Support ?Trait bounds in supertraits and dyn Trait under a feature gate

compiler/rustc_ast_lowering/src/item.rs

@@ -1469,8 +1469,14 @@ 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

@@ -1252,6 +1252,7 @@ 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)
@@ -1383,21 +1384,17 @@ 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));
@@ -2495,6 +2492,7 @@ 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

@@ -1266,14 +1266,28 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
         if let GenericBound::Trait(poly, modifiers) = bound {
             match (ctxt, modifiers.constness, modifiers.polarity) {
-                (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                    self.dcx().emit_err(errors::OptionalTraitSupertrait {
-                        span: poly.span,
-                        path_str: pprust::path_to_string(&poly.trait_ref.path),
-                    });
+                (BoundKind::SuperTraits, BoundConstness::Never, BoundPolarity::Maybe(_))
+                    if !self.features.more_maybe_bounds =>
+                {
+                    self.session
+                        .create_feature_err(
+                            errors::OptionalTraitSupertrait {
+                                span: poly.span,
+                                path_str: pprust::path_to_string(&poly.trait_ref.path),
+                            },
+                            sym::more_maybe_bounds,
+                        )
+                        .emit();
                 }
-                (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                    self.dcx().emit_err(errors::OptionalTraitObject { span: poly.span });
+                (BoundKind::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_))
+                    if !self.features.more_maybe_bounds =>
+                {
+                    self.session
+                        .create_feature_err(
+                            errors::OptionalTraitObject { span: poly.span },
+                            sym::more_maybe_bounds,
+                        )
+                        .emit();
                 }
                 (BoundKind::TraitObject, BoundConstness::Always(_), BoundPolarity::Positive) => {
                     self.dcx().emit_err(errors::ConstBoundTraitObject { span: poly.span });

compiler/rustc_feature/src/unstable.rs

@@ -207,6 +207,8 @@ 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

@@ -2610,7 +2610,11 @@ 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(AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been

compiler/rustc_hir/src/intravisit.rs

@@ -872,7 +872,9 @@ 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_analysis/src/astconv/bounds.rs

@@ -5,7 +5,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::{self as ty, Ty};
 use rustc_span::symbol::Ident;
-use rustc_span::{ErrorGuaranteed, Span};
+use rustc_span::{sym, ErrorGuaranteed, Span};
 use rustc_trait_selection::traits;
 use smallvec::SmallVec;
 
@@ -66,10 +66,16 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             }
         }
 
-        if unbounds.len() > 1 {
-            tcx.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
-                spans: unbounds.iter().map(|ptr| ptr.span).collect(),
-            });
+        if unbounds.len() > 1 && !tcx.features().more_maybe_bounds {
+            self.tcx()
+                .sess
+                .create_feature_err(
+                    errors::MultipleRelaxedDefaultBounds {
+                        spans: unbounds.iter().map(|ptr| ptr.span).collect(),
+                    },
+                    sym::more_maybe_bounds,
+                )
+                .emit();
         }
 
         let mut seen_sized_unbound = false;

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -592,7 +592,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         associated_types: FxIndexMap<Span, FxIndexSet<DefId>>,
         potential_assoc_types: Vec<Span>,
-        trait_bounds: &[hir::PolyTraitRef<'_>],
+        trait_bounds: &[(hir::PolyTraitRef<'_>, hir::TraitBoundModifier)],
     ) {
         if associated_types.values().all(|v| v.is_empty()) {
             return;
@@ -634,7 +634,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if object_safety_violations {
             return;
         }
-        if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
+        if let ([], [(bound, _)]) = (&potential_assoc_types[..], &trait_bounds) {
             match bound.trait_ref.path.segments {
                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
@@ -676,7 +676,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // and we can then use their span to indicate this to the user.
         let bound_names = trait_bounds
             .iter()
-            .filter_map(|poly_trait_ref| {
+            .filter_map(|(poly_trait_ref, _)| {
                 let path = poly_trait_ref.trait_ref.path.segments.last()?;
                 let args = path.args?;
 

compiler/rustc_hir_analysis/src/astconv/lint.rs

@@ -97,7 +97,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let mut is_downgradable = true;
         let is_object_safe = match self_ty.kind {
             hir::TyKind::TraitObject(objects, ..) => {
-                objects.iter().all(|o| match o.trait_ref.path.res {
+                objects.iter().all(|(o, _)| match o.trait_ref.path.res {
                     Res::Def(DefKind::Trait, id) => {
                         if Some(id) == owner {
                             // For recursive traits, don't downgrade the error. (#119652)
@@ -188,7 +188,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
     pub(super) fn maybe_lint_bare_trait(&self, self_ty: &hir::Ty<'_>, in_path: bool) {
         let tcx = self.tcx();
-        if let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
+        if let hir::TyKind::TraitObject([(poly_trait_ref, _), ..], _, TraitObjectSyntax::None) =
             self_ty.kind
         {
             let needs_bracket = in_path

compiler/rustc_hir_analysis/src/astconv/object_safety.rs

@@ -22,7 +22,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         span: Span,
         hir_id: hir::HirId,
-        hir_trait_bounds: &[hir::PolyTraitRef<'tcx>],
+        hir_trait_bounds: &[(hir::PolyTraitRef<'tcx>, hir::TraitBoundModifier)],
         lifetime: &hir::Lifetime,
         borrowed: bool,
         representation: DynKind,
@@ -32,7 +32,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let mut bounds = Bounds::default();
         let mut potential_assoc_types = Vec::new();
         let dummy_self = self.tcx().types.trait_object_dummy_self;
-        for trait_bound in hir_trait_bounds.iter().rev() {
+        for (trait_bound, modifier) in hir_trait_bounds.iter().rev() {
+            if *modifier == hir::TraitBoundModifier::Maybe {
+                continue;
+            }
             if let GenericArgCountResult {
                 correct:
                     Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
@@ -277,7 +280,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 let args = tcx.mk_args(&args);
 
                 let span = i.bottom().1;
-                let empty_generic_args = hir_trait_bounds.iter().any(|hir_bound| {
+                let empty_generic_args = hir_trait_bounds.iter().any(|(hir_bound, _)| {
                     hir_bound.trait_ref.path.res == Res::Def(DefKind::Trait, trait_ref.def_id)
                         && hir_bound.span.contains(span)
                 });

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -844,7 +844,7 @@ 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,
         },

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -606,7 +606,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                 debug!(?bounds, ?lifetime, "TraitObject");
                 let scope = Scope::TraitRefBoundary { s: self.scope };
                 self.with(scope, |this| {
-                    for bound in bounds {
+                    for (bound, _) in bounds {
                         this.visit_poly_trait_ref_inner(
                             bound,
                             NonLifetimeBinderAllowed::Deny("trait object types"),

compiler/rustc_hir_pretty/src/lib.rs

@@ -293,13 +293,16 @@ impl<'a> State<'a> {
                     self.word_space("dyn");
                 }
                 let mut first = true;
-                for bound in bounds {
+                for (bound, modifier) in bounds {
                     if first {
                         first = false;
                     } else {
                         self.nbsp();
                         self.word_space("+");
                     }
+                    if *modifier == TraitBoundModifier::Maybe {
+                        self.word("?");
+                    }
                     self.print_poly_trait_ref(bound);
                 }
                 if !lifetime.is_elided() {

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs

@@ -82,7 +82,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
             }
 
             hir::TyKind::TraitObject(bounds, ..) => {
-                for bound in bounds {
+                for (bound, _) in bounds {
                     self.current_index.shift_in(1);
                     self.visit_poly_trait_ref(bound);
                     self.current_index.shift_out(1);

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -567,7 +567,7 @@ impl<'a, 'tcx> Visitor<'tcx> for HirTraitObjectVisitor<'a> {
             _,
         ) = t.kind
         {
-            for ptr in poly_trait_refs {
+            for (ptr, _) in poly_trait_refs {
                 if Some(self.1) == ptr.trait_ref.trait_def_id() {
                     self.0.push(ptr.span);
                 }

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

@@ -62,7 +62,7 @@ pub fn report_object_safety_error<'tcx>(
 
     if let Some(hir_id) = hir_id
         && let hir::Node::Ty(ty) = tcx.hir_node(hir_id)
-        && let hir::TyKind::TraitObject([trait_ref, ..], ..) = ty.kind
+        && let hir::TyKind::TraitObject([(trait_ref, _), ..], ..) = ty.kind
     {
         let mut hir_id = hir_id;
         while let hir::Node::Ty(ty) = tcx.parent_hir_node(hir_id) {

compiler/rustc_lint/src/non_local_def.rs

@@ -125,7 +125,7 @@ impl<'tcx> LateLintPass<'tcx> for NonLocalDefinitions {
                     TyKind::Path(QPath::Resolved(_, ty_path)) => {
                         path_has_local_parent(ty_path, cx, parent, parent_parent)
                     }
-                    TyKind::TraitObject([principle_poly_trait_ref, ..], _, _) => {
+                    TyKind::TraitObject([(principle_poly_trait_ref, _), ..], _, _) => {
                         path_has_local_parent(
                             principle_poly_trait_ref.trait_ref.path,
                             cx,

compiler/rustc_lint/src/traits.rs

@@ -112,9 +112,11 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
 
     fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx>) {
         let hir::TyKind::TraitObject(bounds, _lifetime, _syntax) = &ty.kind else { return };
-        for bound in &bounds[..] {
+        for (bound, modifier) in &bounds[..] {
             let def_id = bound.trait_ref.trait_def_id();
-            if cx.tcx.lang_items().drop_trait() == def_id {
+            if cx.tcx.lang_items().drop_trait() == def_id
+                && *modifier != hir::TraitBoundModifier::Maybe
+            {
                 let Some(def_id) = cx.tcx.get_diagnostic_item(sym::needs_drop) else { return };
                 cx.emit_span_lint(DYN_DROP, bound.span, DropGlue { tcx: cx.tcx, def_id });
             }

compiler/rustc_span/src/symbol.rs

@@ -1170,6 +1170,7 @@ symbols! {
         modifiers,
         module,
         module_path,
+        more_maybe_bounds,
         more_qualified_paths,
         more_struct_aliases,
         movbe_target_feature,

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

@@ -2983,11 +2983,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     match ty.kind {
                         hir::TyKind::TraitObject(traits, _, _) => {
                             let (span, kw) = match traits {
-                                [first, ..] if first.span.lo() == ty.span.lo() => {
+                                [(first, _), ..] if first.span.lo() == ty.span.lo() => {
                                     // Missing `dyn` in front of trait object.
                                     (ty.span.shrink_to_lo(), "dyn ")
                                 }
-                                [first, ..] => (ty.span.until(first.span), ""),
+                                [(first, _), ..] => (ty.span.until(first.span), ""),
                                 [] => span_bug!(ty.span, "trait object with no traits: {ty:?}"),
                             };
                             let needs_parens = traits.len() != 1;

src/librustdoc/clean/mod.rs

@@ -1859,7 +1859,7 @@ pub(crate) fn clean_ty<'tcx>(ty: &hir::Ty<'tcx>, cx: &mut DocContext<'tcx>) -> T
         }
         TyKind::Path(_) => clean_qpath(ty, cx),
         TyKind::TraitObject(bounds, ref lifetime, _) => {
-            let bounds = bounds.iter().map(|bound| clean_poly_trait_ref(bound, cx)).collect();
+            let bounds = bounds.iter().map(|(bound, _)| clean_poly_trait_ref(bound, cx)).collect();
             let lifetime =
                 if !lifetime.is_elided() { Some(clean_lifetime(*lifetime, cx)) } else { None };
             DynTrait(bounds, lifetime)

src/tools/clippy/clippy_lints/src/lifetimes.rs

@@ -549,7 +549,7 @@ impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
                 if !lt.is_elided() {
                     self.unelided_trait_object_lifetime = true;
                 }
-                for bound in bounds {
+                for (bound, _) in bounds {
                     self.visit_poly_trait_ref(bound);
                 }
             },

src/tools/clippy/clippy_lints/src/trait_bounds.rs

@@ -181,7 +181,7 @@ impl<'tcx> LateLintPass<'tcx> for TraitBounds {
 
             // Iterate the bounds and add them to our seen hash
             // If we haven't yet seen it, add it to the fixed traits
-            for bound in bounds {
+            for (bound, _) in bounds {
                 let Some(def_id) = bound.trait_ref.trait_def_id() else {
                     continue;
                 };
@@ -196,9 +196,9 @@ impl<'tcx> LateLintPass<'tcx> for TraitBounds {
             // If the number of unique traits isn't the same as the number of traits in the bounds,
             // there must be 1 or more duplicates
             if bounds.len() != unique_traits.len() {
-                let mut bounds_span = bounds[0].span;
+                let mut bounds_span = bounds[0].0.span;
 
-                for bound in bounds.iter().skip(1) {
+                for (bound, _) in bounds.iter().skip(1) {
                     bounds_span = bounds_span.to(bound.span);
                 }