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)

src/librustdoc/clean/mod.rs

@@ -1858,7 +1858,7 @@ 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

@@ -543,7 +543,7 @@ 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

@@ -183,7 +183,7 @@ 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;
                 };
@@ -198,9 +198,9 @@ 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);
                 }
 

src/tools/clippy/clippy_lints/src/types/borrowed_box.rs

@@ -81,14 +81,17 @@ pub(super) fn check(cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>, lt: &Lifetime, m
 
 // Returns true if given type is `Any` trait.
 fn is_any_trait(cx: &LateContext<'_>, t: &hir::Ty<'_>) -> bool {
-    if let TyKind::TraitObject(traits, ..) = t.kind
-        && !traits.is_empty()
-        && let Some(trait_did) = traits[0].trait_ref.trait_def_id()
-        // Only Send/Sync can be used as additional traits, so it is enough to
-        // check only the first trait.
-        && cx.tcx.is_diagnostic_item(sym::Any, trait_did)
-    {
-        return true;
+    if let TyKind::TraitObject(traits, ..) = t.kind {
+        return traits
+            .iter()
+            .any(|(bound, _)| {
+                if let Some(trait_did) = bound.trait_ref.trait_def_id()
+                    && cx.tcx.is_diagnostic_item(sym::Any, trait_did)
+                {
+                    return true;
+                }
+                false
+            });
     }
 
     false

src/tools/clippy/clippy_lints/src/types/type_complexity.rs

@@ -55,6 +55,7 @@ impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
             TyKind::TraitObject(param_bounds, _, _) => {
                 let has_lifetime_parameters = param_bounds.iter().any(|bound| {
                     bound
+                        .0
                         .bound_generic_params
                         .iter()
                         .any(|param| matches!(param.kind, GenericParamKind::Lifetime { .. }))