Make ptr_cast_add_auto_to_object lint into hard error

In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter which was needed to restabilize trait upcasting. We considered just making this a hard error at the time, but opted against it due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's now make a hard error of this.
rust-lang · workingjubilee · Feb 13, 2025 · Feb 18, 2025 · Feb 20, 2025 · Feb 20, 2025
commit ef337a6599145766fc73b2896c6ff3e0fef65b0b
diff --git a/compiler/rustc_error_codes/src/error_codes/E0804.md b/compiler/rustc_error_codes/src/error_codes/E0804.md
@@ -0,0 +1,41 @@
+An auto trait cannot be added to the bounds of a `dyn Trait` type via
+a pointer cast.
+
+Erroneous code example:
+
+```rust,edition2021,compile_fail,E0804
+let ptr: *const dyn core::any::Any = &();
+_ = ptr as *const (dyn core::any::Any + Send);
+```
+
+Adding an auto trait can make the vtable invalid, potentially causing
+UB in safe code afterwards. For example:
+
+```rust,edition2021,no_run
+use core::{mem::transmute, ptr::NonNull};
+
+trait Trait {
+    fn f(&self)
+    where
+        Self: Send;
+}
+
+impl Trait for NonNull<()> {
+    fn f(&self) {
+        unreachable!()
+    }
+}
+
+fn main() {
+    let unsend: &dyn Trait = &NonNull::dangling();
+    let bad: &(dyn Trait + Send) = unsafe { transmute(unsend) };
+    // This crashes, since the vtable for `NonNull as dyn Trait` does
+    // not have an entry for `Trait::f`.
+    bad.f();
+}
+```
+
+To fix this error, you can use `transmute` rather than pointer casts,
+but you must ensure that the vtable is valid for the pointer's type
+before calling a method on the trait object or allowing other code to
+do so.
diff --git a/compiler/rustc_error_codes/src/lib.rs b/compiler/rustc_error_codes/src/lib.rs
@@ -547,6 +547,7 @@ E0800: 0800,
 E0801: 0801,
 E0802: 0802,
 E0803: 0803,
+E0804: 0804,
         );
     )
 }

diff --git a/compiler/rustc_hir_typeck/messages.ftl b/compiler/rustc_hir_typeck/messages.ftl
@@ -154,10 +154,13 @@ hir_typeck_pass_to_variadic_function = can't pass `{$ty}` to variadic function
     .suggestion = cast the value to `{$cast_ty}`
     .teach_help = certain types, like `{$ty}`, must be casted before passing them to a variadic function, because of arcane ABI rules dictated by the C standard
 
-hir_typeck_ptr_cast_add_auto_to_object = adding {$traits_len ->
-    [1] an auto trait {$traits}
+hir_typeck_ptr_cast_add_auto_to_object = cannot add {$traits_len ->
+    [1] auto trait {$traits}
     *[other] auto traits {$traits}
-} to a trait object in a pointer cast may cause UB later on
+} to dyn bound via pointer cast
+    .note = this could allow UB elsewhere
+    .help = use `transmute` if you're sure this is sound
+    .label = unsupported cast
 
 hir_typeck_remove_semi_for_coerce = you might have meant to return the `match` expression
 hir_typeck_remove_semi_for_coerce_expr = this could be implicitly returned but it is a statement, not a tail expression

diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs
@@ -936,23 +936,19 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             .collect::<Vec<_>>();
 
                         if !added.is_empty() {
-                            tcx.emit_node_span_lint(
-                                lint::builtin::PTR_CAST_ADD_AUTO_TO_OBJECT,
-                                self.expr.hir_id,
-                                self.span,
-                                errors::PtrCastAddAutoToObject {
-                                    traits_len: added.len(),
-                                    traits: {
-                                        let mut traits: Vec<_> = added
-                                            .into_iter()
-                                            .map(|trait_did| tcx.def_path_str(trait_did))
-                                            .collect();
-
-                                        traits.sort();
-                                        traits.into()
-                                    },
+                            tcx.dcx().emit_err(errors::PtrCastAddAutoToObject {
+                                span: self.span,
+                                traits_len: added.len(),
+                                traits: {
+                                    let mut traits: Vec<_> = added
+                                        .into_iter()
+                                        .map(|trait_did| tcx.def_path_str(trait_did))
+                                        .collect();
+
+                                    traits.sort();
+                                    traits.into()
                                 },
-                            )
+                            });
                         }
 
                         Ok(CastKind::PtrPtrCast)

diff --git a/compiler/rustc_hir_typeck/src/errors.rs b/compiler/rustc_hir_typeck/src/errors.rs
@@ -373,9 +373,14 @@ pub(crate) struct LossyProvenanceInt2Ptr<'tcx> {
     pub sugg: LossyProvenanceInt2PtrSuggestion,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(hir_typeck_ptr_cast_add_auto_to_object)]
+#[derive(Diagnostic)]
+#[diag(hir_typeck_ptr_cast_add_auto_to_object, code = E0804)]
+#[note]
+#[help]
 pub(crate) struct PtrCastAddAutoToObject {
+    #[primary_span]
+    #[label]
+    pub span: Span,
     pub traits_len: usize,
     pub traits: DiagSymbolList<String>,
 }

diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
@@ -601,6 +601,11 @@ fn register_builtins(store: &mut LintStore) {
         "converted into hard error, \
          see <https://github.com/rust-lang/rust/issues/73333> for more information",
     );
+    store.register_removed(
+        "ptr_cast_add_auto_to_object",
+        "converted into hard error, see issue #127323 \
+         <https://github.com/rust-lang/rust/issues/127323> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
@@ -80,7 +80,6 @@ declare_lint_pass! {
         PRIVATE_BOUNDS,
         PRIVATE_INTERFACES,
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
-        PTR_CAST_ADD_AUTO_TO_OBJECT,
         PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
         REDUNDANT_IMPORTS,
@@ -4827,58 +4826,6 @@ declare_lint! {
     };
 }
 
-declare_lint! {
-    /// The `ptr_cast_add_auto_to_object` lint detects casts of raw pointers to trait
-    /// objects, which add auto traits.
-    ///
-    /// ### Example
-    ///
-    /// ```rust,edition2021,compile_fail
-    /// let ptr: *const dyn core::any::Any = &();
-    /// _ = ptr as *const dyn core::any::Any + Send;
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Adding an auto trait can make the vtable invalid, potentially causing
-    /// UB in safe code afterwards. For example:
-    ///
-    /// ```ignore (causes a warning)
-    /// #![feature(arbitrary_self_types)]
-    ///
-    /// trait Trait {
-    ///     fn f(self: *const Self)
-    ///     where
-    ///         Self: Send;
-    /// }
-    ///
-    /// impl Trait for *const () {
-    ///     fn f(self: *const Self) {
-    ///         unreachable!()
-    ///     }
-    /// }
-    ///
-    /// fn main() {
-    ///     let unsend: *const () = &();
-    ///     let unsend: *const dyn Trait = &unsend;
-    ///     let send_bad: *const (dyn Trait + Send) = unsend as _;
-    ///     send_bad.f(); // this crashes, since vtable for `*const ()` does not have an entry for `f`
-    /// }
-    /// ```
-    ///
-    /// Generally you must ensure that vtable is right for the pointer's type,
-    /// before passing the pointer to safe code.
-    pub PTR_CAST_ADD_AUTO_TO_OBJECT,
-    Warn,
-    "detects `as` casts from pointers to `dyn Trait` to pointers to `dyn Trait + Auto`",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
-        reference: "issue #127323 <https://github.com/rust-lang/rust/issues/127323>",
-    };
-}
-
 declare_lint! {
     /// The `out_of_scope_macro_calls` lint detects `macro_rules` called when they are not in scope,
     /// above their definition, which may happen in key-value attributes.

diff --git a/tests/ui/cast/ptr-to-trait-obj-add-auto.rs b/tests/ui/cast/ptr-to-trait-obj-add-auto.rs
@@ -1,18 +1,20 @@
-//@ check-pass
-
 trait Trait<'a> {}
 
 fn add_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send) {
     x as _
-    //~^ warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
-    //~| warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~^ ERROR cannot add auto trait `Send` to dyn bound via pointer cast
+    //~| NOTE unsupported cast
+    //~| NOTE this could allow UB elsewhere
+    //~| HELP use `transmute` if you're sure this is sound
 }
 
 // (to test diagnostic list formatting)
 fn add_multiple_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send + Sync + Unpin) {
     x as _
-    //~^ warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
-    //~| warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~^ ERROR cannot add auto traits `Send`, `Sync`, and `Unpin` to dyn bound via pointer cast
+    //~| NOTE unsupported cast
+    //~| NOTE this could allow UB elsewhere
+    //~| HELP use `transmute` if you're sure this is sound
 }
 
 fn main() {}
diff --git a/tests/ui/cast/ptr-to-trait-obj-add-auto.stderr b/tests/ui/cast/ptr-to-trait-obj-add-auto.stderr
@@ -1,43 +1,21 @@
-warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
-  --> $DIR/ptr-to-trait-obj-add-auto.rs:6:5
+error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast
+  --> $DIR/ptr-to-trait-obj-add-auto.rs:4:5
    |
 LL |     x as _
-   |     ^^^^^^
+   |     ^^^^^^ unsupported cast
    |
-   = 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 #127323 <https://github.com/rust-lang/rust/issues/127323>
-   = note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
 
-warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
+error[E0804]: cannot add auto traits `Send`, `Sync`, and `Unpin` to dyn bound via pointer cast
   --> $DIR/ptr-to-trait-obj-add-auto.rs:13:5
    |
 LL |     x as _
-   |     ^^^^^^
+   |     ^^^^^^ unsupported cast
    |
-   = 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 #127323 <https://github.com/rust-lang/rust/issues/127323>
+   = note: this could allow UB elsewhere
+   = help: use `transmute` if you're sure this is sound
 
-warning: 2 warnings emitted
-
-Future incompatibility report: Future breakage diagnostic:
-warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
-  --> $DIR/ptr-to-trait-obj-add-auto.rs:6:5
-   |
-LL |     x as _
-   |     ^^^^^^
-   |
-   = 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 #127323 <https://github.com/rust-lang/rust/issues/127323>
-   = note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
-
-Future breakage diagnostic:
-warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
-  --> $DIR/ptr-to-trait-obj-add-auto.rs:13:5
-   |
-LL |     x as _
-   |     ^^^^^^
-   |
-   = 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 #127323 <https://github.com/rust-lang/rust/issues/127323>
-   = note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0804`.
diff --git a/tests/ui/lint/removed-lints/README.md b/tests/ui/lint/removed-lints/README.md
@@ -0,0 +1,4 @@
+# Removed lints
+
+This directory contains tests to confirm that lints that have been
+removed do not cause errors and produce the appropriate warnings.
diff --git a/tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.rs b/tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.rs
@@ -0,0 +1,5 @@
+//@ check-pass
+
+#![deny(ptr_cast_add_auto_to_object)]
+//~^ WARN  lint `ptr_cast_add_auto_to_object` has been removed
+fn main() {}
diff --git a/tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.stderr b/tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.stderr
@@ -0,0 +1,10 @@
+warning: lint `ptr_cast_add_auto_to_object` has been removed: converted into hard error, see issue #127323 <https://github.com/rust-lang/rust/issues/127323> for more information
+  --> $DIR/ptr_cast_add_auto_to_object.rs:3:9
+   |
+LL | #![deny(ptr_cast_add_auto_to_object)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(renamed_and_removed_lints)]` on by default
+
+warning: 1 warning emitted
+