Initial support for auto traits with default bounds

compiler/rustc_hir/src/lang_items.rs

@@ -430,6 +430,12 @@ language_item_table! {
     // Experimental lang items for implementing contract pre- and post-condition checking.
     ContractBuildCheckEnsures, sym::contract_build_check_ensures, contract_build_check_ensures_fn, Target::Fn, GenericRequirement::None;
     ContractCheckRequires,     sym::contract_check_requires,      contract_check_requires_fn,      Target::Fn, GenericRequirement::None;
+
+    // Experimental lang items for `MCP: Low level components for async drop`(https://github.com/rust-lang/compiler-team/issues/727)
+    DefaultTrait4,           sym::default_trait4,      default_trait4_trait,       Target::Trait,          GenericRequirement::None;
+    DefaultTrait3,           sym::default_trait3,      default_trait3_trait,       Target::Trait,          GenericRequirement::None;
+    DefaultTrait2,           sym::default_trait2,      default_trait2_trait,       Target::Trait,          GenericRequirement::None;
+    DefaultTrait1,           sym::default_trait1,      default_trait1_trait,       Target::Trait,          GenericRequirement::None;
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -344,9 +344,8 @@ fn bounds_from_generic_predicates<'tcx>(
             ty::ClauseKind::Trait(trait_predicate) => {
                 let entry = types.entry(trait_predicate.self_ty()).or_default();
                 let def_id = trait_predicate.def_id();
-                if Some(def_id) != tcx.lang_items().sized_trait() {
-                    // Type params are `Sized` by default, do not add that restriction to the list
-                    // if it is a positive requirement.
+                if !tcx.is_default_trait(def_id) {
+                    // Do not add that restriction to the list if it is a positive requirement.
                     entry.push(trait_predicate.def_id());
                 }
             }

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -39,13 +39,13 @@ fn associated_type_bounds<'tcx>(
         let icx = ItemCtxt::new(tcx, assoc_item_def_id);
         let mut bounds = Vec::new();
         icx.lowerer().lower_bounds(item_ty, hir_bounds, &mut bounds, ty::List::empty(), filter);
-        // Associated types are implicitly sized unless a `?Sized` bound is found
+        // Implicit bounds are added to associated types unless a `?Trait` bound is found
         match filter {
             PredicateFilter::All
             | PredicateFilter::SelfOnly
             | PredicateFilter::SelfTraitThatDefines(_)
             | PredicateFilter::SelfAndAssociatedTypeBounds => {
-                icx.lowerer().add_sized_bound(&mut bounds, item_ty, hir_bounds, None, span);
+                icx.lowerer().add_implicit_traits(&mut bounds, item_ty, hir_bounds, None, span);
             }
             // `ConstIfConst` is only interested in `~const` bounds.
             PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
@@ -328,14 +328,13 @@ fn opaque_type_bounds<'tcx>(
         let icx = ItemCtxt::new(tcx, opaque_def_id);
         let mut bounds = Vec::new();
         icx.lowerer().lower_bounds(item_ty, hir_bounds, &mut bounds, ty::List::empty(), filter);
-        // Opaque types are implicitly sized unless a `?Sized` bound is found
+        // Implicit bounds are added to opaque types unless a `?Trait` bound is found
         match filter {
             PredicateFilter::All
             | PredicateFilter::SelfOnly
             | PredicateFilter::SelfTraitThatDefines(_)
             | PredicateFilter::SelfAndAssociatedTypeBounds => {
-                // Associated types are implicitly sized unless a `?Sized` bound is found
-                icx.lowerer().add_sized_bound(&mut bounds, item_ty, hir_bounds, None, span);
+                icx.lowerer().add_implicit_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

@@ -164,12 +164,41 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             }
 
             ItemKind::Trait(_, _, _, self_bounds, ..) | ItemKind::TraitAlias(_, self_bounds) => {
-                is_trait = Some(self_bounds);
+                is_trait = Some((self_bounds, item.span));
             }
             _ => {}
         }
     };
 
+    if let Node::TraitItem(item) = node {
+        let parent = tcx.local_parent(item.hir_id().owner.def_id);
+        let Node::Item(parent_trait) = tcx.hir_node_by_def_id(parent) else {
+            unreachable!();
+        };
+
+        let (trait_generics, trait_bounds) = match parent_trait.kind {
+            hir::ItemKind::Trait(.., generics, supertraits, _) => (generics, supertraits),
+            hir::ItemKind::TraitAlias(generics, supertraits) => (generics, supertraits),
+            _ => unreachable!(),
+        };
+
+        // Implicitly add `Self: Trait` clauses on trait associated items.
+        // See comment on `add_implicit_super_traits` for more details.
+        if !icx.lowerer().requires_implicit_supertraits(parent, trait_bounds, trait_generics) {
+            let mut bounds = Vec::new();
+            let self_ty_where_predicates = (parent, item.generics.predicates);
+            icx.lowerer().add_implicit_traits_with_filter(
+                &mut bounds,
+                tcx.types.self_param,
+                &[],
+                Some(self_ty_where_predicates),
+                item.span,
+                |tr| tr != hir::LangItem::Sized,
+            );
+            predicates.extend(bounds);
+        }
+    }
+
     let generics = tcx.generics_of(def_id);
 
     // Below we'll consider the bounds on the type parameters (including `Self`)
@@ -180,11 +209,18 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
         let mut bounds = Vec::new();
         icx.lowerer().lower_bounds(
             tcx.types.self_param,
-            self_bounds,
+            self_bounds.0,
             &mut bounds,
             ty::List::empty(),
             PredicateFilter::All,
         );
+        icx.lowerer().add_implicit_super_traits(
+            def_id,
+            &mut bounds,
+            self_bounds.0,
+            hir_generics,
+            self_bounds.1,
+        );
         predicates.extend(bounds);
     }
 
@@ -209,8 +245,8 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             GenericParamKind::Type { .. } => {
                 let param_ty = icx.lowerer().lower_ty_param(param.hir_id);
                 let mut bounds = Vec::new();
-                // Params are implicitly sized unless a `?Sized` bound is found
-                icx.lowerer().add_sized_bound(
+                // // Implicit bounds are added to type params unless a `?Trait` bound is found
+                icx.lowerer().add_implicit_traits(
                     &mut bounds,
                     param_ty,
                     &[],
@@ -624,6 +660,22 @@ pub(super) fn implied_predicates_with_filter<'tcx>(
     let self_param_ty = tcx.types.self_param;
     let mut bounds = Vec::new();
     icx.lowerer().lower_bounds(self_param_ty, superbounds, &mut bounds, ty::List::empty(), filter);
+    match filter {
+        PredicateFilter::All
+        | PredicateFilter::SelfOnly
+        | PredicateFilter::SelfTraitThatDefines(_)
+        | PredicateFilter::SelfAndAssociatedTypeBounds => {
+            icx.lowerer().add_implicit_super_traits(
+                trait_def_id,
+                &mut bounds,
+                superbounds,
+                generics,
+                item.span,
+            );
+        }
+        //`ConstIfConst` is only interested in `~const` bounds.
+        PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
+    }
 
     let where_bounds_that_match =
         icx.probe_ty_param_bounds_in_generics(generics, item.owner_id.def_id, filter);