Auto merge of #121676 - Bryanskiy:polarity, r=petrochenkov

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

This patch allows `maybe` polarity bounds under a feature gate. The only language change here is that corresponding hard errors are replaced by feature gates. Example:
```rust
#![feature(allow_maybe_polarity)]
...
trait Trait1 : ?Trait { ... } // ok
fn foo(_: Box<(dyn Trait2 + ?Trait)>) {} // ok
fn bar<T: ?Sized + ?Trait>(_: &T) {} // ok
```
Maybe bounds still don't do anything (except for `Sized` trait), however this patch will allow us to [experiment with default auto traits](https://github.com/rust-lang/rust/pull/120706#issuecomment-1934006762).

This is a part of the [MCP: Low level components for async drop](https://github.com/rust-lang/compiler-team/issues/727)

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,
         },

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -652,7 +652,7 @@ 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/hir_ty_lowering/bounds.rs

@@ -8,7 +8,7 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::bug;
 use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TyCtxt};
 use rustc_span::symbol::Ident;
-use rustc_span::{ErrorGuaranteed, Span, Symbol};
+use rustc_span::{sym, ErrorGuaranteed, Span, Symbol};
 use rustc_trait_selection::traits;
 use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
 use smallvec::SmallVec;
@@ -73,10 +73,22 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
         }
 
+        let mut unique_bounds = FxIndexSet::default();
+        let mut seen_repeat = false;
+        for unbound in &unbounds {
+            if let Res::Def(DefKind::Trait, unbound_def_id) = unbound.trait_ref.path.res {
+                seen_repeat |= !unique_bounds.insert(unbound_def_id);
+            }
+        }
         if unbounds.len() > 1 {
-            self.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
+            let err = errors::MultipleRelaxedDefaultBounds {
                 spans: unbounds.iter().map(|ptr| ptr.span).collect(),
-            });
+            };
+            if seen_repeat {
+                self.dcx().emit_err(err);
+            } else if !tcx.features().more_maybe_bounds {
+                self.tcx().sess.create_feature_err(err, sym::more_maybe_bounds).emit();
+            };
         }
 
         let mut seen_sized_unbound = false;

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -719,7 +719,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         associated_types: FxIndexMap<Span, FxIndexSet<DefId>>,
         potential_assoc_types: Vec<usize>,
-        trait_bounds: &[hir::PolyTraitRef<'_>],
+        trait_bounds: &[(hir::PolyTraitRef<'_>, hir::TraitBoundModifier)],
     ) {
         if associated_types.values().all(|v| v.is_empty()) {
             return;
@@ -765,12 +765,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // related to issue #91997, turbofishes added only when in an expr or pat
         let mut in_expr_or_pat = false;
         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
-            let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.trait_ref.hir_ref_id));
+            let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.0.trait_ref.hir_ref_id));
             in_expr_or_pat = match grandparent {
                 hir::Node::Expr(_) | hir::Node::Pat(_) => true,
                 _ => false,
             };
-            match bound.trait_ref.path.segments {
+            match bound.0.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
                 // around that bug here, even though it should be fixed elsewhere.
@@ -811,7 +811,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // 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/hir_ty_lowering/lint.rs

@@ -34,7 +34,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 .ok()
                 .is_some_and(|s| s.trim_end().ends_with('<'));
 
-        let is_global = poly_trait_ref.trait_ref.path.is_global();
+        let is_global = poly_trait_ref.0.trait_ref.path.is_global();
 
         let mut sugg = vec![(
             self_ty.span.shrink_to_lo(),
@@ -176,7 +176,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         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)

compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs

@@ -27,7 +27,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &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,
@@ -37,7 +37,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         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, .. }),
@@ -249,7 +252,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 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)
                 });