HACK: Unlike for associated types, `normalizes-to` for opaques
+                    // is currently not treated as a function. We do not erase the
+                    // expected term.
+                    this.add_goal(GoalSource::Misc, Goal::new(tcx, param_env, normalizes_to));
+                }
+                ty::AliasKind::Projection | ty::AliasKind::Inherent | ty::AliasKind::Weak => {
+                    this.add_normalizes_to_goal(Goal::new(tcx, param_env, normalizes_to))
+                }
+            }
             this.try_evaluate_added_goals()?;
             Ok(this.resolve_vars_if_possible(normalized_ty))
         }) {

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

@@ -9,6 +9,7 @@
 //!
 //! [c]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html
 use super::{CanonicalInput, Certainty, EvalCtxt, Goal};
+use crate::solve::eval_ctxt::NestedGoals;
 use crate::solve::{
     inspect, response_no_constraints_raw, CanonicalResponse, QueryResult, Response,
 };
@@ -19,6 +20,7 @@ use rustc_infer::infer::canonical::CanonicalVarValues;
 use rustc_infer::infer::canonical::{CanonicalExt, QueryRegionConstraints};
 use rustc_infer::infer::resolve::EagerResolver;
 use rustc_infer::infer::{InferCtxt, InferOk};
+use rustc_infer::traits::solve::NestedNormalizationGoals;
 use rustc_middle::infer::canonical::Canonical;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{
@@ -28,6 +30,7 @@ use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{self, BoundVar, GenericArgKind, Ty, TyCtxt, TypeFoldable};
 use rustc_next_trait_solver::canonicalizer::{CanonicalizeMode, Canonicalizer};
 use rustc_span::DUMMY_SP;
+use std::assert_matches::assert_matches;
 use std::iter;
 use std::ops::Deref;
 
@@ -93,13 +96,31 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             previous call to `try_evaluate_added_goals!`"
         );
 
-        let certainty = certainty.unify_with(goals_certainty);
-
-        let var_values = self.var_values;
-        let external_constraints = self.compute_external_query_constraints()?;
-
+        // When normalizing, we've replaced the expected term with an unconstrained
+        // inference variable. This means that we dropped information which could
+        // have been important. We handle this by instead returning the nested goals
+        // to the caller, where they are then handled.
+        //
+        // As we return all ambiguous nested goals, we can ignore the certainty returned
+        // by `try_evaluate_added_goals()`.
+        let (certainty, normalization_nested_goals) = if self.is_normalizes_to_goal {
+            let NestedGoals { normalizes_to_goals, goals } = std::mem::take(&mut self.nested_goals);
+            if cfg!(debug_assertions) {
+                assert!(normalizes_to_goals.is_empty());
+                if goals.is_empty() {
+                    assert_matches!(goals_certainty, Certainty::Yes);
+                }
+            }
+            (certainty, NestedNormalizationGoals(goals))
+        } else {
+            let certainty = certainty.unify_with(goals_certainty);
+            (certainty, NestedNormalizationGoals::empty())
+        };
+
+        let external_constraints =
+            self.compute_external_query_constraints(normalization_nested_goals)?;
         let (var_values, mut external_constraints) =
-            (var_values, external_constraints).fold_with(&mut EagerResolver::new(self.infcx));
+            (self.var_values, external_constraints).fold_with(&mut EagerResolver::new(self.infcx));
         // Remove any trivial region constraints once we've resolved regions
         external_constraints
             .region_constraints
@@ -146,6 +167,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
     fn compute_external_query_constraints(
         &self,
+        normalization_nested_goals: NestedNormalizationGoals<'tcx>,
     ) -> Result<ExternalConstraintsData<'tcx>, NoSolution> {
         // We only check for leaks from universes which were entered inside
         // of the query.
@@ -176,7 +198,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             self.predefined_opaques_in_body.opaque_types.iter().all(|(pa, _)| pa != a)
         });
 
-        Ok(ExternalConstraintsData { region_constraints, opaque_types })
+        Ok(ExternalConstraintsData { region_constraints, opaque_types, normalization_nested_goals })
     }
 
     /// After calling a canonical query, we apply the constraints returned
@@ -185,13 +207,14 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     /// This happens in three steps:
     /// - we instantiate the bound variables of the query response
     /// - we unify the `var_values` of the response with the `original_values`
-    /// - we apply the `external_constraints` returned by the query
+    /// - we apply the `external_constraints` returned by the query, returning
+    ///   the `normalization_nested_goals`
     pub(super) fn instantiate_and_apply_query_response(
         &mut self,
         param_env: ty::ParamEnv<'tcx>,
         original_values: Vec<ty::GenericArg<'tcx>>,
         response: CanonicalResponse<'tcx>,
-    ) -> Certainty {
+    ) -> (NestedNormalizationGoals<'tcx>, Certainty) {
         let instantiation = Self::compute_query_response_instantiation_values(
             self.infcx,
             &original_values,
@@ -203,11 +226,14 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         Self::unify_query_var_values(self.infcx, param_env, &original_values, var_values);
 
-        let ExternalConstraintsData { region_constraints, opaque_types } =
-            external_constraints.deref();
+        let ExternalConstraintsData {
+            region_constraints,
+            opaque_types,
+            normalization_nested_goals,
+        } = external_constraints.deref();
         self.register_region_constraints(region_constraints);
         self.register_new_opaque_types(param_env, opaque_types);
-        certainty
+        (normalization_nested_goals.clone(), certainty)
     }
 
     /// This returns the canoncial variable values to instantiate the bound variables of

compiler/rustc_trait_selection/src/solve/eval_ctxt/commit_if_ok.rs

@@ -11,6 +11,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             infcx: self.infcx,
             variables: self.variables,
             var_values: self.var_values,
+            is_normalizes_to_goal: self.is_normalizes_to_goal,
             predefined_opaques_in_body: self.predefined_opaques_in_body,
             max_input_universe: self.max_input_universe,
             search_graph: self.search_graph,
@@ -25,6 +26,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 infcx: _,
                 variables: _,
                 var_values: _,
+                is_normalizes_to_goal: _,
                 predefined_opaques_in_body: _,
                 max_input_universe: _,
                 search_graph: _,