Rollup merge of #108319 - compiler-errors:dont-project-to-specializable-rpitits, r=lcnr

Don't project specializable RPITIT projection

This effective rejects specialization + RPITIT/AFIT (usages of `impl Trait` in traits) because the implementation is significantly complicated over making regular "default" trait method bodies work.

I have another PR that experimentally fixes all this, but the code may not be worth investing in.

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -648,6 +648,13 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
             tcx.fn_sig(trait_m.def_id).subst(tcx, trait_to_placeholder_substs),
         )
         .fold_with(&mut collector);
+
+    debug_assert_ne!(
+        collector.types.len(),
+        0,
+        "expect >1 RPITITs in call to `collect_return_position_impl_trait_in_trait_tys`"
+    );
+
     let trait_sig = ocx.normalize(&norm_cause, param_env, unnormalized_trait_sig);
     trait_sig.error_reported()?;
     let trait_return_ty = trait_sig.output();