Rollup merge of #129725 - compiler-errors:predicates-of, r=fmease

Stop using `ty::GenericPredicates` for non-predicates_of queries

`GenericPredicates` is a struct of several parts: A list of of an item's own predicates, and a parent def id (and some effects related stuff, but ignore that since it's kinda irrelevant). When instantiating these generic predicates, it calls `predicates_of` on the parent and instantiates its predicates, and appends the item's own instantiated predicates too:

acb4e8b625/compiler/rustc_middle/src/ty/generics.rs (L407-L413)

Notice how this should result in a recursive set of calls to `predicates_of`... However, `GenericPredicates` is *also* misused by a bunch of *other* queries as a convenient way of passing around a list of predicates. For these queries, we don't ever set the parent def id of the `GenericPredicates`, but if we did, then this would be very easy to mistakenly call `predicates_of` instead of some other intended parent query.

Given that footgun, and the fact that we don't ever even *use* the parent def id in the `GenericPredicates` returned from queries like `explicit_super_predicates_of`, It really has no benefit over just returning `&'tcx [(Clause<'tcx>, Span)]`.

This PR additionally opts to wrap the results of `EarlyBinder`, as we've tended to use that in the return type of these kinds of queries to properly convey that the user has params to deal with, and it also gives a convenient way of iterating over a slice of things after instantiating.

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -185,12 +185,11 @@ fn predicates_reference_self(
 ) -> SmallVec<[Span; 1]> {
     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
     let predicates = if supertraits_only {
-        tcx.explicit_super_predicates_of(trait_def_id)
+        tcx.explicit_super_predicates_of(trait_def_id).skip_binder()
     } else {
-        tcx.predicates_of(trait_def_id)
+        tcx.predicates_of(trait_def_id).predicates
     };
     predicates
-        .predicates
         .iter()
         .map(|&(predicate, sp)| (predicate.instantiate_supertrait(tcx, trait_ref), sp))
         .filter_map(|(clause, sp)| {
@@ -266,9 +265,8 @@ fn super_predicates_have_non_lifetime_binders(
         return SmallVec::new();
     }
     tcx.explicit_super_predicates_of(trait_def_id)
-        .predicates
-        .iter()
-        .filter_map(|(pred, span)| pred.has_non_region_bound_vars().then_some(*span))
+        .iter_identity_copied()
+        .filter_map(|(pred, span)| pred.has_non_region_bound_vars().then_some(span))
         .collect()
 }
 

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

@@ -600,21 +600,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // Check supertraits hold. This is so that their associated type bounds
         // will be checked in the code below.
-        for super_trait in tcx
+        for (supertrait, _) in tcx
             .explicit_super_predicates_of(trait_predicate.def_id())
-            .instantiate(tcx, trait_predicate.trait_ref.args)
-            .predicates
-            .into_iter()
+            .iter_instantiated_copied(tcx, trait_predicate.trait_ref.args)
         {
-            let normalized_super_trait = normalize_with_depth_to(
+            let normalized_supertrait = normalize_with_depth_to(
                 self,
                 obligation.param_env,
                 obligation.cause.clone(),
                 obligation.recursion_depth + 1,
-                super_trait,
+                supertrait,
                 &mut nested,
             );
-            nested.push(obligation.with(tcx, normalized_super_trait));
+            nested.push(obligation.with(tcx, normalized_supertrait));
         }
 
         let assoc_types: Vec<_> = tcx

compiler/rustc_trait_selection/src/traits/util.rs

@@ -131,7 +131,7 @@ impl<'tcx> TraitAliasExpander<'tcx> {
         let predicates = tcx.explicit_super_predicates_of(trait_ref.def_id());
         debug!(?predicates);
 
-        let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
+        let items = predicates.skip_binder().iter().rev().filter_map(|(pred, span)| {
             pred.instantiate_supertrait(tcx, trait_ref)
                 .as_trait_clause()
                 .map(|trait_ref| item.clone_and_push(trait_ref.map_bound(|t| t.trait_ref), *span))

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -120,8 +120,7 @@ fn prepare_vtable_segments_inner<'tcx, T>(
 
             let mut direct_super_traits_iter = tcx
                 .explicit_super_predicates_of(inner_most_trait_ref.def_id())
-                .predicates
-                .into_iter()
+                .iter_identity_copied()
                 .filter_map(move |(pred, _)| {
                     pred.instantiate_supertrait(tcx, inner_most_trait_ref).as_trait_clause()
                 });