Small improvements in object lifetime default code

I found those while trying to understand how the code works.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2845,6 +2845,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     /// provided, if they provided one, and otherwise search the supertypes of trait bounds
     /// for region bounds. It may be that we can derive no bound at all, in which case
     /// we return `None`.
+    #[instrument(level = "debug", skip(self, span), ret)]
     fn compute_object_lifetime_bound(
         &self,
         span: Span,
@@ -2853,8 +2854,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     {
         let tcx = self.tcx();
 
-        debug!("compute_opt_region_bound(existential_predicates={:?})", existential_predicates);
-
         // No explicit region bound specified. Therefore, examine trait
         // bounds and see if we can derive region bounds from those.
         let derived_region_bounds = object_region_bounds(tcx, existential_predicates);

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1848,8 +1848,8 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
-        debug!("resolve_object_lifetime_default(lifetime_ref={:?})", lifetime_ref);
         let mut late_depth = 0;
         let mut scope = self.scope;
         let lifetime = loop {

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -913,20 +913,15 @@ pub fn object_region_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
     existential_predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
 ) -> Vec<ty::Region<'tcx>> {
-    // Since we don't actually *know* the self type for an object,
-    // this "open(err)" serves as a kind of dummy standin -- basically
-    // a placeholder type.
-    let open_ty = Ty::new_fresh(tcx, 0);
-
     let predicates = existential_predicates.iter().filter_map(|predicate| {
         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
             None
         } else {
-            Some(predicate.with_self_ty(tcx, open_ty))
+            Some(predicate.with_self_ty(tcx, tcx.types.trait_object_dummy_self))
         }
     });
 
-    required_region_bounds(tcx, open_ty, predicates)
+    required_region_bounds(tcx, tcx.types.trait_object_dummy_self, predicates)
 }
 
 /// Given a set of predicates that apply to an object type, returns