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

Revert stabilization of trait_upcasting feature

Reverts #118133

This reverts commit 6d2b84b3ed7848fd91b8d6151d4451b3103ed816, reversing changes made to 73bc12199ea8c7651ed98b069c0dd6b0bb5fabcf.

The feature has a soundness bug:

* #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

src/lib.rs

@@ -10,7 +10,7 @@
 #![feature(nonzero_ops)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
-#![feature(int_roundings)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,

tests/fail/dyn-upcast-trait-mismatch.rs

@@ -1,3 +1,6 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
 trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
     fn a(&self) -> i32 {
         10

tests/pass/box-custom-alloc.rs

@@ -1,6 +1,7 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-#![feature(allocator_api)]
+#![allow(incomplete_features)] // for trait upcasting
+#![feature(allocator_api, trait_upcasting)]
 
 use std::alloc::Layout;
 use std::alloc::{AllocError, Allocator};

tests/pass/dyn-upcast.rs

@@ -1,3 +1,6 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
 fn main() {
     basic();
     diamond();