add polarity

compiler/rustc_ast_lowering/src/lib.rs

@@ -1255,6 +1255,7 @@ 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)
@@ -1386,21 +1387,17 @@ 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));
@@ -2502,6 +2499,7 @@ 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/messages.ftl

@@ -202,11 +202,6 @@ ast_passes_nomangle_ascii = `#[no_mangle]` requires ASCII identifier
 ast_passes_obsolete_auto = `impl Trait for .. {"{}"}` is an obsolete syntax
     .help = use `auto trait Trait {"{}"}` instead
 
-ast_passes_optional_trait_object = `?Trait` is not permitted in trait object types
-
-ast_passes_optional_trait_supertrait = `?Trait` is not permitted in supertraits
-    .note = traits are `?{$path_str}` by default
-
 ast_passes_out_of_order_params = {$param_ord} parameters must be declared prior to {$max_param} parameters
     .suggestion = reorder the parameters: lifetimes, then consts and types
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1260,16 +1260,8 @@ 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 {
+            // Some of the arms are feature-gated. See `feature_gate::PostExpansionVisitor`.
             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::TraitObject, BoundConstness::Never, BoundPolarity::Maybe(_)) => {
-                    self.dcx().emit_err(errors::OptionalTraitObject { span: poly.span });
-                }
                 (BoundKind::TraitObject, BoundConstness::Always(_), BoundPolarity::Positive) => {
                     self.dcx().emit_err(errors::ConstBoundTraitObject { span: poly.span });
                 }

compiler/rustc_ast_passes/src/errors.rs

@@ -543,22 +543,6 @@ pub struct NestedLifetimes {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_optional_trait_supertrait)]
-#[note]
-pub struct OptionalTraitSupertrait {
-    #[primary_span]
-    pub span: Span,
-    pub path_str: String,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_optional_trait_object)]
-pub struct OptionalTraitObject {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_const_bound_trait_object)]
 pub struct ConstBoundTraitObject {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -1,5 +1,5 @@
 use rustc_ast as ast;
-use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
+use rustc_ast::visit::{self, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
 use rustc_ast::{attr, AssocConstraint, AssocConstraintKind, NodeId};
 use rustc_ast::{PatKind, RangeEnd};
 use rustc_feature::{AttributeGate, BuiltinAttribute, Features, GateIssue, BUILTIN_ATTRIBUTE_MAP};
@@ -169,6 +169,25 @@ impl<'a> PostExpansionVisitor<'a> {
 }
 
 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
+    fn visit_param_bound(&mut self, bound: &'a ast::GenericBound, ctxt: BoundKind) {
+        if let ast::GenericBound::Trait(poly, modifiers) = bound {
+            let gate = |descr| {
+                gate!(&self, more_maybe_bounds, poly.span, descr);
+            };
+
+            match (ctxt, modifiers.polarity) {
+                (BoundKind::TraitObject, ast::BoundPolarity::Maybe(_)) => {
+                    gate("`?Trait` is not permitted in trait object types");
+                }
+                (BoundKind::SuperTraits, ast::BoundPolarity::Maybe(_)) => {
+                    gate("`?Trait` is not permitted in supertraits");
+                }
+                _ => {}
+            }
+        }
+        visit::walk_param_bound(self, bound);
+    }
+
     fn visit_attribute(&mut self, attr: &ast::Attribute) {
         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
         // Check feature gates for built-in attributes.

compiler/rustc_error_codes/src/error_codes/E0203.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 Having multiple relaxed default bounds is unsupported.
 
-Erroneous code example:
+Previously erroneous code example:
 
-```compile_fail,E0203
+```compile_fail
 struct Bad<T: ?Sized + ?Send>{
     inner: T
 }

compiler/rustc_error_codes/src/lib.rs

@@ -575,6 +575,7 @@ E0797: 0797,
 //  E0190, // deprecated: can only cast a &-pointer to an &-object
 //  E0194, // merged into E0403
 //  E0196, // cannot determine a type for this closure
+//  E0203  // replaced with a `more_maybe_bounds` feature gate
 //  E0209, // builtin traits can only be implemented on structs or enums
 //  E0213, // associated types are not accepted in this context
 //  E0215, // angle-bracket notation is not stable with `Fn`

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

@@ -2603,7 +2603,11 @@ 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

@@ -873,7 +873,9 @@ 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/messages.ftl

@@ -275,9 +275,6 @@ hir_analysis_missing_type_params =
     } to {$parameters}
     .note = because of the default `Self` reference, type parameters must be specified on object types
 
-hir_analysis_multiple_relaxed_default_bounds =
-    type parameter has more than one relaxed default bound, only one is supported
-
 hir_analysis_must_be_name_of_associated_function = must be a name of an associated function
 
 hir_analysis_must_implement_not_function = not a function

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -4,8 +4,9 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::{self as ty, Ty};
+use rustc_session::parse::feature_err;
 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 +67,14 @@ 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 {
+            feature_err(
+                tcx.sess,
+                sym::more_maybe_bounds,
+                unbounds.iter().map(|ptr| ptr.span).collect::<Vec<Span>>(),
+                "type parameter has more than one relaxed default bound, only one is supported",
+            )
+            .emit();
         }
 
         let mut seen_sized_unbound = false;
@@ -84,7 +89,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             tcx.dcx().span_warn(
                 unbound.span,
                 "relaxing a default bound only does something for `?Sized`; \
-                all other traits are not bound by default",
+            all other traits are not bound by default",
             );
         }
 

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

@@ -101,7 +101,7 @@ 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)
@@ -192,7 +192,7 @@ 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, .. }),
@@ -279,7 +282,7 @@ 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

@@ -834,7 +834,7 @@ 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

@@ -638,7 +638,7 @@ 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_analysis/src/errors.rs

@@ -246,13 +246,6 @@ pub struct CopyImplOnTypeWithDtor {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_multiple_relaxed_default_bounds, code = E0203)]
-pub struct MultipleRelaxedDefaultBounds {
-    #[primary_span]
-    pub spans: Vec<Span>,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_copy_impl_on_non_adt, code = E0206)]
 pub struct CopyImplOnNonAdt {

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

@@ -88,7 +88,7 @@ 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

@@ -569,7 +569,7 @@ 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

@@ -127,7 +127,7 @@ impl<'tcx> LateLintPass<'tcx> for NonLocalDefinitions {
                     TyKind::Path(QPath::Resolved(_, ty_path)) => {
                         path_has_local_parent(ty_path, cx, &*local_parents)
                     }
-                    TyKind::TraitObject([principle_poly_trait_ref, ..], _, _) => {
+                    TyKind::TraitObject([(principle_poly_trait_ref, _), ..], _, _) => {
                         path_has_local_parent(
                             principle_poly_trait_ref.trait_ref.path,
                             cx,