message: slightly cleaner, if more verbose, vtable handling in codegen backends
diff: @@ -195,11 +195,6 @@ pub(crate) fn codegen_const_value<'tcx>(
  }
  Scalar::Ptr(ptr, _size) => {
  let (alloc_id, offset) = ptr.into_parts(); // we know the `offset` is relative
  - // For vtables, get the underlying data allocation.
  - let alloc_id = match fx.tcx.global_alloc(alloc_id) {
  - GlobalAlloc::VTable(ty, trait_ref) => fx.tcx.vtable_allocation((ty, trait_ref)),
  - _ => alloc_id,
  - };
  let base_addr = match fx.tcx.global_alloc(alloc_id) {
  GlobalAlloc::Memory(alloc) => {
  let data_id = data_id_for_alloc_id(
  @@ -221,7 +216,20 @@ pub(crate) fn codegen_const_value<'tcx>(
  fx.module.declare_func_in_func(func_id, &mut fx.bcx.func);
  fx.bcx.ins().func_addr(fx.pointer_type, local_func_id)
  }
  - GlobalAlloc::VTable(..) => bug!("vtables are already handled"),
  + GlobalAlloc::VTable(ty, trait_ref) => {
  + let alloc_id = fx.tcx.vtable_allocation((ty, trait_ref));
  + let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
  + // FIXME: factor this common code with the `Memory` arm into a function?
  + let data_id = data_id_for_alloc_id(
  + &mut fx.constants_cx,
  + fx.module,
  + alloc_id,
  + alloc.inner().mutability,
  + );
  + let local_data_id =
  + fx.module.declare_data_in_func(data_id, &mut fx.bcx.func);
  + fx.bcx.ins().global_value(fx.pointer_type, local_data_id)
  + }
  GlobalAlloc::Static(def_id) => {
  assert!(fx.tcx.is_static(def_id));
  let data_id = data_id_for_static(fx.tcx, fx.module, def_id, false);
  