1 //! Implements "Stacked Borrows". See <https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md>
2 //! for further information.
4 use std::cell::RefCell;
6 use std::num::NonZeroU64;
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
12 use rustc_hir::Mutability;
13 use rustc_middle::mir::RetagKind;
15 use rustc_target::abi::{Align, LayoutOf, Size};
19 pub type PtrId = NonZeroU64;
20 pub type CallId = NonZeroU64;
21 pub type AllocExtra = Stacks;
23 /// Tracking pointer provenance
24 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
30 impl fmt::Debug for Tag {
31 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
33 Tag::Tagged(id) => write!(f, "<{}>", id),
34 Tag::Untagged => write!(f, "<untagged>"),
39 /// Indicates which permission is granted (by this item to some pointers)
40 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
42 /// Grants unique mutable access.
44 /// Grants shared mutable access.
46 /// Grants shared read-only access.
48 /// Grants no access, but separates two groups of SharedReadWrite so they are not
49 /// all considered mutually compatible.
53 /// An item in the per-location borrow stack.
54 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
56 /// The permission this item grants.
58 /// The pointers the permission is granted to.
60 /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
61 protector: Option<CallId>,
64 impl fmt::Debug for Item {
65 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
66 write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
67 if let Some(call) = self.protector {
68 write!(f, " (call {})", call)?;
75 /// Extra per-location state.
76 #[derive(Clone, Debug, PartialEq, Eq)]
78 /// Used *mostly* as a stack; never empty.
80 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
81 /// * Except for `Untagged`, no tag occurs in the stack more than once.
85 /// Extra per-allocation state.
86 #[derive(Clone, Debug)]
88 // Even reading memory can have effects on the stack, so we need a `RefCell` here.
89 stacks: RefCell<RangeMap<Stack>>,
90 // Pointer to global state
94 /// Extra global state, available to the memory access hooks.
96 pub struct GlobalState {
97 /// Next unused pointer ID (tag).
99 /// Table storing the "base" tag for each allocation.
100 /// The base tag is the one used for the initial pointer.
101 /// We need this in a separate table to handle cyclic statics.
102 base_ptr_ids: FxHashMap<AllocId, Tag>,
103 /// Next unused call ID (for protectors).
104 next_call_id: CallId,
105 /// Those call IDs corresponding to functions that are still running.
106 active_calls: FxHashSet<CallId>,
107 /// The pointer id to trace
108 tracked_pointer_tag: Option<PtrId>,
109 /// The call id to trace
110 tracked_call_id: Option<CallId>,
111 /// Whether to track raw pointers.
114 /// Memory extra state gives us interior mutable access to the global state.
115 pub type MemoryExtra = Rc<RefCell<GlobalState>>;
117 /// Indicates which kind of access is being performed.
118 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
119 pub enum AccessKind {
124 impl fmt::Display for AccessKind {
125 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
127 AccessKind::Read => write!(f, "read access"),
128 AccessKind::Write => write!(f, "write access"),
133 /// Indicates which kind of reference is being created.
134 /// Used by high-level `reborrow` to compute which permissions to grant to the
136 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
138 /// `&mut` and `Box`.
139 Unique { two_phase: bool },
140 /// `&` with or without interior mutability.
142 /// `*mut`/`*const` (raw pointers).
143 Raw { mutable: bool },
146 impl fmt::Display for RefKind {
147 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
149 RefKind::Unique { two_phase: false } => write!(f, "unique"),
150 RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
151 RefKind::Shared => write!(f, "shared"),
152 RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
153 RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
158 /// Utilities for initialization and ID generation
161 tracked_pointer_tag: Option<PtrId>,
162 tracked_call_id: Option<CallId>,
166 next_ptr_id: NonZeroU64::new(1).unwrap(),
167 base_ptr_ids: FxHashMap::default(),
168 next_call_id: NonZeroU64::new(1).unwrap(),
169 active_calls: FxHashSet::default(),
176 fn new_ptr(&mut self) -> PtrId {
177 let id = self.next_ptr_id;
178 if Some(id) == self.tracked_pointer_tag {
179 register_diagnostic(NonHaltingDiagnostic::CreatedPointerTag(id));
181 self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
185 pub fn new_call(&mut self) -> CallId {
186 let id = self.next_call_id;
187 trace!("new_call: Assigning ID {}", id);
188 if Some(id) == self.tracked_call_id {
189 register_diagnostic(NonHaltingDiagnostic::CreatedCallId(id));
191 assert!(self.active_calls.insert(id));
192 self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
196 pub fn end_call(&mut self, id: CallId) {
197 assert!(self.active_calls.remove(&id));
200 fn is_active(&self, id: CallId) -> bool {
201 self.active_calls.contains(&id)
204 pub fn global_base_ptr(&mut self, id: AllocId) -> Tag {
205 self.base_ptr_ids.get(&id).copied().unwrap_or_else(|| {
206 let tag = Tag::Tagged(self.new_ptr());
207 trace!("New allocation {:?} has base tag {:?}", id, tag);
208 self.base_ptr_ids.try_insert(id, tag).unwrap();
215 fn err_sb_ub(msg: String) -> InterpError<'static> {
216 err_machine_stop!(TerminationInfo::ExperimentalUb {
219 "https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md"
224 // # Stacked Borrows Core Begin
226 /// We need to make at least the following things true:
228 /// U1: After creating a `Uniq`, it is at the top.
229 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
230 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
232 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
233 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
234 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
236 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
237 /// F3: If an access happens with an `&` outside `UnsafeCell`,
238 /// it requires the `SharedReadOnly` to still be in the stack.
240 /// Core relation on `Permission` to define which accesses are allowed
242 /// This defines for a given permission, whether it permits the given kind of access.
243 fn grants(self, access: AccessKind) -> bool {
244 // Disabled grants nothing. Otherwise, all items grant read access, and except for SharedReadOnly they grant write access.
245 self != Permission::Disabled
246 && (access == AccessKind::Read || self != Permission::SharedReadOnly)
250 /// Core per-location operations: access, dealloc, reborrow.
252 /// Find the item granting the given kind of access to the given tag, and return where
253 /// it is on the stack.
254 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
257 .enumerate() // we also need to know *where* in the stack
258 .rev() // search top-to-bottom
259 // Return permission of first item that grants access.
260 // We require a permission with the right tag, ensuring U3 and F3.
263 if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
268 /// Find the first write-incompatible item above the given one --
269 /// i.e, find the height to which the stack will be truncated when writing to `granting`.
270 fn find_first_write_incompatible(&self, granting: usize) -> usize {
271 let perm = self.borrows[granting].perm;
273 Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
274 Permission::Disabled => bug!("Cannot use Disabled for anything"),
275 // On a write, everything above us is incompatible.
276 Permission::Unique => granting + 1,
277 Permission::SharedReadWrite => {
278 // The SharedReadWrite *just* above us are compatible, to skip those.
279 let mut idx = granting + 1;
280 while let Some(item) = self.borrows.get(idx) {
281 if item.perm == Permission::SharedReadWrite {
285 // Found first incompatible!
294 /// Check if the given item is protected.
295 fn check_protector(item: &Item, tag: Option<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
296 if let Tag::Tagged(id) = item.tag {
297 if Some(id) == global.tracked_pointer_tag {
298 register_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(item.clone()));
301 if let Some(call) = item.protector {
302 if global.is_active(call) {
303 if let Some(tag) = tag {
304 Err(err_sb_ub(format!(
305 "not granting access to tag {:?} because incompatible item is protected: {:?}",
309 Err(err_sb_ub(format!("deallocating while item is protected: {:?}", item)))?
316 /// Test if a memory `access` using pointer tagged `tag` is granted.
317 /// If yes, return the index of the item that granted it.
322 global: &GlobalState,
323 ) -> InterpResult<'tcx> {
324 // Two main steps: Find granting item, remove incompatible items above.
326 // Step 1: Find granting item.
327 let granting_idx = self.find_granting(access, ptr.tag).ok_or_else(|| {
329 "no item granting {} to tag {:?} at {} found in borrow stack.",
336 // Step 2: Remove incompatible items above them. Make sure we do not remove protected
337 // items. Behavior differs for reads and writes.
338 if access == AccessKind::Write {
339 // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
340 // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
341 let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
342 for item in self.borrows.drain(first_incompatible_idx..).rev() {
343 trace!("access: popping item {:?}", item);
344 Stack::check_protector(&item, Some(ptr.tag), global)?;
347 // On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
348 // The reason this is not following the stack discipline (by removing the first Unique and
349 // everything on top of it) is that in `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
350 // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
351 // `SharedReadWrite` for `raw`.
352 // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
353 // reference and use that.
354 // We *disable* instead of removing `Unique` to avoid "connecting" two neighbouring blocks of SRWs.
355 for idx in ((granting_idx + 1)..self.borrows.len()).rev() {
356 let item = &mut self.borrows[idx];
357 if item.perm == Permission::Unique {
358 trace!("access: disabling item {:?}", item);
359 Stack::check_protector(item, Some(ptr.tag), global)?;
360 item.perm = Permission::Disabled;
369 /// Deallocate a location: Like a write access, but also there must be no
370 /// active protectors at all because we will remove all items.
371 fn dealloc(&mut self, ptr: Pointer<Tag>, global: &GlobalState) -> InterpResult<'tcx> {
372 // Step 1: Find granting item.
373 self.find_granting(AccessKind::Write, ptr.tag).ok_or_else(|| {
375 "no item granting write access for deallocation to tag {:?} at {} found in borrow stack",
376 ptr.tag, ptr.erase_tag(),
380 // Step 2: Remove all items. Also checks for protectors.
381 for item in self.borrows.drain(..).rev() {
382 Stack::check_protector(&item, None, global)?;
388 /// Derive a new pointer from one with the given tag.
389 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
390 /// an access, and they add the new item directly on top of the one it is derived
391 /// from instead of all the way at the top of the stack.
394 derived_from: Pointer<Tag>,
396 global: &GlobalState,
397 ) -> InterpResult<'tcx> {
398 // Figure out which access `perm` corresponds to.
400 if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
401 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
402 // We use that to determine where to put the new item.
403 let granting_idx = self.find_granting(access, derived_from.tag)
404 .ok_or_else(|| err_sb_ub(format!(
405 "trying to reborrow for {:?} at {}, but parent tag {:?} does not have an appropriate item in the borrow stack",
406 new.perm, derived_from.erase_tag(), derived_from.tag,
409 // Compute where to put the new item.
410 // Either way, we ensure that we insert the new item in a way such that between
411 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
412 let new_idx = if new.perm == Permission::SharedReadWrite {
414 access == AccessKind::Write,
415 "this case only makes sense for stack-like accesses"
417 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
418 // access. Instead of popping the stack, we insert the item at the place the stack would
419 // be popped to (i.e., we insert it above all the write-compatible items).
420 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
421 self.find_first_write_incompatible(granting_idx)
423 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
424 // Here, creating a reference actually counts as an access.
425 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
426 self.access(access, derived_from, global)?;
428 // We insert "as far up as possible": We know only compatible items are remaining
429 // on top of `derived_from`, and we want the new item at the top so that we
430 // get the strongest possible guarantees.
431 // This ensures U1 and F1.
435 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
436 if self.borrows[new_idx - 1] == new || self.borrows.get(new_idx) == Some(&new) {
437 // Optimization applies, done.
438 trace!("reborrow: avoiding adding redundant item {:?}", new);
440 trace!("reborrow: adding item {:?}", new);
441 self.borrows.insert(new_idx, new);
447 // # Stacked Borrows Core End
449 /// Map per-stack operations to higher-level per-location-range operations.
451 /// Creates new stack with initial tag.
452 fn new(size: Size, perm: Permission, tag: Tag, extra: MemoryExtra) -> Self {
453 let item = Item { perm, tag, protector: None };
454 let stack = Stack { borrows: vec![item] };
456 Stacks { stacks: RefCell::new(RangeMap::new(size, stack)), global: extra }
459 /// Call `f` on every stack in the range.
464 f: impl Fn(Pointer<Tag>, &mut Stack, &GlobalState) -> InterpResult<'tcx>,
465 ) -> InterpResult<'tcx> {
466 let global = self.global.borrow();
467 let mut stacks = self.stacks.borrow_mut();
468 for (offset, stack) in stacks.iter_mut(ptr.offset, size) {
469 let mut cur_ptr = ptr;
470 cur_ptr.offset = offset;
471 f(cur_ptr, stack, &*global)?;
477 /// Glue code to connect with Miri Machine Hooks
479 pub fn new_allocation(
483 kind: MemoryKind<MiriMemoryKind>,
485 let (tag, perm) = match kind {
486 // New unique borrow. This tag is not accessible by the program,
487 // so it will only ever be used when using the local directly (i.e.,
488 // not through a pointer). That is, whenever we directly write to a local, this will pop
489 // everything else off the stack, invalidating all previous pointers,
490 // and in particular, *all* raw pointers.
491 MemoryKind::Stack => (Tag::Tagged(extra.borrow_mut().new_ptr()), Permission::Unique),
492 // `Global` memory can be referenced by global pointers from `tcx`.
493 // Thus we call `global_base_ptr` such that the global pointers get the same tag
494 // as what we use here.
495 // `ExternStatic` is used for extern statics, and thus must also be listed here.
496 // `Env` we list because we can get away with precise tracking there.
497 // The base pointer is not unique, so the base permission is `SharedReadWrite`.
499 MiriMemoryKind::Global
500 | MiriMemoryKind::ExternStatic
501 | MiriMemoryKind::Tls
502 | MiriMemoryKind::Env,
503 ) => (extra.borrow_mut().global_base_ptr(id), Permission::SharedReadWrite),
504 // Everything else we handle like raw pointers for now.
506 let mut extra = extra.borrow_mut();
508 if extra.track_raw { Tag::Tagged(extra.new_ptr()) } else { Tag::Untagged };
509 (tag, Permission::SharedReadWrite)
512 (Stacks::new(size, perm, tag, extra), tag)
516 pub fn memory_read<'tcx>(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
517 trace!("read access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
518 self.for_each(ptr, size, |ptr, stack, global| stack.access(AccessKind::Read, ptr, global))
522 pub fn memory_written<'tcx>(&mut self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
523 trace!("write access with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
524 self.for_each(ptr, size, |ptr, stack, global| stack.access(AccessKind::Write, ptr, global))
528 pub fn memory_deallocated<'tcx>(
532 ) -> InterpResult<'tcx> {
533 trace!("deallocation with tag {:?}: {:?}, size {}", ptr.tag, ptr.erase_tag(), size.bytes());
534 self.for_each(ptr, size, |ptr, stack, global| stack.dealloc(ptr, global))
538 /// Retagging/reborrowing. There is some policy in here, such as which permissions
539 /// to grant for which references, and when to add protectors.
540 impl<'mir, 'tcx: 'mir> EvalContextPrivExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
541 trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
544 place: &MPlaceTy<'tcx, Tag>,
549 ) -> InterpResult<'tcx> {
550 let this = self.eval_context_mut();
551 let protector = if protect { Some(this.frame().extra.call_id) } else { None };
552 let ptr = place.ptr.assert_ptr();
554 "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
563 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
564 let extra = &this.memory.get_raw(ptr.alloc_id)?.extra;
565 let stacked_borrows =
566 extra.stacked_borrows.as_ref().expect("we should have Stacked Borrows data");
567 // Update the stacks.
568 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
569 // There could be existing unique pointers reborrowed from them that should remain valid!
570 let perm = match kind {
571 RefKind::Unique { two_phase: false } => Permission::Unique,
572 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
573 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
574 RefKind::Shared | RefKind::Raw { mutable: false } => {
575 // Shared references and *const are a whole different kind of game, the
576 // permission is not uniform across the entire range!
577 // We need a frozen-sensitive reborrow.
578 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
579 // We are only ever `SharedReadOnly` inside the frozen bits.
580 let perm = if frozen {
581 Permission::SharedReadOnly
583 Permission::SharedReadWrite
585 let item = Item { perm, tag: new_tag, protector };
586 stacked_borrows.for_each(cur_ptr, size, |cur_ptr, stack, global| {
587 stack.grant(cur_ptr, item, global)
592 let item = Item { perm, tag: new_tag, protector };
593 stacked_borrows.for_each(ptr, size, |ptr, stack, global| stack.grant(ptr, item, global))
596 /// Retags an indidual pointer, returning the retagged version.
597 /// `mutbl` can be `None` to make this a raw pointer.
600 val: &ImmTy<'tcx, Tag>,
603 ) -> InterpResult<'tcx, ImmTy<'tcx, Tag>> {
604 let this = self.eval_context_mut();
605 // We want a place for where the ptr *points to*, so we get one.
606 let place = this.ref_to_mplace(val)?;
607 let size = this.size_and_align_of_mplace(&place)?.map(|(size, _)| size);
608 // FIXME: If we cannot determine the size (because the unsized tail is an `extern type`),
609 // bail out -- we cannot reasonably figure out which memory range to reborrow.
610 // See https://github.com/rust-lang/unsafe-code-guidelines/issues/276.
611 let size = match size {
613 None => return Ok(*val),
615 // `reborrow` relies on getting a `Pointer` and everything being in-bounds,
616 // so let's ensure that. However, we do not care about alignment.
617 // We can see dangling ptrs in here e.g. after a Box's `Unique` was
618 // updated using "self.0 = ..." (can happen in Box::from_raw) so we cannot ICE; see miri#1050.
619 let place = this.mplace_access_checked(place, Some(Align::from_bytes(1).unwrap()))?;
620 // Nothing to do for ZSTs. We use `is_bits` here because we *do* need to retag even ZSTs
621 // when there actually is a tag (to avoid inheriting a tag that would let us access more
623 if size == Size::ZERO && place.ptr.is_bits() {
627 // Compute new borrow.
629 let mut mem_extra = this.memory.extra.stacked_borrows.as_ref().unwrap().borrow_mut();
631 // Give up tracking for raw pointers.
632 RefKind::Raw { .. } if !mem_extra.track_raw => Tag::Untagged,
633 // All other pointers are properly tracked.
634 _ => Tag::Tagged(mem_extra.new_ptr()),
639 this.reborrow(&place, size, kind, new_tag, protect)?;
640 let new_place = place.replace_tag(new_tag);
642 // Return new pointer.
643 Ok(ImmTy::from_immediate(new_place.to_ref(), val.layout))
647 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
648 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
649 fn retag(&mut self, kind: RetagKind, place: &PlaceTy<'tcx, Tag>) -> InterpResult<'tcx> {
650 let this = self.eval_context_mut();
651 // Determine mutability and whether to add a protector.
652 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
653 // making it useless.
654 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
656 // References are simple.
657 ty::Ref(_, _, Mutability::Mut) => Some((
658 RefKind::Unique { two_phase: kind == RetagKind::TwoPhase },
659 kind == RetagKind::FnEntry,
661 ty::Ref(_, _, Mutability::Not) =>
662 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
663 // Raw pointers need to be enabled.
664 ty::RawPtr(tym) if kind == RetagKind::Raw =>
665 Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
666 // Boxes do not get a protector: protectors reflect that references outlive the call
667 // they were passed in to; that's just not the case for boxes.
668 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
673 // We only reborrow "bare" references/boxes.
674 // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
675 // but might also cost us optimization and analyses. We will have to experiment more with this.
676 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
678 let val = this.read_immediate(&this.place_to_op(place)?)?;
679 let val = this.retag_reference(&val, mutbl, protector)?;
680 this.write_immediate(*val, place)?;
686 /// After a stack frame got pushed, retag the return place so that we are sure
687 /// it does not alias with anything.
689 /// This is a HACK because there is nothing in MIR that would make the retag
690 /// explicit. Also see https://github.com/rust-lang/rust/issues/71117.
691 fn retag_return_place(&mut self) -> InterpResult<'tcx> {
692 let this = self.eval_context_mut();
693 let return_place = if let Some(return_place) = this.frame_mut().return_place {
696 // No return place, nothing to do.
699 if return_place.layout.is_zst() {
700 // There may not be any memory here, nothing to do.
703 // We need this to be in-memory to use tagged pointers.
704 let return_place = this.force_allocation(&return_place)?;
706 // We have to turn the place into a pointer to use the existing code.
707 // (The pointer type does not matter, so we use a raw pointer.)
708 let ptr_layout = this.layout_of(this.tcx.mk_mut_ptr(return_place.layout.ty))?;
709 let val = ImmTy::from_immediate(return_place.to_ref(), ptr_layout);
711 let val = this.retag_reference(
713 RefKind::Unique { two_phase: false },
716 // And use reborrowed pointer for return place.
717 let return_place = this.ref_to_mplace(&val)?;
718 this.frame_mut().return_place = Some(return_place.into());