initial implementation for default auto traits

Author: Bryanskiy

compiler/rustc_ast_lowering/src/item.rs

@@ -1525,8 +1525,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     continue;
                 }
                 let is_param = *is_param.get_or_insert_with(compute_is_param);
-                if !is_param {
-                    self.dcx().emit_err(MisplacedRelaxTraitBound { span: bound.span() });
+                if !is_param && !self.tcx.features().more_maybe_bounds {
+                    self.tcx
+                        .sess
+                        .create_feature_err(
+                            MisplacedRelaxTraitBound { span: bound.span() },
+                            sym::more_maybe_bounds,
+                        )
+                        .emit();
                 }
             }
         }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1216,6 +1216,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     itctx,
                     TraitBoundModifiers::NONE,
                 );
+                let bound = (bound, hir::TraitBoundModifier::None);
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
                 (bounds, lifetime_bound)
@@ -1348,21 +1349,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             // We can safely ignore constness here since AST validation
                             // takes care of rejecting invalid modifier combinations and
                             // const trait bounds in trait object types.
-                            GenericBound::Trait(ty, modifiers) => match modifiers.polarity {
-                                BoundPolarity::Positive | BoundPolarity::Negative(_) => {
-                                    Some(this.lower_poly_trait_ref(
-                                        ty,
-                                        itctx,
-                                        // Still, don't pass along the constness here; we don't want to
-                                        // synthesize any host effect args, it'd only cause problems.
-                                        TraitBoundModifiers {
-                                            constness: BoundConstness::Never,
-                                            ..*modifiers
-                                        },
-                                    ))
-                                }
-                                BoundPolarity::Maybe(_) => None,
-                            },
+                            GenericBound::Trait(ty, modifiers) => {
+                                // Still, don't pass along the constness here; we don't want to
+                                // synthesize any host effect args, it'd only cause problems.
+                                let modifiers = TraitBoundModifiers {
+                                    constness: BoundConstness::Never,
+                                    ..*modifiers
+                                };
+                                let trait_ref = this.lower_poly_trait_ref(ty, itctx, modifiers);
+                                let polarity = this.lower_trait_bound_modifiers(modifiers);
+                                Some((trait_ref, polarity))
+                            }
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
                                     lifetime_bound = Some(this.lower_lifetime(lifetime));
@@ -2688,6 +2685,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             trait_ref: hir::TraitRef { path, hir_ref_id: hir_id },
                             span: self.lower_span(span),
                         };
+                        let principal = (principal, hir::TraitBoundModifier::None);
 
                         // The original ID is taken by the `PolyTraitRef`,
                         // so the `Ty` itself needs a different one.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1345,14 +1345,28 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         match bound {
             GenericBound::Trait(trait_ref, modifiers) => {
                 match (ctxt, modifiers.constness, modifiers.polarity) {
-                    (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                        self.dcx().emit_err(errors::OptionalTraitSupertrait {
-                            span: trait_ref.span,
-                            path_str: pprust::path_to_string(&trait_ref.trait_ref.path),
-                        });
+                    (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_))
+                        if !self.features.more_maybe_bounds =>
+                    {
+                        self.session
+                            .create_feature_err(
+                                errors::OptionalTraitSupertrait {
+                                    span: trait_ref.span,
+                                    path_str: pprust::path_to_string(&trait_ref.trait_ref.path),
+                                },
+                                sym::more_maybe_bounds,
+                            )
+                            .emit();
                     }
-                    (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                        self.dcx().emit_err(errors::OptionalTraitObject { span: trait_ref.span });
+                    (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_))
+                        if !self.features.more_maybe_bounds =>
+                    {
+                        self.session
+                            .create_feature_err(
+                                errors::OptionalTraitObject { span: trait_ref.span },
+                                sym::more_maybe_bounds,
+                            )
+                            .emit();
                     }
                     (
                         BoundKind::TraitObject,

compiler/rustc_feature/src/unstable.rs

@@ -205,6 +205,8 @@ declare_features! (
     (unstable, lifetime_capture_rules_2024, "1.76.0", None),
     /// Allows `#[link(..., cfg(..))]`; perma-unstable per #37406
     (unstable, link_cfg, "1.14.0", None),
+    /// Allows using `?Trait` trait bounds in more contexts.
+    (internal, more_maybe_bounds, "CURRENT_RUSTC_VERSION", None),
     /// Allows the `multiple_supertrait_upcastable` lint.
     (unstable, multiple_supertrait_upcastable, "1.69.0", None),
     /// Allow negative trait bounds. This is an internal-only feature for testing the trait solver!

compiler/rustc_hir/src/hir.rs

@@ -2832,7 +2832,11 @@ pub enum TyKind<'hir> {
     OpaqueDef(ItemId, &'hir [GenericArg<'hir>], bool),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
-    TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),
+    TraitObject(
+        &'hir [(PolyTraitRef<'hir>, TraitBoundModifier)],
+        &'hir Lifetime,
+        TraitObjectSyntax,
+    ),
     /// Unused for now.
     Typeof(&'hir AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been

compiler/rustc_hir/src/intravisit.rs

@@ -902,7 +902,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) -> V::Resul
             try_visit!(visitor.visit_array_length(length));
         }
         TyKind::TraitObject(bounds, ref lifetime, _syntax) => {
-            walk_list!(visitor, visit_poly_trait_ref, bounds);
+            for (bound, _modifier) in bounds {
+                try_visit!(visitor.visit_poly_trait_ref(bound));
+            }
             try_visit!(visitor.visit_lifetime(lifetime));
         }
         TyKind::Typeof(ref expression) => try_visit!(visitor.visit_anon_const(expression)),

compiler/rustc_hir/src/lang_items.rs

@@ -413,6 +413,12 @@ language_item_table! {
     EffectsIntersectionOutput, sym::EffectsIntersectionOutput, effects_intersection_output, Target::AssocTy, GenericRequirement::None;
     EffectsCompat,           sym::EffectsCompat,       effects_compat,             Target::Trait,          GenericRequirement::Exact(1);
     EffectsTyCompat,         sym::EffectsTyCompat,     effects_ty_compat,          Target::Trait,          GenericRequirement::Exact(1);
+
+    // 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;
 }
 
 pub enum GenericRequirement {

compiler/rustc_hir_analysis/src/bounds.rs

@@ -166,11 +166,27 @@ impl<'tcx> Bounds<'tcx> {
         ));
     }
 
-    pub fn push_sized(&mut self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) {
-        let sized_def_id = tcx.require_lang_item(LangItem::Sized, Some(span));
-        let trait_ref = ty::TraitRef::new(tcx, sized_def_id, [ty]);
-        // Preferable to put this obligation first, since we report better errors for sized ambiguity.
-        self.clauses.insert(0, (trait_ref.upcast(tcx), span));
+    pub fn push_lang_item_trait(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        lang_item: LangItem,
+        span: Span,
+    ) {
+        assert_eq!(lang_item.target(), rustc_hir::Target::Trait);
+        if lang_item == LangItem::Sized {
+            let sized_def_id = tcx.require_lang_item(LangItem::Sized, Some(span));
+            let trait_ref = ty::TraitRef::new(tcx, sized_def_id, [ty]);
+            // Preferable to put this obligation first, since we report better errors for sized ambiguity.
+            self.clauses.insert(0, (trait_ref.upcast(tcx), span));
+        } else {
+            // Do not generate default bounds if lang item was not defined
+            let Some(trait_def_id) = tcx.lang_items().get(lang_item) else {
+                return;
+            };
+            let trait_ref = ty::TraitRef::new(tcx, trait_def_id, [ty]);
+            self.clauses.push((trait_ref.upcast(tcx), span));
+        }
     }
 
     pub fn clauses(

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -353,9 +353,8 @@ 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/check/wfcheck.rs

@@ -831,7 +831,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GATArgsCollector<'tcx> {
 
 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
     match ty.kind {
-        hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
+        hir::TyKind::TraitObject([(trait_ref, _)], ..) => match trait_ref.trait_ref.path.segments {
             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
             _ => false,
         },