downgrade mutable-ptr-in-final-value from hard-error to future-incompat lint to address issue 121610.

compiler/rustc_const_eval/src/errors.rs

@@ -25,10 +25,13 @@ pub(crate) struct DanglingPtrInFinal {
     pub kind: InternKind,
 }
 
-#[derive(Diagnostic)]
+#[derive(LintDiagnostic)]
 #[diag(const_eval_mutable_ptr_in_final)]
 pub(crate) struct MutablePtrInFinal {
-    #[primary_span]
+    // rust-lang/rust#122153: This was marked as `#[primary_span]` under
+    // `derive(Diagnostic)`. Since we expect we may hard-error in future, we are
+    // keeping the field (and skipping it under `derive(LintDiagnostic)`).
+    #[skip_arg]
     pub span: Span,
     pub kind: InternKind,
 }

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -23,6 +23,7 @@ use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::sym;
+use rustc_session::lint;
 
 use super::{AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy};
 use crate::const_eval;
@@ -262,10 +263,13 @@ pub fn intern_const_alloc_recursive<
         })?);
     }
     if found_bad_mutable_pointer {
-        return Err(ecx
-            .tcx
-            .dcx()
-            .emit_err(MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind }));
+        let err_diag = MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind };
+        ecx.tcx.emit_node_span_lint(
+            lint::builtin::CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE,
+            ecx.best_lint_scope(),
+            err_diag.span,
+            err_diag,
+        )
     }
 
     Ok(())

compiler/rustc_lint_defs/src/builtin.rs

@@ -30,6 +30,7 @@ declare_lint_pass! {
         CENUM_IMPL_DROP_CAST,
         COHERENCE_LEAK_CHECK,
         CONFLICTING_REPR_HINTS,
+        CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE,
         CONST_EVALUATABLE_UNCHECKED,
         CONST_ITEM_MUTATION,
         DEAD_CODE,
@@ -2796,6 +2797,41 @@ declare_lint! {
     @feature_gate = sym::strict_provenance;
 }
 
+declare_lint! {
+    /// The `const_eval_mutable_ptr_in_final_value` lint detects if a mutable pointer
+    /// has leaked into the final value of a const expression.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// pub enum JsValue {
+    ///     Undefined,
+    ///     Object(std::cell::Cell<bool>),
+    /// }
+    ///
+    /// impl ::std::ops::Drop for JsValue {
+    ///     fn drop(&mut self) {}
+    /// }
+    ///
+    /// const UNDEFINED: &JsValue = &JsValue::Undefined;
+    ///
+    /// fn main() {
+    /// }
+    /// ```
+    ///
+    /// This is a [future-incompatible] lint to ease the transition to an error.
+    /// See [issue #122153] for more details.
+    ///
+    /// [issue #122153]: https://github.com/rust-lang/rust/issues/122153
+    pub CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE,
+    Warn,
+    "detects a mutable pointer that has leaked into final value of a const expression",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #122153 <https://github.com/rust-lang/rust/issues/122153>",
+    };
+}
+
 declare_lint! {
     /// The `const_evaluatable_unchecked` lint detects a generic constant used
     /// in a type.