Avoid explicit trait object lifetimes

Stylistic-only. Increases legibility.
Use explicitly elided lifetimes in impl headers, they get elaborated to fresh lifetime.
Make use of object lifetime defaulting for a trait object type inside a reference type.

Author: fmease

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -22,7 +22,7 @@ use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, Symbol, DUMMY_SP};
 use rustc_trait_selection::traits::object_safety_violations_for_assoc_item;
 
-impl<'o, 'tcx> dyn HirTyLowerer<'tcx> + 'o {
+impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// On missing type parameters, emit an E0393 error and provide a structured suggestion using
     /// the type parameter's name as a placeholder.
     pub(crate) fn complain_about_missing_type_params(

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -8,7 +8,7 @@ use rustc_trait_selection::traits::error_reporting::suggestions::NextTypeParamNa
 
 use super::HirTyLowerer;
 
-impl<'o, 'tcx> dyn HirTyLowerer<'tcx> + 'o {
+impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Make sure that we are in the condition to suggest the blanket implementation.
     pub(super) fn maybe_lint_blanket_trait_impl<G: EmissionGuarantee>(
         &self,

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -241,7 +241,7 @@ pub trait GenericArgsLowerer<'a, 'tcx> {
     ) -> ty::GenericArg<'tcx>;
 }
 
-impl<'o, 'tcx> dyn HirTyLowerer<'tcx> + 'o {
+impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Lower a lifetime from the HIR to our internal notion of a lifetime called a *region*.
     #[instrument(level = "debug", skip(self), ret)]
     pub fn lower_lifetime(
@@ -413,7 +413,7 @@ impl<'o, 'tcx> dyn HirTyLowerer<'tcx> + 'o {
         }
 
         struct GenericArgsCtxt<'a, 'tcx> {
-            lowerer: &'a (dyn HirTyLowerer<'tcx> + 'a),
+            lowerer: &'a dyn HirTyLowerer<'tcx>,
             def_id: DefId,
             generic_args: &'a GenericArgs<'tcx>,
             span: Span,

compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs

@@ -17,7 +17,7 @@ use smallvec::{smallvec, SmallVec};
 
 use super::HirTyLowerer;
 
-impl<'o, 'tcx> dyn HirTyLowerer<'tcx> + 'o {
+impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Lower a trait object type from the HIR to our internal notion of a type.
     #[instrument(level = "debug", skip_all, ret)]
     pub(super) fn lower_trait_object_ty(