Add an (perma-)unstable option to disable vtable vptr

This flag is intended for evaluation of trait upcasting
space cost for embedded use cases.

compiler/rustc_interface/src/tests.rs

@@ -807,6 +807,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(no_jump_tables, true);
     tracked!(no_link, true);
     tracked!(no_profiler_runtime, true);
+    tracked!(no_trait_vptr, true);
     tracked!(no_unique_section_names, true);
     tracked!(oom, OomStrategy::Panic);
     tracked!(osx_rpath_install_name, true);

compiler/rustc_session/src/options.rs

@@ -1631,6 +1631,8 @@ options! {
         "run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"),
     no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED],
         "prevent automatic injection of the profiler_builtins crate"),
+    no_trait_vptr: bool = (false, parse_no_flag, [TRACKED],
+        "disable generation of trait vptr in vtable for upcasting"),
     no_unique_section_names: bool = (false, parse_bool, [TRACKED],
         "do not use unique names for text and data sections when -Z function-sections is used"),
     normalize_docs: bool = (false, parse_bool, [TRACKED],

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -152,7 +152,7 @@ fn prepare_vtable_segments_inner<'tcx, T>(
         while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
             segment_visitor(VtblSegment::TraitOwnEntries {
                 trait_ref: inner_most_trait_ref,
-                emit_vptr,
+                emit_vptr: emit_vptr && !tcx.sess.opts.unstable_opts.no_trait_vptr,
             })?;
 
             // If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,