Rollup merge of rust-lang#109641 - compiler-errors:dont-elaborate-non…

…-obl, r=oli-obk

Don't elaborate non-obligations into obligations

It's suspicious to elaborate a `PolyTraitRef` or `Predicate` into an `Obligation`, since the former does not have a param-env associated with it, but the latter does. This is a footgun that, while not being misused *currently* in the compiler, easily could be misused by someone less familiar with the elaborator's inner workings.

This PR just changes the API -- ideally, the elaborator wouldn't even have to deal with obligations if we're not elaborating obligations, but that would require a bit more abstraction than I could be bothered with today.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1427,13 +1427,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         for (base_trait_ref, span, constness) in regular_traits_refs_spans {
             assert_eq!(constness, ty::BoundConstness::NotConst);
 
-            for obligation in traits::elaborate_trait_ref(tcx, base_trait_ref) {
-                debug!(
-                    "conv_object_ty_poly_trait_ref: observing object predicate `{:?}`",
-                    obligation.predicate
-                );
+            for pred in traits::elaborate_trait_ref(tcx, base_trait_ref) {
+                debug!("conv_object_ty_poly_trait_ref: observing object predicate `{:?}`", pred);
 
-                let bound_predicate = obligation.predicate.kind();
+                let bound_predicate = pred.kind();
                 match bound_predicate.skip_binder() {
                     ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => {
                         let pred = bound_predicate.rebind(pred);

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1912,14 +1912,13 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
         // Check elaborated bounds.
         let implied_obligations = traits::elaborate_predicates_with_span(tcx, predicates_with_span);
 
-        for obligation in implied_obligations {
+        for (pred, obligation_span) in implied_obligations {
             // We lower empty bounds like `Vec<dyn Copy>:` as
             // `WellFormed(Vec<dyn Copy>)`, which will later get checked by
             // regular WF checking
-            if let ty::PredicateKind::WellFormed(..) = obligation.predicate.kind().skip_binder() {
+            if let ty::PredicateKind::WellFormed(..) = pred.kind().skip_binder() {
                 continue;
             }
-            let pred = obligation.predicate;
             // Match the existing behavior.
             if pred.is_global() && !pred.has_late_bound_vars() {
                 let pred = self.normalize(span, None, pred);
@@ -1930,8 +1929,6 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
                 if let Some(hir::Generics { predicates, .. }) =
                     hir_node.and_then(|node| node.generics())
                 {
-                    let obligation_span = obligation.cause.span();
-
                     span = predicates
                         .iter()
                         // There seems to be no better way to find out which predicate we are in

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -130,12 +130,9 @@ pub(super) fn item_bounds(
     tcx: TyCtxt<'_>,
     def_id: DefId,
 ) -> ty::EarlyBinder<&'_ ty::List<ty::Predicate<'_>>> {
-    let bounds = tcx.mk_predicates_from_iter(
-        util::elaborate_predicates(
-            tcx,
-            tcx.explicit_item_bounds(def_id).iter().map(|&(bound, _span)| bound),
-        )
-        .map(|obligation| obligation.predicate),
-    );
+    let bounds = tcx.mk_predicates_from_iter(util::elaborate_predicates(
+        tcx,
+        tcx.explicit_item_bounds(def_id).iter().map(|&(bound, _span)| bound),
+    ));
     ty::EarlyBinder(bounds)
 }

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -318,16 +318,8 @@ fn check_predicates<'tcx>(
     span: Span,
 ) {
     let instantiated = tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_substs);
-    let impl1_predicates: Vec<_> = traits::elaborate_predicates_with_span(
-        tcx,
-        std::iter::zip(
-            instantiated.predicates,
-            // Don't drop predicates (unsound!) because `spans` is too short
-            instantiated.spans.into_iter().chain(std::iter::repeat(span)),
-        ),
-    )
-    .map(|obligation| (obligation.predicate, obligation.cause.span))
-    .collect();
+    let impl1_predicates: Vec<_> =
+        traits::elaborate_predicates_with_span(tcx, instantiated.into_iter()).collect();
 
     let mut impl2_predicates = if impl2_node.is_from_trait() {
         // Always applicable traits have to be always applicable without any
@@ -341,7 +333,6 @@ fn check_predicates<'tcx>(
                 .predicates
                 .into_iter(),
         )
-        .map(|obligation| obligation.predicate)
         .collect()
     };
     debug!(?impl1_predicates, ?impl2_predicates);
@@ -361,12 +352,16 @@ fn check_predicates<'tcx>(
     // which is sound because we forbid impls like the following
     //
     // impl<D: Debug> AlwaysApplicable for D { }
-    let always_applicable_traits = impl1_predicates.iter().copied().filter(|&(predicate, _)| {
-        matches!(
-            trait_predicate_kind(tcx, predicate),
-            Some(TraitSpecializationKind::AlwaysApplicable)
-        )
-    });
+    let always_applicable_traits = impl1_predicates
+        .iter()
+        .copied()
+        .filter(|&(predicate, _)| {
+            matches!(
+                trait_predicate_kind(tcx, predicate),
+                Some(TraitSpecializationKind::AlwaysApplicable)
+            )
+        })
+        .map(|(pred, _span)| pred);
 
     // Include the well-formed predicates of the type parameters of the impl.
     for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().subst_identity().substs {
@@ -380,10 +375,7 @@ fn check_predicates<'tcx>(
             traits::elaborate_obligations(tcx, obligations).map(|obligation| obligation.predicate),
         )
     }
-    impl2_predicates.extend(
-        traits::elaborate_predicates_with_span(tcx, always_applicable_traits)
-            .map(|obligation| obligation.predicate),
-    );
+    impl2_predicates.extend(traits::elaborate_predicates(tcx, always_applicable_traits));
 
     for (predicate, span) in impl1_predicates {
         if !impl2_predicates.iter().any(|pred2| trait_predicates_eq(tcx, predicate, *pred2, span)) {

compiler/rustc_hir_typeck/src/closure.rs

@@ -204,25 +204,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mut expected_sig = None;
         let mut expected_kind = None;
 
-        for obligation in traits::elaborate_predicates_with_span(
+        for (pred, span) in traits::elaborate_predicates_with_span(
             self.tcx,
             // Reverse the obligations here, since `elaborate_*` uses a stack,
             // and we want to keep inference generally in the same order of
             // the registered obligations.
             predicates.rev(),
         ) {
-            debug!(?obligation.predicate);
-            let bound_predicate = obligation.predicate.kind();
+            debug!(?pred);
+            let bound_predicate = pred.kind();
 
             // Given a Projection predicate, we can potentially infer
             // the complete signature.
             if expected_sig.is_none()
                 && let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) = bound_predicate.skip_binder()
             {
                 let inferred_sig = self.normalize(
-                    obligation.cause.span,
+                    span,
                     self.deduce_sig_from_projection(
-                    Some(obligation.cause.span),
+                    Some(span),
                         bound_predicate.rebind(proj_predicate),
                     ),
                 );

compiler/rustc_hir_typeck/src/method/confirm.rs

@@ -576,17 +576,13 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
 
         traits::elaborate_predicates(self.tcx, predicates.predicates.iter().copied())
             // We don't care about regions here.
-            .filter_map(|obligation| match obligation.predicate.kind().skip_binder() {
+            .filter_map(|pred| match pred.kind().skip_binder() {
                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred))
                     if trait_pred.def_id() == sized_def_id =>
                 {
                     let span = predicates
                         .iter()
-                        .find_map(
-                            |(p, span)| {
-                                if p == obligation.predicate { Some(span) } else { None }
-                            },
-                        )
+                        .find_map(|(p, span)| if p == pred { Some(span) } else { None })
                         .unwrap_or(rustc_span::DUMMY_SP);
                     Some((trait_pred, span))
                 }

compiler/rustc_infer/src/traits/util.rs

@@ -74,44 +74,58 @@ pub struct Elaborator<'tcx> {
 pub fn elaborate_trait_ref<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>,
-) -> Elaborator<'tcx> {
+) -> impl Iterator<Item = ty::Predicate<'tcx>> {
     elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate(tcx)))
 }
 
 pub fn elaborate_trait_refs<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
-) -> Elaborator<'tcx> {
-    let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate(tcx));
+) -> impl Iterator<Item = ty::Predicate<'tcx>> {
+    let predicates = trait_refs.map(move |trait_ref| trait_ref.without_const().to_predicate(tcx));
     elaborate_predicates(tcx, predicates)
 }
 
 pub fn elaborate_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
-) -> Elaborator<'tcx> {
-    let obligations = predicates
-        .map(|predicate| {
-            predicate_obligation(predicate, ty::ParamEnv::empty(), ObligationCause::dummy())
-        })
-        .collect();
-    elaborate_obligations(tcx, obligations)
+) -> impl Iterator<Item = ty::Predicate<'tcx>> {
+    elaborate_obligations(
+        tcx,
+        predicates
+            .map(|predicate| {
+                Obligation::new(
+                    tcx,
+                    // We'll dump the cause/param-env later
+                    ObligationCause::dummy(),
+                    ty::ParamEnv::empty(),
+                    predicate,
+                )
+            })
+            .collect(),
+    )
+    .map(|obl| obl.predicate)
 }
 
 pub fn elaborate_predicates_with_span<'tcx>(
     tcx: TyCtxt<'tcx>,
     predicates: impl Iterator<Item = (ty::Predicate<'tcx>, Span)>,
-) -> Elaborator<'tcx> {
-    let obligations = predicates
-        .map(|(predicate, span)| {
-            predicate_obligation(
-                predicate,
-                ty::ParamEnv::empty(),
-                ObligationCause::dummy_with_span(span),
-            )
-        })
-        .collect();
-    elaborate_obligations(tcx, obligations)
+) -> impl Iterator<Item = (ty::Predicate<'tcx>, Span)> {
+    elaborate_obligations(
+        tcx,
+        predicates
+            .map(|(predicate, span)| {
+                Obligation::new(
+                    tcx,
+                    // We'll dump the cause/param-env later
+                    ObligationCause::dummy_with_span(span),
+                    ty::ParamEnv::empty(),
+                    predicate,
+                )
+            })
+            .collect(),
+    )
+    .map(|obl| (obl.predicate, obl.cause.span))
 }
 
 pub fn elaborate_obligations<'tcx>(
@@ -141,10 +155,6 @@ impl<'tcx> Elaborator<'tcx> {
         self.stack.extend(obligations.into_iter().filter(|o| self.visited.insert(o.predicate)));
     }
 
-    pub fn filter_to_traits(self) -> FilterToTraits<Self> {
-        FilterToTraits::new(self)
-    }
-
     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
         let tcx = self.visited.tcx;
 
@@ -325,20 +335,18 @@ impl<'tcx> Iterator for Elaborator<'tcx> {
 // Supertrait iterator
 ///////////////////////////////////////////////////////////////////////////
 
-pub type Supertraits<'tcx> = FilterToTraits<Elaborator<'tcx>>;
-
 pub fn supertraits<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>,
-) -> Supertraits<'tcx> {
-    elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
+) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
+    FilterToTraits::new(elaborate_trait_ref(tcx, trait_ref))
 }
 
 pub fn transitive_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
-    bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
-) -> Supertraits<'tcx> {
-    elaborate_trait_refs(tcx, bounds).filter_to_traits()
+    trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
+) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
+    FilterToTraits::new(elaborate_trait_refs(tcx, trait_refs))
 }
 
 /// A specialized variant of `elaborate_trait_refs` that only elaborates trait references that may
@@ -393,12 +401,12 @@ impl<I> FilterToTraits<I> {
     }
 }
 
-impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToTraits<I> {
+impl<'tcx, I: Iterator<Item = ty::Predicate<'tcx>>> Iterator for FilterToTraits<I> {
     type Item = ty::PolyTraitRef<'tcx>;
 
     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
-        while let Some(obligation) = self.base_iterator.next() {
-            if let Some(data) = obligation.predicate.to_opt_poly_trait_pred() {
+        while let Some(pred) = self.base_iterator.next() {
+            if let Some(data) = pred.to_opt_poly_trait_pred() {
                 return Some(data.map_bound(|t| t.trait_ref));
             }
         }

compiler/rustc_lint/src/unused.rs

@@ -258,11 +258,11 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                         cx.tcx,
                         cx.tcx.explicit_item_bounds(def).iter().cloned(),
                     )
-                    .find_map(|obligation| {
+                    .find_map(|(pred, _span)| {
                         // We only look at the `DefId`, so it is safe to skip the binder here.
                         if let ty::PredicateKind::Clause(ty::Clause::Trait(
                             ref poly_trait_predicate,
-                        )) = obligation.predicate.kind().skip_binder()
+                        )) = pred.kind().skip_binder()
                         {
                             let def_id = poly_trait_predicate.trait_ref.def_id;
 

compiler/rustc_mir_transform/src/const_prop.rs

@@ -117,7 +117,7 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
         if traits::impossible_predicates(
             tcx,
-            traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
+            traits::elaborate_predicates(tcx, predicates).collect(),
         ) {
             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
             return;

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -93,7 +93,7 @@ impl<'tcx> MirLint<'tcx> for ConstProp {
             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
         if traits::impossible_predicates(
             tcx,
-            traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
+            traits::elaborate_predicates(tcx, predicates).collect(),
         ) {
             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
             return;

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -470,7 +470,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         for assumption in
             elaborate_predicates(tcx, bounds.iter().map(|bound| bound.with_self_ty(tcx, self_ty)))
         {
-            match G::consider_object_bound_candidate(self, goal, assumption.predicate) {
+            match G::consider_object_bound_candidate(self, goal, assumption) {
                 Ok(result) => {
                     candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result })
                 }

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -349,8 +349,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             let normalized_preds = elaborate_predicates(
                 tcx,
                 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
-            )
-            .map(|o| o.predicate);
+            );
             new_env = ty::ParamEnv::new(
                 tcx.mk_predicates_from_iter(normalized_preds),
                 param_env.reveal(),

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -367,8 +367,8 @@ fn negative_impl_exists<'tcx>(
     }
 
     // Try to prove a negative obligation exists for super predicates
-    for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
-        if resolve_negative_obligation(infcx.fork(), &o, body_def_id) {
+    for pred in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
+        if resolve_negative_obligation(infcx.fork(), &o.with(infcx.tcx, pred), body_def_id) {
             return true;
         }
     }

compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs

@@ -82,15 +82,15 @@ pub fn recompute_applicable_impls<'tcx>(
 
     let predicates =
         tcx.predicates_of(obligation.cause.body_id.to_def_id()).instantiate_identity(tcx);
-    for obligation in elaborate_predicates_with_span(tcx, predicates.into_iter()) {
-        let kind = obligation.predicate.kind();
+    for (pred, span) in elaborate_predicates_with_span(tcx, predicates.into_iter()) {
+        let kind = pred.kind();
         if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = kind.skip_binder()
             && param_env_candidate_may_apply(kind.rebind(trait_pred))
         {
             if kind.rebind(trait_pred.trait_ref) == ty::TraitRef::identity(tcx, trait_pred.def_id()) {
                 ambiguities.push(Ambiguity::ParamEnv(tcx.def_span(trait_pred.def_id())))
             } else {
-                ambiguities.push(Ambiguity::ParamEnv(obligation.cause.span))
+                ambiguities.push(Ambiguity::ParamEnv(span))
             }
         }
     }

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1624,8 +1624,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
         };
 
-        for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
-            let bound_predicate = obligation.predicate.kind();
+        for pred in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
+            let bound_predicate = pred.kind();
             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
                 bound_predicate.skip_binder()
             {