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_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs

@@ -84,7 +84,7 @@ 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_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs

@@ -607,7 +607,7 @@ 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_trait_selection/src/error_reporting/traits/mod.rs

@@ -437,7 +437,7 @@ pub fn report_object_safety_error<'tcx>(
         if tcx.parent_hir_node(hir_id).fn_sig().is_some() {
             // Do not suggest `impl Trait` when dealing with things like super-traits.
             err.span_suggestion_verbose(
-                ty.span.until(trait_ref.span),
+                ty.span.until(trait_ref.0.span),
                 "consider using an opaque type instead",
                 "impl ",
                 Applicability::MaybeIncorrect,

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

@@ -3040,11 +3040,11 @@ impl<'a, 'tcx> TypeErrCtxt<'a, '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;