Guard us against degenerate default traits

Author: fmease

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -717,16 +717,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         trait_ref: &hir::TraitRef<'tcx>,
         self_ty: Ty<'tcx>,
     ) -> ty::TraitRef<'tcx> {
-        let _ = self.prohibit_generic_args(
-            trait_ref.path.segments.split_last().unwrap().1.iter(),
-            GenericsArgsErrExtend::None,
-        );
+        let [leading_segments @ .., segment] = trait_ref.path.segments else { bug!() };
+
+        let _ = self.prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
 
         self.lower_mono_trait_ref(
             trait_ref.path.span,
             trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
             self_ty,
-            trait_ref.path.segments.last().unwrap(),
+            segment,
             true,
         )
     }
@@ -757,7 +756,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     #[instrument(level = "debug", skip(self, bounds))]
     pub(crate) fn lower_poly_trait_ref(
         &self,
-        poly_trait_ref: &hir::PolyTraitRef<'tcx>,
+        &hir::PolyTraitRef {
+            bound_generic_params,
+            modifiers: hir::TraitBoundModifiers { constness, polarity },
+            trait_ref,
+            span,
+        }: &hir::PolyTraitRef<'tcx>,
         self_ty: Ty<'tcx>,
         bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
         predicate_filter: PredicateFilter,
@@ -767,52 +771,67 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         // We use the *resolved* bound vars later instead of the HIR ones since the former
         // also include the bound vars of the overarching predicate if applicable.
-        let hir::PolyTraitRef { bound_generic_params: _, modifiers, trait_ref, span } =
-            *poly_trait_ref;
-        let hir::TraitBoundModifiers { constness, polarity } = modifiers;
+        let _ = bound_generic_params;
 
         let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());
 
-        // Relaxed bounds `?Trait` and `PointeeSized` bounds aren't represented in the `middle::ty` IR
+        // Relaxed bounds `?Trait` and `PointeeSized` bounds aren't represented in the middle::ty IR
         // as they denote the *absence* of a default bound. However, we can't bail out early here since
         // we still need to perform several validation steps (see below). Instead, simply "pour" all
         // resulting bounds "down the drain", i.e., into a new `Vec` that just gets dropped at the end.
-        let (polarity, bounds) = match polarity {
-            rustc_ast::BoundPolarity::Positive
-                if tcx.is_lang_item(trait_def_id, hir::LangItem::PointeeSized) =>
-            {
+        let transient = match polarity {
+            hir::BoundPolarity::Positive => {
                 // To elaborate on the comment directly above, regarding `PointeeSized` specifically,
                 // we don't "reify" such bounds to avoid trait system limitations -- namely,
                 // non-global where-clauses being preferred over item bounds (where `PointeeSized`
                 // bounds would be proven) -- which can result in errors when a `PointeeSized`
                 // supertrait / bound / predicate is added to some items.
-                (ty::PredicatePolarity::Positive, &mut Vec::new())
+                tcx.is_lang_item(trait_def_id, hir::LangItem::PointeeSized)
             }
-            rustc_ast::BoundPolarity::Positive => (ty::PredicatePolarity::Positive, bounds),
-            rustc_ast::BoundPolarity::Negative(_) => (ty::PredicatePolarity::Negative, bounds),
-            rustc_ast::BoundPolarity::Maybe(_) => {
+            hir::BoundPolarity::Negative(_) => false,
+            hir::BoundPolarity::Maybe(_) => {
                 self.require_bound_to_relax_default_trait(trait_ref, span);
+                true
+            }
+        };
+        let bounds = if transient { &mut Vec::new() } else { bounds };
 
-                (ty::PredicatePolarity::Positive, &mut Vec::new())
+        let polarity = match polarity {
+            hir::BoundPolarity::Positive | hir::BoundPolarity::Maybe(_) => {
+                ty::PredicatePolarity::Positive
             }
+            hir::BoundPolarity::Negative(_) => ty::PredicatePolarity::Negative,
         };
 
-        let trait_segment = trait_ref.path.segments.last().unwrap();
+        let [leading_segments @ .., segment] = trait_ref.path.segments else { bug!() };
 
-        let _ = self.prohibit_generic_args(
-            trait_ref.path.segments.split_last().unwrap().1.iter(),
-            GenericsArgsErrExtend::None,
-        );
-        self.report_internal_fn_trait(span, trait_def_id, trait_segment, false);
+        let _ = self.prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
+        self.report_internal_fn_trait(span, trait_def_id, segment, false);
 
         let (generic_args, arg_count) = self.lower_generic_args_of_path(
             trait_ref.path.span,
             trait_def_id,
             &[],
-            trait_segment,
+            segment,
             Some(self_ty),
         );
 
+        let constraints = segment.args().constraints;
+
+        if transient && (!generic_args[1..].is_empty() || !constraints.is_empty()) {
+            // Since the bound won't be present in the middle::ty IR as established above, any
+            // arguments or constraints won't be checked for well-formedness in later passes.
+            //
+            // This is only an issue if the trait ref is otherwise valid which can only happen if
+            // the corresponding default trait has generic parameters or associated items. Such a
+            // trait would be degenerate. We delay a bug to detect and guard us against these.
+            //
+            // E.g: Given `/*default*/ trait Bound<'a: 'static, T, const N: usize> {}`,
+            // `?Bound<Vec<str>, { panic!() }>` won't be wfchecked.
+            self.dcx()
+                .span_delayed_bug(span, "transient bound should not have args or constraints");
+        }
+
         let bound_vars = tcx.late_bound_vars(trait_ref.hir_ref_id);
         debug!(?bound_vars);
 
@@ -924,7 +943,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             == OverlappingAsssocItemConstraints::Forbidden)
             .then_some(FxIndexMap::default());
 
-        for constraint in trait_segment.args().constraints {
+        for constraint in constraints {
             // Don't register any associated item constraints for negative bounds,
             // since we should have emitted an error for them earlier, and they
             // would not be well-formed!
@@ -1916,10 +1935,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             Res::Def(DefKind::OpaqueTy, did) => {
                 // Check for desugared `impl Trait`.
                 assert_matches!(tcx.opaque_ty_origin(did), hir::OpaqueTyOrigin::TyAlias { .. });
-                let item_segment = path.segments.split_last().unwrap();
-                let _ = self
-                    .prohibit_generic_args(item_segment.1.iter(), GenericsArgsErrExtend::OpaqueTy);
-                let args = self.lower_generic_args_of_path_segment(span, did, item_segment.0);
+                let [leading_segments @ .., segment] = path.segments else { bug!() };
+                let _ = self.prohibit_generic_args(
+                    leading_segments.iter(),
+                    GenericsArgsErrExtend::OpaqueTy,
+                );
+                let args = self.lower_generic_args_of_path_segment(span, did, segment);
                 Ty::new_opaque(tcx, did, args)
             }
             Res::Def(
@@ -1931,11 +1952,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 did,
             ) => {
                 assert_eq!(opt_self_ty, None);
-                let _ = self.prohibit_generic_args(
-                    path.segments.split_last().unwrap().1.iter(),
-                    GenericsArgsErrExtend::None,
-                );
-                self.lower_path_segment(span, did, path.segments.last().unwrap())
+                let [leading_segments @ .., segment] = path.segments else { bug!() };
+                let _ = self
+                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
+                self.lower_path_segment(span, did, segment)
             }
             Res::Def(kind @ DefKind::Variant, def_id)
                 if let PermitVariants::Yes = permit_variants =>
@@ -1955,8 +1975,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     GenericsArgsErrExtend::DefVariant(&path.segments),
                 );
 
-                let GenericPathSegment(def_id, index) = generic_segments.last().unwrap();
-                self.lower_path_segment(span, *def_id, &path.segments[*index])
+                let &GenericPathSegment(def_id, index) = generic_segments.last().unwrap();
+                self.lower_path_segment(span, def_id, &path.segments[index])
             }
             Res::Def(DefKind::TyParam, def_id) => {
                 assert_eq!(opt_self_ty, None);
@@ -2242,15 +2262,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
             Res::Def(DefKind::Const | DefKind::Ctor(_, CtorKind::Const), did) => {
                 assert_eq!(opt_self_ty, None);
-                let _ = self.prohibit_generic_args(
-                    path.segments.split_last().unwrap().1.iter(),
-                    GenericsArgsErrExtend::None,
-                );
-                let args = self.lower_generic_args_of_path_segment(
-                    span,
-                    did,
-                    path.segments.last().unwrap(),
-                );
+                let [leading_segments @ .., segment] = path.segments else { bug!() };
+                let _ = self
+                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
+                let args = self.lower_generic_args_of_path_segment(span, did, segment);
                 ty::Const::new_unevaluated(tcx, ty::UnevaluatedConst::new(did, args))
             }
             Res::Def(DefKind::AssocConst, did) => {