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

Revert stabilization of trait_upcasting feature

Reverts #118133

This reverts commit 6d2b84b3ed, reversing changes made to 73bc12199e.

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.

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -5,6 +5,7 @@ use crate::{
 
 use rustc_hir as hir;
 use rustc_middle::ty;
+use rustc_session::lint::FutureIncompatibilityReason;
 use rustc_span::sym;
 use rustc_trait_selection::traits::supertraits;
 
@@ -12,6 +13,9 @@ declare_lint! {
     /// The `deref_into_dyn_supertrait` lint is output whenever there is a use of the
     /// `Deref` implementation with a `dyn SuperTrait` type as `Output`.
     ///
+    /// These implementations will become shadowed when the `trait_upcasting` feature is stabilized.
+    /// The `deref` functions will no longer be called implicitly, so there might be behavior change.
+    ///
     /// ### Example
     ///
     /// ```rust,compile_fail
@@ -40,10 +44,15 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The implicit dyn upcasting coercion take priority over those `Deref` impls.
+    /// The dyn upcasting coercion feature adds new coercion rules, taking priority
+    /// over certain other coercion rules, which will cause some behavior change.
     pub DEREF_INTO_DYN_SUPERTRAIT,
     Warn,
-    "`Deref` implementation usage with a supertrait trait object for output are shadow by implicit coercion",
+    "`Deref` implementation usage with a supertrait trait object for output might be shadowed in the future",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseSemanticsChange,
+        reference: "issue #89460 <https://github.com/rust-lang/rust/issues/89460>",
+    };
 }
 
 declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);

compiler/rustc_lint/src/lib.rs

@@ -35,6 +35,7 @@
 #![feature(iter_intersperse)]
 #![feature(iter_order_by)]
 #![feature(let_chains)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_lint/src/lints.rs

@@ -532,7 +532,6 @@ pub enum BuiltinSpecialModuleNameUsed {
 // deref_into_dyn_supertrait.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_supertrait_as_deref_target)]
-#[help]
 pub struct SupertraitAsDerefTarget<'a> {
     pub self_ty: Ty<'a>,
     pub supertrait_principal: PolyExistentialTraitRef<'a>,