regression test from 121610.

tests/ui/consts/future-incompat-mutable-in-final-value-issue-121610.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+use std::cell::Cell;
+
+pub enum JsValue {
+    Undefined,
+    Object(Cell<bool>),
+}
+
+impl ::std::ops::Drop for JsValue {
+    fn drop(&mut self) {}
+}
+
+const UNDEFINED: &JsValue = &JsValue::Undefined;
+    //~^ WARN encountered mutable pointer in final value of constant
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+fn main() {
+}

tests/ui/consts/future-incompat-mutable-in-final-value-issue-121610.stderr

@@ -0,0 +1,23 @@
+warning: encountered mutable pointer in final value of constant
+  --> $DIR/future-incompat-mutable-in-final-value-issue-121610.rs:13:1
+   |
+LL | const UNDEFINED: &JsValue = &JsValue::Undefined;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #122153 <https://github.com/rust-lang/rust/issues/122153>
+   = note: `#[warn(const_eval_mutable_ptr_in_final_value)]` on by default
+
+warning: 1 warning emitted
+
+Future incompatibility report: Future breakage diagnostic:
+warning: encountered mutable pointer in final value of constant
+  --> $DIR/future-incompat-mutable-in-final-value-issue-121610.rs:13:1
+   |
+LL | const UNDEFINED: &JsValue = &JsValue::Undefined;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #122153 <https://github.com/rust-lang/rust/issues/122153>
+   = note: `#[warn(const_eval_mutable_ptr_in_final_value)]` on by default
+