Rollup merge of #126232 - RalfJung:dyn-trait-equality, r=oli-obk

interpret: dyn trait metadata check: equate traits in a proper way

Hopefully fixes https://github.com/rust-lang/miri/issues/3541... unfortunately we don't have a testcase.

The first commit is just a refactor without functional change.

r? `@oli-obk`

9 files changed, 129 insertions, 116 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index 0a45bbb3edb..6961e13c239 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -387,7 +387,6 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         match (&src_pointee_ty.kind(), &dest_pointee_ty.kind()) {
             (&ty::Array(_, length), &ty::Slice(_)) => {
                 let ptr = self.read_pointer(src)?;
-                // u64 cast is from usize to u64, which is always good
                 let val = Immediate::new_slice(
                     ptr,
                     length.eval_target_usize(*self.tcx, self.param_env),
@@ -405,13 +404,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let (old_data, old_vptr) = val.to_scalar_pair();
                 let old_data = old_data.to_pointer(self)?;
                 let old_vptr = old_vptr.to_pointer(self)?;
-                let (ty, old_trait) = self.get_ptr_vtable(old_vptr)?;
-                if old_trait != data_a.principal() {
-                    throw_ub!(InvalidVTableTrait {
-                        expected_trait: data_a,
-                        vtable_trait: old_trait,
-                    });
-                }
+                let ty = self.get_ptr_vtable_ty(old_vptr, Some(data_a))?;
                 let new_vptr = self.get_vtable_ptr(ty, data_b.principal())?;
                 self.write_immediate(Immediate::new_dyn_trait(old_data, new_vptr, self), dest)
             }
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 7c2100fcbe3..e28cc05cc2a 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -765,10 +765,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 }
                 Ok(Some((full_size, full_align)))
             }
-            ty::Dynamic(_, _, ty::Dyn) => {
+            ty::Dynamic(expected_trait, _, ty::Dyn) => {
                 let vtable = metadata.unwrap_meta().to_pointer(self)?;
                 // Read size and align from vtable (already checks size).
-                Ok(Some(self.get_vtable_size_and_align(vtable)?))
+                Ok(Some(self.get_vtable_size_and_align(vtable, Some(expected_trait))?))
             }
 
             ty::Slice(_) | ty::Str => {
diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index 7f16d441606..1d54da267ee 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -432,12 +432,14 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
             sym::vtable_size => {
                 let ptr = self.read_pointer(&args[0])?;
-                let (size, _align) = self.get_vtable_size_and_align(ptr)?;
+                // `None` because we don't know which trait to expect here; any vtable is okay.
+                let (size, _align) = self.get_vtable_size_and_align(ptr, None)?;
                 self.write_scalar(Scalar::from_target_usize(size.bytes(), self), dest)?;
             }
             sym::vtable_align => {
                 let ptr = self.read_pointer(&args[0])?;
-                let (_size, align) = self.get_vtable_size_and_align(ptr)?;
+                // `None` because we don't know which trait to expect here; any vtable is okay.
+                let (_size, align) = self.get_vtable_size_and_align(ptr, None)?;
                 self.write_scalar(Scalar::from_target_usize(align.bytes(), self), dest)?;
             }
 
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 7eb73e9b52f..5461e9c6ad3 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -867,19 +867,26 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             .ok_or_else(|| err_ub!(InvalidFunctionPointer(Pointer::new(alloc_id, offset))).into())
     }
 
-    pub fn get_ptr_vtable(
+    /// Get the dynamic type of the given vtable pointer.
+    /// If `expected_trait` is `Some`, it must be a vtable for the given trait.
+    pub fn get_ptr_vtable_ty(
         &self,
         ptr: Pointer<Option<M::Provenance>>,
-    ) -> InterpResult<'tcx, (Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>)> {
+        expected_trait: Option<&'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>>,
+    ) -> InterpResult<'tcx, Ty<'tcx>> {
         trace!("get_ptr_vtable({:?})", ptr);
         let (alloc_id, offset, _tag) = self.ptr_get_alloc_id(ptr)?;
         if offset.bytes() != 0 {
             throw_ub!(InvalidVTablePointer(Pointer::new(alloc_id, offset)))
         }
-        match self.tcx.try_get_global_alloc(alloc_id) {
-            Some(GlobalAlloc::VTable(ty, trait_ref)) => Ok((ty, trait_ref)),
-            _ => throw_ub!(InvalidVTablePointer(Pointer::new(alloc_id, offset))),
+        let Some(GlobalAlloc::VTable(ty, vtable_trait)) = self.tcx.try_get_global_alloc(alloc_id)
+        else {
+            throw_ub!(InvalidVTablePointer(Pointer::new(alloc_id, offset)))
+        };
+        if let Some(expected_trait) = expected_trait {
+            self.check_vtable_for_type(vtable_trait, expected_trait)?;
         }
+        Ok(ty)
     }
 
     pub fn alloc_mark_immutable(&mut self, id: AllocId) -> InterpResult<'tcx> {
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 046ff34e3d0..08d3165867c 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -9,7 +9,6 @@ use tracing::{instrument, trace};
 
 use rustc_ast::Mutability;
 use rustc_middle::mir;
-use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::ty::Ty;
 use rustc_middle::{bug, span_bug};
@@ -1018,54 +1017,6 @@ where
         let layout = self.layout_of(raw.ty)?;
         Ok(self.ptr_to_mplace(ptr.into(), layout))
     }
-
-    /// Turn a place with a `dyn Trait` type into a place with the actual dynamic type.
-    /// Aso returns the vtable.
-    pub(super) fn unpack_dyn_trait(
-        &self,
-        mplace: &MPlaceTy<'tcx, M::Provenance>,
-        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
-    ) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, Pointer<Option<M::Provenance>>)> {
-        assert!(
-            matches!(mplace.layout.ty.kind(), ty::Dynamic(_, _, ty::Dyn)),
-            "`unpack_dyn_trait` only makes sense on `dyn*` types"
-        );
-        let vtable = mplace.meta().unwrap_meta().to_pointer(self)?;
-        let (ty, vtable_trait) = self.get_ptr_vtable(vtable)?;
-        if expected_trait.principal() != vtable_trait {
-            throw_ub!(InvalidVTableTrait { expected_trait, vtable_trait });
-        }
-        // This is a kind of transmute, from a place with unsized type and metadata to
-        // a place with sized type and no metadata.
-        let layout = self.layout_of(ty)?;
-        let mplace =
-            MPlaceTy { mplace: MemPlace { meta: MemPlaceMeta::None, ..mplace.mplace }, layout };
-        Ok((mplace, vtable))
-    }
-
-    /// Turn a `dyn* Trait` type into an value with the actual dynamic type.
-    /// Also returns the vtable.
-    pub(super) fn unpack_dyn_star<P: Projectable<'tcx, M::Provenance>>(
-        &self,
-        val: &P,
-        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
-    ) -> InterpResult<'tcx, (P, Pointer<Option<M::Provenance>>)> {
-        assert!(
-            matches!(val.layout().ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
-            "`unpack_dyn_star` only makes sense on `dyn*` types"
-        );
-        let data = self.project_field(val, 0)?;
-        let vtable = self.project_field(val, 1)?;
-        let vtable = self.read_pointer(&vtable.to_op(self)?)?;
-        let (ty, vtable_trait) = self.get_ptr_vtable(vtable)?;
-        if expected_trait.principal() != vtable_trait {
-            throw_ub!(InvalidVTableTrait { expected_trait, vtable_trait });
-        }
-        // `data` is already the right thing but has the wrong type. So we transmute it.
-        let layout = self.layout_of(ty)?;
-        let data = data.transmute(layout, self)?;
-        Ok((data, vtable))
-    }
 }
 
 // Some nodes are used a lot. Make sure they don't unintentionally get bigger.
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index cbfe25ca8df..8f76a148679 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -1,6 +1,7 @@
 use std::borrow::Cow;
 
 use either::Either;
+use rustc_middle::ty::TyCtxt;
 use tracing::trace;
 
 use rustc_middle::span_bug;
@@ -827,20 +828,19 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 };
 
                 // Obtain the underlying trait we are working on, and the adjusted receiver argument.
-                let (vptr, dyn_ty, adjusted_receiver) = if let ty::Dynamic(data, _, ty::DynStar) =
+                let (dyn_trait, dyn_ty, adjusted_recv) = if let ty::Dynamic(data, _, ty::DynStar) =
                     receiver_place.layout.ty.kind()
                 {
-                    let (recv, vptr) = self.unpack_dyn_star(&receiver_place, data)?;
-                    let (dyn_ty, _dyn_trait) = self.get_ptr_vtable(vptr)?;
+                    let recv = self.unpack_dyn_star(&receiver_place, data)?;
 
-                    (vptr, dyn_ty, recv.ptr())
+                    (data.principal(), recv.layout.ty, recv.ptr())
                 } else {
                     // Doesn't have to be a `dyn Trait`, but the unsized tail must be `dyn Trait`.
                     // (For that reason we also cannot use `unpack_dyn_trait`.)
                     let receiver_tail = self
                         .tcx
                         .struct_tail_erasing_lifetimes(receiver_place.layout.ty, self.param_env);
-                    let ty::Dynamic(data, _, ty::Dyn) = receiver_tail.kind() else {
+                    let ty::Dynamic(receiver_trait, _, ty::Dyn) = receiver_tail.kind() else {
                         span_bug!(
                             self.cur_span(),
                             "dynamic call on non-`dyn` type {}",
@@ -851,25 +851,24 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
                     // Get the required information from the vtable.
                     let vptr = receiver_place.meta().unwrap_meta().to_pointer(self)?;
-                    let (dyn_ty, dyn_trait) = self.get_ptr_vtable(vptr)?;
-                    if dyn_trait != data.principal() {
-                        throw_ub!(InvalidVTableTrait {
-                            expected_trait: data,
-                            vtable_trait: dyn_trait,
-                        });
-                    }
+                    let dyn_ty = self.get_ptr_vtable_ty(vptr, Some(receiver_trait))?;
 
                     // It might be surprising that we use a pointer as the receiver even if this
                     // is a by-val case; this works because by-val passing of an unsized `dyn
                     // Trait` to a function is actually desugared to a pointer.
-                    (vptr, dyn_ty, receiver_place.ptr())
+                    (receiver_trait.principal(), dyn_ty, receiver_place.ptr())
                 };
 
                 // Now determine the actual method to call. We can do that in two different ways and
                 // compare them to ensure everything fits.
-                let Some(ty::VtblEntry::Method(fn_inst)) =
-                    self.get_vtable_entries(vptr)?.get(idx).copied()
-                else {
+                let vtable_entries = if let Some(dyn_trait) = dyn_trait {
+                    let trait_ref = dyn_trait.with_self_ty(*self.tcx, dyn_ty);
+                    let trait_ref = self.tcx.erase_regions(trait_ref);
+                    self.tcx.vtable_entries(trait_ref)
+                } else {
+                    TyCtxt::COMMON_VTABLE_ENTRIES
+                };
+                let Some(ty::VtblEntry::Method(fn_inst)) = vtable_entries.get(idx).copied() else {
                     // FIXME(fee1-dead) these could be variants of the UB info enum instead of this
                     throw_ub_custom!(fluent::const_eval_dyn_call_not_a_method);
                 };
@@ -898,7 +897,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let receiver_ty = Ty::new_mut_ptr(self.tcx.tcx, dyn_ty);
                 args[0] = FnArg::Copy(
                     ImmTy::from_immediate(
-                        Scalar::from_maybe_pointer(adjusted_receiver, self).into(),
+                        Scalar::from_maybe_pointer(adjusted_recv, self).into(),
                         self.layout_of(receiver_ty)?,
                     )
                     .into(),
@@ -974,11 +973,11 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         let place = match place.layout.ty.kind() {
             ty::Dynamic(data, _, ty::Dyn) => {
                 // Dropping a trait object. Need to find actual drop fn.
-                self.unpack_dyn_trait(&place, data)?.0
+                self.unpack_dyn_trait(&place, data)?
             }
             ty::Dynamic(data, _, ty::DynStar) => {
                 // Dropping a `dyn*`. Need to find actual drop fn.
-                self.unpack_dyn_star(&place, data)?.0
+                self.unpack_dyn_star(&place, data)?
             }
             _ => {
                 debug_assert_eq!(
diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
index 244a6ba48a4..bd2c6519421 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -1,11 +1,14 @@
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::mir::interpret::{InterpResult, Pointer};
 use rustc_middle::ty::layout::LayoutOf;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, Ty};
 use rustc_target::abi::{Align, Size};
+use rustc_trait_selection::traits::ObligationCtxt;
 use tracing::trace;
 
 use super::util::ensure_monomorphic_enough;
-use super::{InterpCx, Machine};
+use super::{throw_ub, InterpCx, MPlaceTy, Machine, MemPlaceMeta, OffsetMode, Projectable};
 
 impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     /// Creates a dynamic vtable for the given type and vtable origin. This is used only for
@@ -33,28 +36,90 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         Ok(vtable_ptr.into())
     }
 
-    /// Returns a high-level representation of the entries of the given vtable.
-    pub fn get_vtable_entries(
-        &self,
-        vtable: Pointer<Option<M::Provenance>>,
-    ) -> InterpResult<'tcx, &'tcx [ty::VtblEntry<'tcx>]> {
-        let (ty, poly_trait_ref) = self.get_ptr_vtable(vtable)?;
-        Ok(if let Some(poly_trait_ref) = poly_trait_ref {
-            let trait_ref = poly_trait_ref.with_self_ty(*self.tcx, ty);
-            let trait_ref = self.tcx.erase_regions(trait_ref);
-            self.tcx.vtable_entries(trait_ref)
-        } else {
-            TyCtxt::COMMON_VTABLE_ENTRIES
-        })
-    }
-
     pub fn get_vtable_size_and_align(
         &self,
         vtable: Pointer<Option<M::Provenance>>,
+        expected_trait: Option<&'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>>,
     ) -> InterpResult<'tcx, (Size, Align)> {
-        let (ty, _trait_ref) = self.get_ptr_vtable(vtable)?;
+        let ty = self.get_ptr_vtable_ty(vtable, expected_trait)?;
         let layout = self.layout_of(ty)?;
         assert!(layout.is_sized(), "there are no vtables for unsized types");
         Ok((layout.size, layout.align.abi))
     }
+
+    /// Check that the given vtable trait is valid for a pointer/reference/place with the given
+    /// expected trait type.
+    pub(super) fn check_vtable_for_type(
+        &self,
+        vtable_trait: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx> {
+        // Fast path: if they are equal, it's all fine.
+        if expected_trait.principal() == vtable_trait {
+            return Ok(());
+        }
+        if let (Some(expected_trait), Some(vtable_trait)) =
+            (expected_trait.principal(), vtable_trait)
+        {
+            // Slow path: spin up an inference context to check if these traits are sufficiently equal.
+            let infcx = self.tcx.infer_ctxt().build();
+            let ocx = ObligationCtxt::new(&infcx);
+            let cause = ObligationCause::dummy_with_span(self.cur_span());
+            // equate the two trait refs after normalization
+            let expected_trait = ocx.normalize(&cause, self.param_env, expected_trait);
+            let vtable_trait = ocx.normalize(&cause, self.param_env, vtable_trait);
+            if ocx.eq(&cause, self.param_env, expected_trait, vtable_trait).is_ok() {
+                if ocx.select_all_or_error().is_empty() {
+                    // All good.
+                    return Ok(());
+                }
+            }
+        }
+        throw_ub!(InvalidVTableTrait { expected_trait, vtable_trait });
+    }
+
+    /// Turn a place with a `dyn Trait` type into a place with the actual dynamic type.
+    pub(super) fn unpack_dyn_trait(
+        &self,
+        mplace: &MPlaceTy<'tcx, M::Provenance>,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
+        assert!(
+            matches!(mplace.layout.ty.kind(), ty::Dynamic(_, _, ty::Dyn)),
+            "`unpack_dyn_trait` only makes sense on `dyn*` types"
+        );
+        let vtable = mplace.meta().unwrap_meta().to_pointer(self)?;
+        let ty = self.get_ptr_vtable_ty(vtable, Some(expected_trait))?;
+        // This is a kind of transmute, from a place with unsized type and metadata to
+        // a place with sized type and no metadata.
+        let layout = self.layout_of(ty)?;
+        let mplace = mplace.offset_with_meta(
+            Size::ZERO,
+            OffsetMode::Wrapping,
+            MemPlaceMeta::None,
+            layout,
+            self,
+        )?;
+        Ok(mplace)
+    }
+
+    /// Turn a `dyn* Trait` type into an value with the actual dynamic type.
+    pub(super) fn unpack_dyn_star<P: Projectable<'tcx, M::Provenance>>(
+        &self,
+        val: &P,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx, P> {
+        assert!(
+            matches!(val.layout().ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
+            "`unpack_dyn_star` only makes sense on `dyn*` types"
+        );
+        let data = self.project_field(val, 0)?;
+        let vtable = self.project_field(val, 1)?;
+        let vtable = self.read_pointer(&vtable.to_op(self)?)?;
+        let ty = self.get_ptr_vtable_ty(vtable, Some(expected_trait))?;
+        // `data` is already the right thing but has the wrong type. So we transmute it.
+        let layout = self.layout_of(ty)?;
+        let data = data.transmute(layout, self)?;
+        Ok(data)
+    }
 }
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index f532f6bbe37..6f75bc2af4e 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -343,20 +343,16 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
         match tail.kind() {
             ty::Dynamic(data, _, ty::Dyn) => {
                 let vtable = meta.unwrap_meta().to_pointer(self.ecx)?;
-                // Make sure it is a genuine vtable pointer.
-                let (_dyn_ty, dyn_trait) = try_validation!(
-                    self.ecx.get_ptr_vtable(vtable),
+                // Make sure it is a genuine vtable pointer for the right trait.
+                try_validation!(
+                    self.ecx.get_ptr_vtable_ty(vtable, Some(data)),
                     self.path,
                     Ub(DanglingIntPointer(..) | InvalidVTablePointer(..)) =>
-                        InvalidVTablePtr { value: format!("{vtable}") }
+                        InvalidVTablePtr { value: format!("{vtable}") },
+                    Ub(InvalidVTableTrait { expected_trait, vtable_trait }) => {
+                        InvalidMetaWrongTrait { expected_trait, vtable_trait: *vtable_trait }
+                    },
                 );
-                // Make sure it is for the right trait.
-                if dyn_trait != data.principal() {
-                    throw_validation_failure!(
-                        self.path,
-                        InvalidMetaWrongTrait { expected_trait: data, vtable_trait: dyn_trait }
-                    );
-                }
             }
             ty::Slice(..) | ty::Str => {
                 let _len = meta.unwrap_meta().to_target_usize(self.ecx)?;
diff --git a/compiler/rustc_const_eval/src/interpret/visitor.rs b/compiler/rustc_const_eval/src/interpret/visitor.rs
index b812e89854b..71c057e549b 100644
--- a/compiler/rustc_const_eval/src/interpret/visitor.rs
+++ b/compiler/rustc_const_eval/src/interpret/visitor.rs
@@ -95,7 +95,7 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 // unsized values are never immediate, so we can assert_mem_place
                 let op = v.to_op(self.ecx())?;
                 let dest = op.assert_mem_place();
-                let inner_mplace = self.ecx().unpack_dyn_trait(&dest, data)?.0;
+                let inner_mplace = self.ecx().unpack_dyn_trait(&dest, data)?;
                 trace!("walk_value: dyn object layout: {:#?}", inner_mplace.layout);
                 // recurse with the inner type
                 return self.visit_field(v, 0, &inner_mplace.into());
@@ -104,7 +104,7 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 // DynStar types. Very different from a dyn type (but strangely part of the
                 // same variant in `TyKind`): These are pairs where the 2nd component is the
                 // vtable, and the first component is the data (which must be ptr-sized).
-                let data = self.ecx().unpack_dyn_star(v, data)?.0;
+                let data = self.ecx().unpack_dyn_star(v, data)?;
                 return self.visit_field(v, 0, &data);
             }
             // Slices do not need special handling here: they have `Array` field