Rollup merge of rust-lang#120233 - oli-obk:revert_trait_obj_upcast_stabilization, r=lcnr

Revert stabilization of trait_upcasting feature

Reverts rust-lang#118133

This reverts commit 6d2b84b, reversing changes made to 73bc121.

The feature has a soundness bug:

* rust-lang#120222

It is unclear to me whether we'll actually want to destabilize, but I thought it was still prudent to open the PR for easy destabilization once we get there.

Author: matthiaskrgr

core/tests/lib.rs

@@ -113,6 +113,7 @@
 #![feature(slice_flatten)]
 #![feature(error_generic_member_access)]
 #![feature(error_in_core)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 #![feature(utf8_chunks)]
 #![feature(is_ascii_octdigit)]
 #![feature(get_many_mut)]