hir_analysis: add missing sizedness bounds

Default sizedness bounds were not being added to
`explicit_super_predicates_of` and `explicit_implied_predicates_of`
which meant that a trait bound added to a associated type projection
would be missing the implied predicate of the default sizedness
supertrait of that trait.

An unexpected consequence of this change was that the check for multiple
principals was now finding an additional `MetaSized` principal when
eagerly expanding trait aliases. Instead of special-casing trait aliases
as different from traits and not adding a `MetaSized` supertrait to trait
aliases, filter out `MetaSized` when lowering `dyn Trait`.

Author: davidtwco

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -12,7 +12,9 @@ use tracing::{debug, instrument};
 
 use super::ItemCtxt;
 use super::predicates_of::assert_only_contains_predicates_from;
-use crate::hir_ty_lowering::{HirTyLowerer, OverlappingAsssocItemConstraints, PredicateFilter};
+use crate::hir_ty_lowering::{
+    HirTyLowerer, ImpliedBoundsContext, OverlappingAsssocItemConstraints, PredicateFilter,
+};
 
 /// For associated types we include both bounds written on the type
 /// (`type X: Trait`) and predicates from the trait: `where Self::X: Trait`.
@@ -52,15 +54,20 @@ fn associated_type_bounds<'tcx>(
             | PredicateFilter::SelfTraitThatDefines(_)
             | PredicateFilter::SelfAndAssociatedTypeBounds => {
                 // Implicit bounds are added to associated types unless a `?Trait` bound is found.
-                icx.lowerer().add_sizedness_bounds(
+                icx.lowerer().add_implicit_sizedness_bounds(
+                    &mut bounds,
+                    item_ty,
+                    hir_bounds,
+                    ImpliedBoundsContext::AssociatedTypeOrImplTrait,
+                    span,
+                );
+                icx.lowerer().add_default_traits(
                     &mut bounds,
                     item_ty,
                     hir_bounds,
-                    None,
-                    None,
+                    ImpliedBoundsContext::AssociatedTypeOrImplTrait,
                     span,
                 );
-                icx.lowerer().add_default_traits(&mut bounds, item_ty, hir_bounds, None, span);
 
                 // Also collect `where Self::Assoc: Trait` from the parent trait's where clauses.
                 let trait_def_id = tcx.local_parent(assoc_item_def_id);
@@ -372,15 +379,20 @@ fn opaque_type_bounds<'tcx>(
             | PredicateFilter::SelfOnly
             | PredicateFilter::SelfTraitThatDefines(_)
             | PredicateFilter::SelfAndAssociatedTypeBounds => {
-                icx.lowerer().add_sizedness_bounds(
+                icx.lowerer().add_implicit_sizedness_bounds(
+                    &mut bounds,
+                    item_ty,
+                    hir_bounds,
+                    ImpliedBoundsContext::AssociatedTypeOrImplTrait,
+                    span,
+                );
+                icx.lowerer().add_default_traits(
                     &mut bounds,
                     item_ty,
                     hir_bounds,
-                    None,
-                    None,
+                    ImpliedBoundsContext::AssociatedTypeOrImplTrait,
                     span,
                 );
-                icx.lowerer().add_default_traits(&mut bounds, item_ty, hir_bounds, None, span);
             }
             //`ConstIfConst` is only interested in `[const]` bounds.
             PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -19,7 +19,8 @@ use crate::collect::ItemCtxt;
 use crate::constrained_generic_params as cgp;
 use crate::delegation::inherit_predicates_for_delegation_item;
 use crate::hir_ty_lowering::{
-    HirTyLowerer, OverlappingAsssocItemConstraints, PredicateFilter, RegionInferReason,
+    HirTyLowerer, ImpliedBoundsContext, OverlappingAsssocItemConstraints, PredicateFilter,
+    RegionInferReason,
 };
 
 /// Returns a list of all type predicates (explicit and implicit) for the definition with
@@ -189,19 +190,18 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             PredicateFilter::All,
             OverlappingAsssocItemConstraints::Allowed,
         );
-        icx.lowerer().add_sizedness_bounds(
+        icx.lowerer().add_implicit_sizedness_bounds(
             &mut bounds,
             tcx.types.self_param,
             self_bounds,
-            None,
-            Some(def_id),
+            ImpliedBoundsContext::TraitDef(def_id),
             span,
         );
-        icx.lowerer().add_default_super_traits(
-            def_id,
+        icx.lowerer().add_default_traits(
             &mut bounds,
+            tcx.types.self_param,
             self_bounds,
-            hir_generics,
+            ImpliedBoundsContext::TraitDef(def_id),
             span,
         );
         predicates.extend(bounds);
@@ -229,19 +229,18 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
                 let param_ty = icx.lowerer().lower_ty_param(param.hir_id);
                 let mut bounds = Vec::new();
                 // Implicit bounds are added to type params unless a `?Trait` bound is found
-                icx.lowerer().add_sizedness_bounds(
+                icx.lowerer().add_implicit_sizedness_bounds(
                     &mut bounds,
                     param_ty,
                     &[],
-                    Some((param.def_id, hir_generics.predicates)),
-                    None,
+                    ImpliedBoundsContext::TyParam(param.def_id, hir_generics.predicates),
                     param.span,
                 );
                 icx.lowerer().add_default_traits(
                     &mut bounds,
                     param_ty,
                     &[],
-                    Some((param.def_id, hir_generics.predicates)),
+                    ImpliedBoundsContext::TyParam(param.def_id, hir_generics.predicates),
                     param.span,
                 );
                 trace!(?bounds);
@@ -676,11 +675,18 @@ pub(super) fn implied_predicates_with_filter<'tcx>(
         | PredicateFilter::SelfOnly
         | PredicateFilter::SelfTraitThatDefines(_)
         | PredicateFilter::SelfAndAssociatedTypeBounds => {
-            icx.lowerer().add_default_super_traits(
-                trait_def_id,
+            icx.lowerer().add_implicit_sizedness_bounds(
+                &mut bounds,
+                self_param_ty,
+                superbounds,
+                ImpliedBoundsContext::TraitDef(trait_def_id),
+                item.span,
+            );
+            icx.lowerer().add_default_traits(
                 &mut bounds,
+                self_param_ty,
                 superbounds,
-                generics,
+                ImpliedBoundsContext::TraitDef(trait_def_id),
                 item.span,
             );
         }