interpret: err instead of ICE on size mismatches in to_bits_or_ptr_internal

RalfJung · RalfJung · commit 38004b72bcfc · 2022-04-07T16:24:48.000-04:00
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -167,17 +167,18 @@ pub(super) fn op_to_const<'tcx>(
             },
             Immediate::ScalarPair(a, b) => {
                 // We know `offset` is relative to the allocation, so we can use `into_parts`.
-                let (data, start) = match ecx.scalar_to_ptr(a.check_init().unwrap()).into_parts() {
-                    (Some(alloc_id), offset) => {
-                        (ecx.tcx.global_alloc(alloc_id).unwrap_memory(), offset.bytes())
-                    }
-                    (None, _offset) => (
-                        ecx.tcx.intern_const_alloc(Allocation::from_bytes_byte_aligned_immutable(
-                            b"" as &[u8],
-                        )),
-                        0,
-                    ),
-                };
+                let (data, start) =
+                    match ecx.scalar_to_ptr(a.check_init().unwrap()).unwrap().into_parts() {
+                        (Some(alloc_id), offset) => {
+                            (ecx.tcx.global_alloc(alloc_id).unwrap_memory(), offset.bytes())
+                        }
+                        (None, _offset) => (
+                            ecx.tcx.intern_const_alloc(
+                                Allocation::from_bytes_byte_aligned_immutable(b"" as &[u8]),
+                            ),
+                            0,
+                        ),
+                    };
                 let len = b.to_machine_usize(ecx).unwrap();
                 let start = start.try_into().unwrap();
                 let len: usize = len.try_into().unwrap();
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -197,8 +197,8 @@ impl interpret::MayLeak for ! {
 }
 
 impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
-    fn guaranteed_eq(&mut self, a: Scalar, b: Scalar) -> bool {
-        match (a, b) {
+    fn guaranteed_eq(&mut self, a: Scalar, b: Scalar) -> InterpResult<'tcx, bool> {
+        Ok(match (a, b) {
             // Comparisons between integers are always known.
             (Scalar::Int { .. }, Scalar::Int { .. }) => a == b,
             // Equality with integers can never be known for sure.
@@ -207,25 +207,25 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
             // some things (like functions and vtables) do not have stable addresses
             // so we need to be careful around them (see e.g. #73722).
             (Scalar::Ptr(..), Scalar::Ptr(..)) => false,
-        }
+        })
     }
 
-    fn guaranteed_ne(&mut self, a: Scalar, b: Scalar) -> bool {
-        match (a, b) {
+    fn guaranteed_ne(&mut self, a: Scalar, b: Scalar) -> InterpResult<'tcx, bool> {
+        Ok(match (a, b) {
             // Comparisons between integers are always known.
             (Scalar::Int(_), Scalar::Int(_)) => a != b,
             // Comparisons of abstract pointers with null pointers are known if the pointer
             // is in bounds, because if they are in bounds, the pointer can't be null.
             // Inequality with integers other than null can never be known for sure.
             (Scalar::Int(int), ptr @ Scalar::Ptr(..))
             | (ptr @ Scalar::Ptr(..), Scalar::Int(int)) => {
-                int.is_null() && !self.scalar_may_be_null(ptr)
+                int.is_null() && !self.scalar_may_be_null(ptr)?
             }
             // FIXME: return `true` for at least some comparisons where we can reliably
             // determine the result of runtime inequality tests at compile-time.
             // Examples include comparison of addresses in different static items.
             (Scalar::Ptr(..), Scalar::Ptr(..)) => false,
-        }
+        })
     }
 }
 
@@ -329,9 +329,9 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 let a = ecx.read_immediate(&args[0])?.to_scalar()?;
                 let b = ecx.read_immediate(&args[1])?.to_scalar()?;
                 let cmp = if intrinsic_name == sym::ptr_guaranteed_eq {
-                    ecx.guaranteed_eq(a, b)
+                    ecx.guaranteed_eq(a, b)?
                 } else {
-                    ecx.guaranteed_ne(a, b)
+                    ecx.guaranteed_ne(a, b)?
                 };
                 ecx.write_scalar(Scalar::from_bool(cmp), dest)?;
             }
diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -283,7 +283,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 if let Some(entry_idx) = vptr_entry_idx {
                     let entry_idx = u64::try_from(entry_idx).unwrap();
                     let (old_data, old_vptr) = val.to_scalar_pair()?;
-                    let old_vptr = self.scalar_to_ptr(old_vptr);
+                    let old_vptr = self.scalar_to_ptr(old_vptr)?;
                     let new_vptr = self
                         .read_new_vtable_after_trait_upcasting_from_vtable(old_vptr, entry_idx)?;
                     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
@@ -640,7 +640,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 Ok(Some((size, align)))
             }
             ty::Dynamic(..) => {
-                let vtable = self.scalar_to_ptr(metadata.unwrap_meta());
+                let vtable = self.scalar_to_ptr(metadata.unwrap_meta())?;
                 // Read size and align from vtable (already checks size).
                 Ok(Some(self.read_size_and_align_from_vtable(vtable)?))
             }
diff --git a/compiler/rustc_const_eval/src/interpret/intern.rs b/compiler/rustc_const_eval/src/interpret/intern.rs
@@ -202,7 +202,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: CompileTimeMachine<'mir, 'tcx, const_eval::Memory
             if let ty::Dynamic(..) =
                 tcx.struct_tail_erasing_lifetimes(referenced_ty, self.ecx.param_env).kind()
             {
-                let ptr = self.ecx.scalar_to_ptr(mplace.meta.unwrap_meta());
+                let ptr = self.ecx.scalar_to_ptr(mplace.meta.unwrap_meta())?;
                 if let Some(alloc_id) = ptr.provenance {
                     // Explicitly choose const mode here, since vtables are immutable, even
                     // if the reference of the fat pointer is mutable.
diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -1102,30 +1102,38 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
 /// Machine pointer introspection.
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
-    pub fn scalar_to_ptr(&self, scalar: Scalar<M::PointerTag>) -> Pointer<Option<M::PointerTag>> {
+    pub fn scalar_to_ptr(
+        &self,
+        scalar: Scalar<M::PointerTag>,
+    ) -> InterpResult<'tcx, Pointer<Option<M::PointerTag>>> {
         // We use `to_bits_or_ptr_internal` since we are just implementing the method people need to
         // call to force getting out a pointer.
-        match scalar.to_bits_or_ptr_internal(self.pointer_size()) {
-            Err(ptr) => ptr.into(),
-            Ok(bits) => {
-                let addr = u64::try_from(bits).unwrap();
-                let ptr = M::ptr_from_addr(&self, addr);
-                if addr == 0 {
-                    assert!(ptr.provenance.is_none(), "null pointer can never have an AllocId");
+        Ok(
+            match scalar
+                .to_bits_or_ptr_internal(self.pointer_size())
+                .map_err(|s| err_ub!(ScalarSizeMismatch(s)))?
+            {
+                Err(ptr) => ptr.into(),
+                Ok(bits) => {
+                    let addr = u64::try_from(bits).unwrap();
+                    let ptr = M::ptr_from_addr(&self, addr);
+                    if addr == 0 {
+                        assert!(ptr.provenance.is_none(), "null pointer can never have an AllocId");
+                    }
+                    ptr
                 }
-                ptr
-            }
-        }
+            },
+        )
     }
 
     /// Test if this value might be null.
     /// If the machine does not support ptr-to-int casts, this is conservative.
-    pub fn scalar_may_be_null(&self, scalar: Scalar<M::PointerTag>) -> bool {
-        match scalar.try_to_int() {
+    pub fn scalar_may_be_null(&self, scalar: Scalar<M::PointerTag>) -> InterpResult<'tcx, bool> {
+        Ok(match scalar.try_to_int() {
             Ok(int) => int.is_null(),
             Err(_) => {
                 // Can only happen during CTFE.
-                let ptr = self.scalar_to_ptr(scalar);
+                let ptr = self.scalar_to_ptr(scalar)?;
                 match self.ptr_try_get_alloc_id(ptr) {
                     Ok((alloc_id, offset, _)) => {
                         let (size, _align) = self
@@ -1138,7 +1146,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     Err(_offset) => bug!("a non-int scalar is always a pointer"),
                 }
             }
-        }
+        })
     }
 
     /// Turning a "maybe pointer" into a proper pointer (and some information
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -342,7 +342,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         &self,
         op: &OpTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx, Pointer<Option<M::PointerTag>>> {
-        Ok(self.scalar_to_ptr(self.read_scalar(op)?.check_init()?))
+        self.scalar_to_ptr(self.read_scalar(op)?.check_init()?)
     }
 
     // Turn the wide MPlace into a string (must already be dereferenced!)
@@ -738,7 +738,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         // okay. Everything else, we conservatively reject.
                         let ptr_valid = niche_start == 0
                             && variants_start == variants_end
-                            && !self.scalar_may_be_null(tag_val);
+                            && !self.scalar_may_be_null(tag_val)?;
                         if !ptr_valid {
                             throw_ub!(InvalidTag(dbg_val))
                         }
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -281,7 +281,7 @@ where
         };
 
         let mplace = MemPlace {
-            ptr: self.scalar_to_ptr(ptr.check_init()?),
+            ptr: self.scalar_to_ptr(ptr.check_init()?)?,
             // We could use the run-time alignment here. For now, we do not, because
             // the point of tracking the alignment here is to make sure that the *static*
             // alignment information emitted with the loads is correct. The run-time
@@ -1104,7 +1104,7 @@ where
         &self,
         mplace: &MPlaceTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx, (ty::Instance<'tcx>, MPlaceTy<'tcx, M::PointerTag>)> {
-        let vtable = self.scalar_to_ptr(mplace.vtable()); // also sanity checks the type
+        let vtable = self.scalar_to_ptr(mplace.vtable())?; // also sanity checks the type
         let (instance, ty) = self.read_drop_type_from_vtable(vtable)?;
         let layout = self.layout_of(ty)?;
 
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -519,7 +519,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         .kind(),
                     ty::Dynamic(..)
                 ));
-                let vtable = self.scalar_to_ptr(receiver_place.meta.unwrap_meta());
+                let vtable = self.scalar_to_ptr(receiver_place.meta.unwrap_meta())?;
                 let fn_val = self.get_vtable_slot(vtable, u64::try_from(idx).unwrap())?;
 
                 // `*mut receiver_place.layout.ty` is almost the layout that we
diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -50,7 +50,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let vtable_slot = self
             .get_ptr_alloc(vtable_slot, ptr_size, self.tcx.data_layout.pointer_align.abi)?
             .expect("cannot be a ZST");
-        let fn_ptr = self.scalar_to_ptr(vtable_slot.read_ptr_sized(Size::ZERO)?.check_init()?);
+        let fn_ptr = self.scalar_to_ptr(vtable_slot.read_ptr_sized(Size::ZERO)?.check_init()?)?;
         self.get_ptr_fn(fn_ptr)
     }
 
@@ -75,7 +75,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             .check_init()?;
         // We *need* an instance here, no other kind of function value, to be able
         // to determine the type.
-        let drop_instance = self.get_ptr_fn(self.scalar_to_ptr(drop_fn))?.as_instance()?;
+        let drop_instance = self.get_ptr_fn(self.scalar_to_ptr(drop_fn)?)?.as_instance()?;
         trace!("Found drop fn: {:?}", drop_instance);
         let fn_sig = drop_instance.ty(*self.tcx, self.param_env).fn_sig(*self.tcx);
         let fn_sig = self.tcx.normalize_erasing_late_bound_regions(self.param_env, fn_sig);
@@ -132,7 +132,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             .get_ptr_alloc(vtable_slot, pointer_size, self.tcx.data_layout.pointer_align.abi)?
             .expect("cannot be a ZST");
 
-        let new_vtable = self.scalar_to_ptr(new_vtable.read_ptr_sized(Size::ZERO)?.check_init()?);
+        let new_vtable =
+            self.scalar_to_ptr(new_vtable.read_ptr_sized(Size::ZERO)?.check_init()?)?;
 
         Ok(new_vtable)
     }
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -312,7 +312,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         let tail = self.ecx.tcx.struct_tail_erasing_lifetimes(pointee.ty, self.ecx.param_env);
         match tail.kind() {
             ty::Dynamic(..) => {
-                let vtable = self.ecx.scalar_to_ptr(meta.unwrap_meta());
+                let vtable = self.ecx.scalar_to_ptr(meta.unwrap_meta())?;
                 // Direct call to `check_ptr_access_align` checks alignment even on CTFE machines.
                 try_validation!(
                     self.ecx.check_ptr_access_align(
@@ -577,7 +577,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
 
                 // If we check references recursively, also check that this points to a function.
                 if let Some(_) = self.ref_tracking {
-                    let ptr = self.ecx.scalar_to_ptr(value);
+                    let ptr = self.ecx.scalar_to_ptr(value)?;
                     let _fn = try_validation!(
                         self.ecx.get_ptr_fn(ptr),
                         self.path,
@@ -590,7 +590,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                     // FIXME: Check if the signature matches
                 } else {
                     // Otherwise (for standalone Miri), we have to still check it to be non-null.
-                    if self.ecx.scalar_may_be_null(value) {
+                    if self.ecx.scalar_may_be_null(value)? {
                         throw_validation_failure!(self.path, { "a null function pointer" });
                     }
                 }
@@ -667,7 +667,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // We support 2 kinds of ranges here: full range, and excluding zero.
                 if start == 1 && end == max_value {
                     // Only null is the niche.  So make sure the ptr is NOT null.
-                    if self.ecx.scalar_may_be_null(value) {
+                    if self.ecx.scalar_may_be_null(value)? {
                         throw_validation_failure!(self.path,
                             { "a potentially null pointer" }
                             expected {
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -15,8 +15,8 @@ use rustc_target::abi::{Align, HasDataLayout, Size};
 
 use super::{
     read_target_uint, write_target_uint, AllocId, InterpError, InterpResult, Pointer, Provenance,
-    ResourceExhaustionInfo, Scalar, ScalarMaybeUninit, UndefinedBehaviorInfo, UninitBytesAccess,
-    UnsupportedOpInfo,
+    ResourceExhaustionInfo, Scalar, ScalarMaybeUninit, ScalarSizeMismatch, UndefinedBehaviorInfo,
+    UninitBytesAccess, UnsupportedOpInfo,
 };
 use crate::ty;
 
@@ -81,6 +81,8 @@ impl<'tcx, Tag, Extra> ConstAllocation<'tcx, Tag, Extra> {
 /// is added when converting to `InterpError`.
 #[derive(Debug)]
 pub enum AllocError {
+    /// A scalar had the wrong size.
+    ScalarSizeMismatch(ScalarSizeMismatch),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
     /// Partially overwriting a pointer.
@@ -90,10 +92,19 @@ pub enum AllocError {
 }
 pub type AllocResult<T = ()> = Result<T, AllocError>;
 
+impl From<ScalarSizeMismatch> for AllocError {
+    fn from(s: ScalarSizeMismatch) -> Self {
+        AllocError::ScalarSizeMismatch(s)
+    }
+}
+
 impl AllocError {
     pub fn to_interp_error<'tcx>(self, alloc_id: AllocId) -> InterpError<'tcx> {
         use AllocError::*;
         match self {
+            ScalarSizeMismatch(s) => {
+                InterpError::UndefinedBehavior(UndefinedBehaviorInfo::ScalarSizeMismatch(s))
+            }
             ReadPointerAsBytes => InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes),
             PartialPointerOverwrite(offset) => InterpError::Unsupported(
                 UnsupportedOpInfo::PartialPointerOverwrite(Pointer::new(alloc_id, offset)),
@@ -425,7 +436,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
 
         // `to_bits_or_ptr_internal` is the right method because we just want to store this data
         // as-is into memory.
-        let (bytes, provenance) = match val.to_bits_or_ptr_internal(range.size) {
+        let (bytes, provenance) = match val.to_bits_or_ptr_internal(range.size)? {
             Err(val) => {
                 let (provenance, offset) = val.into_parts();
                 (u128::from(offset.bytes()), Some(provenance))
diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -221,6 +221,13 @@ pub struct UninitBytesAccess {
     pub uninit_size: Size,
 }
 
+/// Information about a size mismatch.
+#[derive(Debug)]
+pub struct ScalarSizeMismatch {
+    pub target_size: u64,
+    pub data_size: u64,
+}
+
 /// Error information for when the program caused Undefined Behavior.
 pub enum UndefinedBehaviorInfo<'tcx> {
     /// Free-form case. Only for errors that are never caught!
@@ -298,10 +305,7 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     /// Working with a local that is not currently live.
     DeadLocal,
     /// Data size is not equal to target size.
-    ScalarSizeMismatch {
-        target_size: u64,
-        data_size: u64,
-    },
+    ScalarSizeMismatch(ScalarSizeMismatch),
     /// A discriminant of an uninhabited enum variant is written.
     UninhabitedEnumVariantWritten,
 }
@@ -408,7 +412,7 @@ impl fmt::Display for UndefinedBehaviorInfo<'_> {
                 "using uninitialized data, but this operation requires initialized memory"
             ),
             DeadLocal => write!(f, "accessing a dead local variable"),
-            ScalarSizeMismatch { target_size, data_size } => write!(
+            ScalarSizeMismatch(self::ScalarSizeMismatch { target_size, data_size }) => write!(
                 f,
                 "scalar size mismatch: expected {} bytes but got {} bytes instead",
                 target_size, data_size
diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -120,7 +120,8 @@ use crate::ty::{self, Instance, Ty, TyCtxt};
 pub use self::error::{
     struct_error, CheckInAllocMsg, ErrorHandled, EvalToAllocationRawResult, EvalToConstValueResult,
     InterpError, InterpErrorInfo, InterpResult, InvalidProgramInfo, MachineStopType,
-    ResourceExhaustionInfo, UndefinedBehaviorInfo, UninitBytesAccess, UnsupportedOpInfo,
+    ResourceExhaustionInfo, ScalarSizeMismatch, UndefinedBehaviorInfo, UninitBytesAccess,
+    UnsupportedOpInfo,
 };
 
 pub use self::value::{get_slice_bytes, ConstAlloc, ConstValue, Scalar, ScalarMaybeUninit};
diff --git a/compiler/rustc_middle/src/mir/interpret/value.rs b/compiler/rustc_middle/src/mir/interpret/value.rs
diff --git a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

Original file line number	Diff line number	Diff line change
`@@ -640,7 +640,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {`
`640`	`640`	`Ok(Some((size, align)))`
`641`	`641`	`}`
`642`	`642`	`ty::Dynamic(..) => {`
`643`		`- let vtable = self.scalar_to_ptr(metadata.unwrap_meta());`
	`643`	`+ let vtable = self.scalar_to_ptr(metadata.unwrap_meta())?;`
`644`	`644`	`// Read size and align from vtable (already checks size).`
`645`	`645`	`Ok(Some(self.read_size_and_align_from_vtable(vtable)?))`
`646`	`646`	`}`
Original file line number	Diff line number	Diff line change
`@@ -202,7 +202,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: CompileTimeMachine<'mir, 'tcx, const_eval::Memory`
`202`	`202`	`if let ty::Dynamic(..) =`
`203`	`203`	`tcx.struct_tail_erasing_lifetimes(referenced_ty, self.ecx.param_env).kind()`
`204`	`204`	`{`
`205`		`- let ptr = self.ecx.scalar_to_ptr(mplace.meta.unwrap_meta());`
	`205`	`+ let ptr = self.ecx.scalar_to_ptr(mplace.meta.unwrap_meta())?;`
`206`	`206`	`if let Some(alloc_id) = ptr.provenance {`
`207`	`207`	`// Explicitly choose const mode here, since vtables are immutable, even`
`208`	`208`	`// if the reference of the fat pointer is mutable.`