5 files changed, 7 insertions, 42 deletions
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index ffec28a395f..8061e0b83c0 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -10,8 +10,8 @@
 )]
 #![feature(box_syntax)]
 #![feature(box_patterns)]
+// On bootstrap bump, this will likely have to become const_fn_unsize
 #![cfg_attr(bootstrap, feature(const_fn))] // For the `transmute` in `P::new`
-#![cfg_attr(not(bootstrap), feature(const_fn_unsize))] // For the `transmute` in `P::new`
 #![feature(const_fn_transmute)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index eef71e096a5..945406aed4b 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -283,6 +283,8 @@ declare_features! (
     (accepted, non_ascii_idents, "1.53.0", Some(55467), None),
     /// Allows arbitrary expressions in key-value attributes at parse time.
     (accepted, extended_key_value_attributes, "1.54.0", Some(78835), None),
+    /// Allows unsizing coercions in `const fn`.
+    (accepted, const_fn_unsize, "1.54.0", Some(64992), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 2cef46a844a..62d81304134 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -644,9 +644,6 @@ declare_features! (
     /// Allows trait bounds in `const fn`.
     (active, const_fn_trait_bound, "1.53.0", Some(57563), None),
 
-    /// Allows unsizing coercions in `const fn`.
-    (active, const_fn_unsize, "1.53.0", Some(64992), None),
-
     /// Allows `async {}` expressions in const contexts.
     (active, const_async_blocks, "1.53.0", Some(85368), None),
 
diff --git a/compiler/rustc_mir/src/transform/check_consts/ops.rs b/compiler/rustc_mir/src/transform/check_consts/ops.rs
index 16f5f14f563..dcbc9c523dc 100644
--- a/compiler/rustc_mir/src/transform/check_consts/ops.rs
+++ b/compiler/rustc_mir/src/transform/check_consts/ops.rs
@@ -541,30 +541,6 @@ impl NonConstOp for UnionAccess {
     }
 }
 
-/// See [#64992].
-///
-/// [#64992]: https://github.com/rust-lang/rust/issues/64992
-#[derive(Debug)]
-pub struct UnsizingCast;
-impl NonConstOp for UnsizingCast {
-    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-        if ccx.const_kind() != hir::ConstContext::ConstFn {
-            Status::Allowed
-        } else {
-            Status::Unstable(sym::const_fn_unsize)
-        }
-    }
-
-    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        feature_err(
-            &ccx.tcx.sess.parse_sess,
-            sym::const_fn_unsize,
-            span,
-            "unsizing casts to types besides slices are not allowed in const fn",
-        )
-    }
-}
-
 // Types that cannot appear in the signature or locals of a `const fn`.
 pub mod ty {
     use super::*;
diff --git a/compiler/rustc_mir/src/transform/check_consts/validation.rs b/compiler/rustc_mir/src/transform/check_consts/validation.rs
index 63fc66f2b9f..41d9d0d04b5 100644
--- a/compiler/rustc_mir/src/transform/check_consts/validation.rs
+++ b/compiler/rustc_mir/src/transform/check_consts/validation.rs
@@ -10,9 +10,7 @@ use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceC
 use rustc_middle::mir::*;
 use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::subst::GenericArgKind;
-use rustc_middle::ty::{
-    self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt, TypeAndMut,
-};
+use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
 use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
 use rustc_span::{sym, Span, Symbol};
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
@@ -636,17 +634,9 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 _,
             ) => self.check_op(ops::FnPtrCast),
 
-            Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, cast_ty) => {
-                if let Some(TypeAndMut { ty, .. }) = cast_ty.builtin_deref(true) {
-                    let unsized_ty = self.tcx.struct_tail_erasing_lifetimes(ty, self.param_env);
-
-                    // Casting/coercing things to slices is fine.
-                    if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
-                        return;
-                    }
-                }
-
-                self.check_op(ops::UnsizingCast);
+            Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, _) => {
+                // Nothing to check here (`check_local_or_return_ty` ensures no trait objects occur
+                // in the type of any local, which also excludes casts).
             }
 
             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {