Downgrade const eval dangling ptr in final to future incompat lint

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

@@ -21,6 +21,7 @@ use rustc_hir as hir;
 use rustc_middle::mir::interpret::{ConstAllocation, CtfeProvenance, InterpResult};
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::TyAndLayout;
+use rustc_session::lint;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::sym;
 
@@ -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 +2797,51 @@ 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() {
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In the 1.77 release, the const evaluation machinery adopted some
+    /// stricter rules to reject expressions with values that could
+    /// end up holding mutable references to state stored in static memory
+    /// (which is inherently immutable).
+    ///
+    /// 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
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    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.

tests/ui/consts/const-eval/heap/alloc_intrinsic_untyped.rs

@@ -1,9 +1,11 @@
 #![feature(core_intrinsics)]
 #![feature(const_heap)]
 #![feature(const_mut_refs)]
+#![deny(const_eval_mutable_ptr_in_final_value)]
 use std::intrinsics;
 
 const BAR: *mut i32 = unsafe { intrinsics::const_allocate(4, 4) as *mut i32 };
 //~^ error: mutable pointer in final value of constant
+//~| WARNING this was previously accepted by the compiler
 
 fn main() {}

tests/ui/consts/const-eval/heap/alloc_intrinsic_untyped.stderr

@@ -1,8 +1,31 @@
 error: encountered mutable pointer in final value of constant
-  --> $DIR/alloc_intrinsic_untyped.rs:6:1
+  --> $DIR/alloc_intrinsic_untyped.rs:7:1
    |
 LL | const BAR: *mut i32 = unsafe { intrinsics::const_allocate(4, 4) as *mut i32 };
    | ^^^^^^^^^^^^^^^^^^^
+   |
+   = 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: the lint level is defined here
+  --> $DIR/alloc_intrinsic_untyped.rs:4:9
+   |
+LL | #![deny(const_eval_mutable_ptr_in_final_value)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
+Future incompatibility report: Future breakage diagnostic:
+error: encountered mutable pointer in final value of constant
+  --> $DIR/alloc_intrinsic_untyped.rs:7:1
+   |
+LL | const BAR: *mut i32 = unsafe { intrinsics::const_allocate(4, 4) as *mut i32 };
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = 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: the lint level is defined here
+  --> $DIR/alloc_intrinsic_untyped.rs:4:9
+   |
+LL | #![deny(const_eval_mutable_ptr_in_final_value)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+

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
+

tests/ui/consts/miri_unleashed/mutable_references.rs

@@ -1,29 +1,38 @@
 //@ compile-flags: -Zunleash-the-miri-inside-of-you
+//@ normalize-stderr-test "(the raw bytes of the constant) \(size: [0-9]*, align: [0-9]*\)" -> "$1 (size: $$SIZE, align: $$ALIGN)"
+//@ normalize-stderr-test "([0-9a-f][0-9a-f] |╾─*ALLOC[0-9]+(\+[a-z0-9]+)?(<imm>)?─*╼ )+ *│.*" -> "HEX_DUMP"
 
+#![deny(const_eval_mutable_ptr_in_final_value)]
 use std::cell::UnsafeCell;
 
 // a test demonstrating what things we could allow with a smarter const qualification
 
 static FOO: &&mut u32 = &&mut 42;
 //~^ ERROR encountered mutable pointer in final value of static
+//~| WARNING this was previously accepted by the compiler
 
 static BAR: &mut () = &mut ();
 //~^ ERROR encountered mutable pointer in final value of static
+//~| WARNING this was previously accepted by the compiler
 
 struct Foo<T>(T);
 
 static BOO: &mut Foo<()> = &mut Foo(());
 //~^ ERROR encountered mutable pointer in final value of static
+//~| WARNING this was previously accepted by the compiler
 
 struct Meh {
     x: &'static UnsafeCell<i32>,
 }
 unsafe impl Sync for Meh {}
 static MEH: Meh = Meh { x: &UnsafeCell::new(42) };
 //~^ ERROR encountered mutable pointer in final value of static
+//~| WARNING this was previously accepted by the compiler
 
 static OH_YES: &mut i32 = &mut 42;
 //~^ ERROR encountered mutable pointer in final value of static
+//~| WARNING this was previously accepted by the compiler
+//~| ERROR it is undefined behavior to use this value
 
 fn main() {
     unsafe {