Auto merge of #115467 - compiler-errors:assoc-ty-object-safety, r=oli-obk

Do not require associated types with Self: Sized to uphold bounds when confirming object candidate

RPITITs and associated types that have `Self: Sized` bounds are opted out of the `dyn Trait` well-formedness check that happens during confirmation. This ensures that we can actually *use* `dyn Trait`s that have associated types that, e.g., have GATs and RPITITs and other naughty things as long as those are opted-out of object safety via a `Self: Sized` bound.

Fixes #115464

This seems like a natural part of https://github.com/rust-lang/rust/pull/112319#issuecomment-1592574451, and I don't think needs re-litigation.

r? `@oli-obk`

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -354,6 +354,12 @@ pub(in crate::solve) fn predicates_for_object_candidate<'tcx>(
         // FIXME(associated_const_equality): Also add associated consts to
         // the requirements here.
         if item.kind == ty::AssocKind::Type {
+            // associated types that require `Self: Sized` do not show up in the built-in
+            // implementation of `Trait for dyn Trait`, and can be dropped here.
+            if tcx.generics_require_sized_self(item.def_id) {
+                continue;
+            }
+
             requirements
                 .extend(tcx.item_bounds(item.def_id).iter_instantiated(tcx, trait_ref.args));
         }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -535,6 +535,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let assoc_types: Vec<_> = tcx
             .associated_items(trait_predicate.def_id())
             .in_definition_order()
+            // Associated types that require `Self: Sized` do not show up in the built-in
+            // implementation of `Trait for dyn Trait`, and can be dropped here.
+            .filter(|item| !tcx.generics_require_sized_self(item.def_id))
             .filter_map(
                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
             )