1 use crate::ArtificialField;
3 use crate::{AccessDepth, Deep, Shallow};
5 use rustc_middle::mir::{Body, BorrowKind, Local, Place, PlaceElem, PlaceRef, ProjectionElem};
6 use rustc_middle::ty::{self, TyCtxt};
10 /// When checking if a place conflicts with another place, this enum is used to influence decisions
11 /// where a place might be equal or disjoint with another place, such as if `a[i] == a[j]`.
12 /// `PlaceConflictBias::Overlap` would bias toward assuming that `i` might equal `j` and that these
13 /// places overlap. `PlaceConflictBias::NoOverlap` assumes that for the purposes of the predicate
14 /// being run in the calling context, the conservative choice is to assume the compared indices
15 /// are disjoint (and therefore, do not overlap).
16 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
17 pub(crate) enum PlaceConflictBias {
22 /// Helper function for checking if places conflict with a mutable borrow and deep access depth.
23 /// This is used to check for places conflicting outside of the borrow checking code (such as in
25 pub(crate) fn places_conflict<'tcx>(
28 borrow_place: Place<'tcx>,
29 access_place: Place<'tcx>,
30 bias: PlaceConflictBias,
32 borrow_conflicts_with_place(
36 BorrowKind::Mut { allow_two_phase_borrow: true },
37 access_place.as_ref(),
43 /// Checks whether the `borrow_place` conflicts with the `access_place` given a borrow kind and
44 /// access depth. The `bias` parameter is used to determine how the unknowable (comparing runtime
45 /// array indices, for example) should be interpreted - this depends on what the caller wants in
46 /// order to make the conservative choice and preserve soundness.
47 #[instrument(level = "debug", skip(tcx, body))]
48 pub(super) fn borrow_conflicts_with_place<'tcx>(
51 borrow_place: Place<'tcx>,
52 borrow_kind: BorrowKind,
53 access_place: PlaceRef<'tcx>,
55 bias: PlaceConflictBias,
57 // This Local/Local case is handled by the more general code below, but
58 // it's so common that it's a speed win to check for it first.
59 if let Some(l1) = borrow_place.as_local() && let Some(l2) = access_place.as_local() {
63 place_components_conflict(tcx, body, borrow_place, borrow_kind, access_place, access, bias)
66 fn place_components_conflict<'tcx>(
69 borrow_place: Place<'tcx>,
70 borrow_kind: BorrowKind,
71 access_place: PlaceRef<'tcx>,
73 bias: PlaceConflictBias,
75 // The borrowck rules for proving disjointness are applied from the "root" of the
76 // borrow forwards, iterating over "similar" projections in lockstep until
77 // we can prove overlap one way or another. Essentially, we treat `Overlap` as
78 // a monoid and report a conflict if the product ends up not being `Disjoint`.
80 // At each step, if we didn't run out of borrow or place, we know that our elements
81 // have the same type, and that they only overlap if they are the identical.
83 // For example, if we are comparing these:
84 // BORROW: (*x1[2].y).z.a
85 // ACCESS: (*x1[i].y).w.b
87 // Then our steps are:
88 // x1 | x1 -- places are the same
89 // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ)
90 // x1[2].y | x1[i].y -- equal or disjoint
91 // *x1[2].y | *x1[i].y -- equal or disjoint
92 // (*x1[2].y).z | (*x1[i].y).w -- we are disjoint and don't need to check more!
94 // Because `zip` does potentially bad things to the iterator inside, this loop
95 // also handles the case where the access might be a *prefix* of the borrow, e.g.
97 // BORROW: (*x1[2].y).z.a
100 // Then our steps are:
101 // x1 | x1 -- places are the same
102 // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ)
103 // x1[2].y | x1[i].y -- equal or disjoint
105 // -- here we run out of access - the borrow can access a part of it. If this
106 // is a full deep access, then we *know* the borrow conflicts with it. However,
107 // if the access is shallow, then we can proceed:
109 // x1[2].y | (*x1[i].y) -- a deref! the access can't get past this, so we
112 // Our invariant is, that at each step of the iteration:
113 // - If we didn't run out of access to match, our borrow and access are comparable
114 // and either equal or disjoint.
115 // - If we did run out of access, the borrow can access a part of it.
117 let borrow_local = borrow_place.local;
118 let access_local = access_place.local;
120 match place_base_conflict(borrow_local, access_local) {
121 Overlap::Arbitrary => {
122 bug!("Two base can't return Arbitrary");
124 Overlap::EqualOrDisjoint => {
125 // This is the recursive case - proceed to the next element.
127 Overlap::Disjoint => {
128 // We have proven the borrow disjoint - further
129 // projections will remain disjoint.
130 debug!("borrow_conflicts_with_place: disjoint");
135 // loop invariant: borrow_c is always either equal to access_c or disjoint from it.
136 for (i, (borrow_c, &access_c)) in
137 iter::zip(borrow_place.projection, access_place.projection).enumerate()
139 debug!(?borrow_c, ?access_c);
141 let borrow_proj_base = &borrow_place.projection[..i];
143 // Borrow and access path both have more components.
147 // - borrow of `a.(...)`, access to `a.(...)`
148 // - borrow of `a.(...)`, access to `b.(...)`
150 // Here we only see the components we have checked so
151 // far (in our examples, just the first component). We
152 // check whether the components being borrowed vs
153 // accessed are disjoint (as in the second example,
154 // but not the first).
155 match place_projection_conflict(
164 Overlap::Arbitrary => {
165 // We have encountered different fields of potentially
166 // the same union - the borrow now partially overlaps.
168 // There is no *easy* way of comparing the fields
169 // further on, because they might have different types
170 // (e.g., borrows of `u.a.0` and `u.b.y` where `.0` and
171 // `.y` come from different structs).
173 // We could try to do some things here - e.g., count
174 // dereferences - but that's probably not a good
175 // idea, at least for now, so just give up and
176 // report a conflict. This is unsafe code anyway so
177 // the user could always use raw pointers.
178 debug!("arbitrary -> conflict");
181 Overlap::EqualOrDisjoint => {
182 // This is the recursive case - proceed to the next element.
184 Overlap::Disjoint => {
185 // We have proven the borrow disjoint - further
186 // projections will remain disjoint.
193 if borrow_place.projection.len() > access_place.projection.len() {
194 for (i, elem) in borrow_place.projection[access_place.projection.len()..].iter().enumerate()
196 // Borrow path is longer than the access path. Examples:
198 // - borrow of `a.b.c`, access to `a.b`
200 // Here, we know that the borrow can access a part of
201 // our place. This is a conflict if that is a part our
202 // access cares about.
204 let proj_base = &borrow_place.projection[..access_place.projection.len() + i];
205 let base_ty = Place::ty_from(borrow_local, proj_base, body, tcx).ty;
207 match (elem, &base_ty.kind(), access) {
208 (_, _, Shallow(Some(ArtificialField::ArrayLength)))
209 | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => {
210 // The array length is like additional fields on the
211 // type; it does not overlap any existing data there.
212 // Furthermore, if cannot actually be a prefix of any
213 // borrowed place (at least in MIR as it is currently.)
215 // e.g., a (mutable) borrow of `a[5]` while we read the
216 // array length of `a`.
217 debug!("borrow_conflicts_with_place: implicit field");
221 (ProjectionElem::Deref, _, Shallow(None)) => {
222 // e.g., a borrow of `*x.y` while we shallowly access `x.y` or some
223 // prefix thereof - the shallow access can't touch anything behind
225 debug!("borrow_conflicts_with_place: shallow access behind ptr");
228 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Not), _) => {
229 // Shouldn't be tracked
230 bug!("Tracking borrow behind shared reference.");
232 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Mut), AccessDepth::Drop) => {
233 // Values behind a mutable reference are not access either by dropping a
234 // value, or by StorageDead
235 debug!("borrow_conflicts_with_place: drop access behind ptr");
239 (ProjectionElem::Field { .. }, ty::Adt(def, _), AccessDepth::Drop) => {
240 // Drop can read/write arbitrary projections, so places
241 // conflict regardless of further projections.
242 if def.has_dtor(tcx) {
247 (ProjectionElem::Deref, _, Deep)
248 | (ProjectionElem::Deref, _, AccessDepth::Drop)
249 | (ProjectionElem::Field { .. }, _, _)
250 | (ProjectionElem::Index { .. }, _, _)
251 | (ProjectionElem::ConstantIndex { .. }, _, _)
252 | (ProjectionElem::Subslice { .. }, _, _)
253 | (ProjectionElem::Downcast { .. }, _, _) => {
254 // Recursive case. This can still be disjoint on a
255 // further iteration if this a shallow access and
256 // there's a deref later on, e.g., a borrow
257 // of `*x.y` while accessing `x`.
263 // Borrow path ran out but access path may not
266 // - borrow of `a.b`, access to `a.b.c`
267 // - borrow of `a.b`, access to `a.b`
269 // In the first example, where we didn't run out of
270 // access, the borrow can access all of our place, so we
273 // If the second example, where we did, then we still know
274 // that the borrow can access a *part* of our place that
275 // our access cares about, so we still have a conflict.
276 if borrow_kind == BorrowKind::Shallow
277 && borrow_place.projection.len() < access_place.projection.len()
279 debug!("borrow_conflicts_with_place: shallow borrow");
282 debug!("borrow_conflicts_with_place: full borrow, CONFLICT");
287 // Given that the bases of `elem1` and `elem2` are always either equal
288 // or disjoint (and have the same type!), return the overlap situation
289 // between `elem1` and `elem2`.
290 fn place_base_conflict(l1: Local, l2: Local) -> Overlap {
292 // the same local - base case, equal
293 debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL");
294 Overlap::EqualOrDisjoint
296 // different locals - base case, disjoint
297 debug!("place_element_conflict: DISJOINT-LOCAL");
302 // Given that the bases of `elem1` and `elem2` are always either equal
303 // or disjoint (and have the same type!), return the overlap situation
304 // between `elem1` and `elem2`.
305 fn place_projection_conflict<'tcx>(
309 pi1_proj_base: &[PlaceElem<'tcx>],
310 pi1_elem: PlaceElem<'tcx>,
311 pi2_elem: PlaceElem<'tcx>,
312 bias: PlaceConflictBias,
314 match (pi1_elem, pi2_elem) {
315 (ProjectionElem::Deref, ProjectionElem::Deref) => {
316 // derefs (e.g., `*x` vs. `*x`) - recur.
317 debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF");
318 Overlap::EqualOrDisjoint
320 (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => {
322 // same field (e.g., `a.y` vs. `a.y`) - recur.
323 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
324 Overlap::EqualOrDisjoint
326 let ty = Place::ty_from(pi1_local, pi1_proj_base, body, tcx).ty;
328 // Different fields of a union, we are basically stuck.
329 debug!("place_element_conflict: STUCK-UNION");
332 // Different fields of a struct (`a.x` vs. `a.y`). Disjoint!
333 debug!("place_element_conflict: DISJOINT-FIELD");
338 (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => {
339 // different variants are treated as having disjoint fields,
340 // even if they occupy the same "space", because it's
341 // impossible for 2 variants of the same enum to exist
342 // (and therefore, to be borrowed) at the same time.
344 // Note that this is different from unions - we *do* allow
345 // this code to compile:
348 // fn foo(x: &mut Result<i32, i32>) {
350 // if let Ok(ref mut a) = *x {
353 // // here, you would *think* that the
354 // // *entirety* of `x` would be borrowed,
355 // // but in fact only the `Ok` variant is,
356 // // so the `Err` variant is *entirely free*:
357 // if let Err(ref mut a) = *x {
364 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
365 Overlap::EqualOrDisjoint
367 debug!("place_element_conflict: DISJOINT-FIELD");
372 ProjectionElem::Index(..),
373 ProjectionElem::Index(..)
374 | ProjectionElem::ConstantIndex { .. }
375 | ProjectionElem::Subslice { .. },
378 ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. },
379 ProjectionElem::Index(..),
381 // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint
382 // (if the indexes differ) or equal (if they are the same).
384 PlaceConflictBias::Overlap => {
385 // If we are biased towards overlapping, then this is the recursive
386 // case that gives "equal *or* disjoint" its meaning.
387 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX");
388 Overlap::EqualOrDisjoint
390 PlaceConflictBias::NoOverlap => {
391 // If we are biased towards no overlapping, then this is disjoint.
392 debug!("place_element_conflict: DISJOINT-ARRAY-INDEX");
398 ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: false },
399 ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: false },
402 ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: true },
403 ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: true },
406 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX");
407 Overlap::EqualOrDisjoint
409 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX");
414 ProjectionElem::ConstantIndex {
415 offset: offset_from_begin,
416 min_length: min_length1,
419 ProjectionElem::ConstantIndex {
420 offset: offset_from_end,
421 min_length: min_length2,
426 ProjectionElem::ConstantIndex {
427 offset: offset_from_end,
428 min_length: min_length1,
431 ProjectionElem::ConstantIndex {
432 offset: offset_from_begin,
433 min_length: min_length2,
437 // both patterns matched so it must be at least the greater of the two
438 let min_length = max(min_length1, min_length2);
439 // `offset_from_end` can be in range `[1..min_length]`, 1 indicates the last
440 // element (like -1 in Python) and `min_length` the first.
441 // Therefore, `min_length - offset_from_end` gives the minimal possible
442 // offset from the beginning
443 if offset_from_begin >= min_length - offset_from_end {
444 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-FE");
445 Overlap::EqualOrDisjoint
447 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-FE");
452 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
453 ProjectionElem::Subslice { from, to, from_end: false },
456 ProjectionElem::Subslice { from, to, from_end: false },
457 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
459 if (from..to).contains(&offset) {
460 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE");
461 Overlap::EqualOrDisjoint
463 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE");
468 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
469 ProjectionElem::Subslice { from, .. },
472 ProjectionElem::Subslice { from, .. },
473 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
476 debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE");
477 Overlap::EqualOrDisjoint
479 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE");
484 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
485 ProjectionElem::Subslice { to, from_end: true, .. },
488 ProjectionElem::Subslice { to, from_end: true, .. },
489 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
493 "place_element_conflict: \
494 DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE-FE"
496 Overlap::EqualOrDisjoint
498 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE-FE");
503 ProjectionElem::Subslice { from: f1, to: t1, from_end: false },
504 ProjectionElem::Subslice { from: f2, to: t2, from_end: false },
506 if f2 >= t1 || f1 >= t2 {
507 debug!("place_element_conflict: DISJOINT-ARRAY-SUBSLICES");
510 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES");
511 Overlap::EqualOrDisjoint
514 (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => {
515 debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-SUBSLICES");
516 Overlap::EqualOrDisjoint
519 ProjectionElem::Deref
520 | ProjectionElem::Field(..)
521 | ProjectionElem::Index(..)
522 | ProjectionElem::ConstantIndex { .. }
523 | ProjectionElem::Subslice { .. }
524 | ProjectionElem::Downcast(..),
527 "mismatched projections in place_element_conflict: {:?} and {:?}",