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 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 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 pub(super) fn borrow_conflicts_with_place<'tcx>(
50 borrow_place: Place<'tcx>,
51 borrow_kind: BorrowKind,
52 access_place: PlaceRef<'tcx>,
54 bias: PlaceConflictBias,
57 "borrow_conflicts_with_place({:?}, {:?}, {:?}, {:?})",
58 borrow_place, access_place, access, bias,
61 // This Local/Local case is handled by the more general code below, but
62 // it's so common that it's a speed win to check for it first.
63 if let Some(l1) = borrow_place.as_local() && let Some(l2) = access_place.as_local() {
67 place_components_conflict(tcx, body, borrow_place, borrow_kind, access_place, access, bias)
70 fn place_components_conflict<'tcx>(
73 borrow_place: Place<'tcx>,
74 borrow_kind: BorrowKind,
75 access_place: PlaceRef<'tcx>,
77 bias: PlaceConflictBias,
79 // The borrowck rules for proving disjointness are applied from the "root" of the
80 // borrow forwards, iterating over "similar" projections in lockstep until
81 // we can prove overlap one way or another. Essentially, we treat `Overlap` as
82 // a monoid and report a conflict if the product ends up not being `Disjoint`.
84 // At each step, if we didn't run out of borrow or place, we know that our elements
85 // have the same type, and that they only overlap if they are the identical.
87 // For example, if we are comparing these:
88 // BORROW: (*x1[2].y).z.a
89 // ACCESS: (*x1[i].y).w.b
91 // Then our steps are:
92 // x1 | x1 -- places are the same
93 // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ)
94 // x1[2].y | x1[i].y -- equal or disjoint
95 // *x1[2].y | *x1[i].y -- equal or disjoint
96 // (*x1[2].y).z | (*x1[i].y).w -- we are disjoint and don't need to check more!
98 // Because `zip` does potentially bad things to the iterator inside, this loop
99 // also handles the case where the access might be a *prefix* of the borrow, e.g.
101 // BORROW: (*x1[2].y).z.a
104 // Then our steps are:
105 // x1 | x1 -- places are the same
106 // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ)
107 // x1[2].y | x1[i].y -- equal or disjoint
109 // -- here we run out of access - the borrow can access a part of it. If this
110 // is a full deep access, then we *know* the borrow conflicts with it. However,
111 // if the access is shallow, then we can proceed:
113 // x1[2].y | (*x1[i].y) -- a deref! the access can't get past this, so we
116 // Our invariant is, that at each step of the iteration:
117 // - If we didn't run out of access to match, our borrow and access are comparable
118 // and either equal or disjoint.
119 // - If we did run out of access, the borrow can access a part of it.
121 let borrow_local = borrow_place.local;
122 let access_local = access_place.local;
124 match place_base_conflict(borrow_local, access_local) {
125 Overlap::Arbitrary => {
126 bug!("Two base can't return Arbitrary");
128 Overlap::EqualOrDisjoint => {
129 // This is the recursive case - proceed to the next element.
131 Overlap::Disjoint => {
132 // We have proven the borrow disjoint - further
133 // projections will remain disjoint.
134 debug!("borrow_conflicts_with_place: disjoint");
139 // loop invariant: borrow_c is always either equal to access_c or disjoint from it.
140 for (i, (borrow_c, &access_c)) in
141 iter::zip(borrow_place.projection, access_place.projection).enumerate()
143 debug!("borrow_conflicts_with_place: borrow_c = {:?}", borrow_c);
144 let borrow_proj_base = &borrow_place.projection[..i];
146 debug!("borrow_conflicts_with_place: access_c = {:?}", access_c);
148 // Borrow and access path both have more components.
152 // - borrow of `a.(...)`, access to `a.(...)`
153 // - borrow of `a.(...)`, access to `b.(...)`
155 // Here we only see the components we have checked so
156 // far (in our examples, just the first component). We
157 // check whether the components being borrowed vs
158 // accessed are disjoint (as in the second example,
159 // but not the first).
160 match place_projection_conflict(
169 Overlap::Arbitrary => {
170 // We have encountered different fields of potentially
171 // the same union - the borrow now partially overlaps.
173 // There is no *easy* way of comparing the fields
174 // further on, because they might have different types
175 // (e.g., borrows of `u.a.0` and `u.b.y` where `.0` and
176 // `.y` come from different structs).
178 // We could try to do some things here - e.g., count
179 // dereferences - but that's probably not a good
180 // idea, at least for now, so just give up and
181 // report a conflict. This is unsafe code anyway so
182 // the user could always use raw pointers.
183 debug!("borrow_conflicts_with_place: arbitrary -> conflict");
186 Overlap::EqualOrDisjoint => {
187 // This is the recursive case - proceed to the next element.
189 Overlap::Disjoint => {
190 // We have proven the borrow disjoint - further
191 // projections will remain disjoint.
192 debug!("borrow_conflicts_with_place: disjoint");
198 if borrow_place.projection.len() > access_place.projection.len() {
199 for (i, elem) in borrow_place.projection[access_place.projection.len()..].iter().enumerate()
201 // Borrow path is longer than the access path. Examples:
203 // - borrow of `a.b.c`, access to `a.b`
205 // Here, we know that the borrow can access a part of
206 // our place. This is a conflict if that is a part our
207 // access cares about.
209 let proj_base = &borrow_place.projection[..access_place.projection.len() + i];
210 let base_ty = Place::ty_from(borrow_local, proj_base, body, tcx).ty;
212 match (elem, &base_ty.kind(), access) {
213 (_, _, Shallow(Some(ArtificialField::ArrayLength)))
214 | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => {
215 // The array length is like additional fields on the
216 // type; it does not overlap any existing data there.
217 // Furthermore, if cannot actually be a prefix of any
218 // borrowed place (at least in MIR as it is currently.)
220 // e.g., a (mutable) borrow of `a[5]` while we read the
221 // array length of `a`.
222 debug!("borrow_conflicts_with_place: implicit field");
226 (ProjectionElem::Deref, _, Shallow(None)) => {
227 // e.g., a borrow of `*x.y` while we shallowly access `x.y` or some
228 // prefix thereof - the shallow access can't touch anything behind
230 debug!("borrow_conflicts_with_place: shallow access behind ptr");
233 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Not), _) => {
234 // Shouldn't be tracked
235 bug!("Tracking borrow behind shared reference.");
237 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Mut), AccessDepth::Drop) => {
238 // Values behind a mutable reference are not access either by dropping a
239 // value, or by StorageDead
240 debug!("borrow_conflicts_with_place: drop access behind ptr");
244 (ProjectionElem::Field { .. }, ty::Adt(def, _), AccessDepth::Drop) => {
245 // Drop can read/write arbitrary projections, so places
246 // conflict regardless of further projections.
247 if def.has_dtor(tcx) {
252 (ProjectionElem::Deref, _, Deep)
253 | (ProjectionElem::Deref, _, AccessDepth::Drop)
254 | (ProjectionElem::Field { .. }, _, _)
255 | (ProjectionElem::Index { .. }, _, _)
256 | (ProjectionElem::ConstantIndex { .. }, _, _)
257 | (ProjectionElem::Subslice { .. }, _, _)
258 | (ProjectionElem::Downcast { .. }, _, _) => {
259 // Recursive case. This can still be disjoint on a
260 // further iteration if this a shallow access and
261 // there's a deref later on, e.g., a borrow
262 // of `*x.y` while accessing `x`.
268 // Borrow path ran out but access path may not
271 // - borrow of `a.b`, access to `a.b.c`
272 // - borrow of `a.b`, access to `a.b`
274 // In the first example, where we didn't run out of
275 // access, the borrow can access all of our place, so we
278 // If the second example, where we did, then we still know
279 // that the borrow can access a *part* of our place that
280 // our access cares about, so we still have a conflict.
281 if borrow_kind == BorrowKind::Shallow
282 && borrow_place.projection.len() < access_place.projection.len()
284 debug!("borrow_conflicts_with_place: shallow borrow");
287 debug!("borrow_conflicts_with_place: full borrow, CONFLICT");
292 // Given that the bases of `elem1` and `elem2` are always either equal
293 // or disjoint (and have the same type!), return the overlap situation
294 // between `elem1` and `elem2`.
295 fn place_base_conflict(l1: Local, l2: Local) -> Overlap {
297 // the same local - base case, equal
298 debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL");
299 Overlap::EqualOrDisjoint
301 // different locals - base case, disjoint
302 debug!("place_element_conflict: DISJOINT-LOCAL");
307 // Given that the bases of `elem1` and `elem2` are always either equal
308 // or disjoint (and have the same type!), return the overlap situation
309 // between `elem1` and `elem2`.
310 fn place_projection_conflict<'tcx>(
314 pi1_proj_base: &[PlaceElem<'tcx>],
315 pi1_elem: PlaceElem<'tcx>,
316 pi2_elem: PlaceElem<'tcx>,
317 bias: PlaceConflictBias,
319 match (pi1_elem, pi2_elem) {
320 (ProjectionElem::Deref, ProjectionElem::Deref) => {
321 // derefs (e.g., `*x` vs. `*x`) - recur.
322 debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF");
323 Overlap::EqualOrDisjoint
325 (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => {
327 // same field (e.g., `a.y` vs. `a.y`) - recur.
328 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
329 Overlap::EqualOrDisjoint
331 let ty = Place::ty_from(pi1_local, pi1_proj_base, body, tcx).ty;
333 // Different fields of a union, we are basically stuck.
334 debug!("place_element_conflict: STUCK-UNION");
337 // Different fields of a struct (`a.x` vs. `a.y`). Disjoint!
338 debug!("place_element_conflict: DISJOINT-FIELD");
343 (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => {
344 // different variants are treated as having disjoint fields,
345 // even if they occupy the same "space", because it's
346 // impossible for 2 variants of the same enum to exist
347 // (and therefore, to be borrowed) at the same time.
349 // Note that this is different from unions - we *do* allow
350 // this code to compile:
353 // fn foo(x: &mut Result<i32, i32>) {
355 // if let Ok(ref mut a) = *x {
358 // // here, you would *think* that the
359 // // *entirety* of `x` would be borrowed,
360 // // but in fact only the `Ok` variant is,
361 // // so the `Err` variant is *entirely free*:
362 // if let Err(ref mut a) = *x {
369 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
370 Overlap::EqualOrDisjoint
372 debug!("place_element_conflict: DISJOINT-FIELD");
377 ProjectionElem::Index(..),
378 ProjectionElem::Index(..)
379 | ProjectionElem::ConstantIndex { .. }
380 | ProjectionElem::Subslice { .. },
383 ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. },
384 ProjectionElem::Index(..),
386 // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint
387 // (if the indexes differ) or equal (if they are the same).
389 PlaceConflictBias::Overlap => {
390 // If we are biased towards overlapping, then this is the recursive
391 // case that gives "equal *or* disjoint" its meaning.
392 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX");
393 Overlap::EqualOrDisjoint
395 PlaceConflictBias::NoOverlap => {
396 // If we are biased towards no overlapping, then this is disjoint.
397 debug!("place_element_conflict: DISJOINT-ARRAY-INDEX");
403 ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: false },
404 ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: false },
407 ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: true },
408 ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: true },
411 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX");
412 Overlap::EqualOrDisjoint
414 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX");
419 ProjectionElem::ConstantIndex {
420 offset: offset_from_begin,
421 min_length: min_length1,
424 ProjectionElem::ConstantIndex {
425 offset: offset_from_end,
426 min_length: min_length2,
431 ProjectionElem::ConstantIndex {
432 offset: offset_from_end,
433 min_length: min_length1,
436 ProjectionElem::ConstantIndex {
437 offset: offset_from_begin,
438 min_length: min_length2,
442 // both patterns matched so it must be at least the greater of the two
443 let min_length = max(min_length1, min_length2);
444 // `offset_from_end` can be in range `[1..min_length]`, 1 indicates the last
445 // element (like -1 in Python) and `min_length` the first.
446 // Therefore, `min_length - offset_from_end` gives the minimal possible
447 // offset from the beginning
448 if offset_from_begin >= min_length - offset_from_end {
449 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-FE");
450 Overlap::EqualOrDisjoint
452 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-FE");
457 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
458 ProjectionElem::Subslice { from, to, from_end: false },
461 ProjectionElem::Subslice { from, to, from_end: false },
462 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
464 if (from..to).contains(&offset) {
465 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE");
466 Overlap::EqualOrDisjoint
468 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE");
473 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
474 ProjectionElem::Subslice { from, .. },
477 ProjectionElem::Subslice { from, .. },
478 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
481 debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE");
482 Overlap::EqualOrDisjoint
484 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE");
489 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
490 ProjectionElem::Subslice { to, from_end: true, .. },
493 ProjectionElem::Subslice { to, from_end: true, .. },
494 ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
498 "place_element_conflict: \
499 DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE-FE"
501 Overlap::EqualOrDisjoint
503 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE-FE");
508 ProjectionElem::Subslice { from: f1, to: t1, from_end: false },
509 ProjectionElem::Subslice { from: f2, to: t2, from_end: false },
511 if f2 >= t1 || f1 >= t2 {
512 debug!("place_element_conflict: DISJOINT-ARRAY-SUBSLICES");
515 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES");
516 Overlap::EqualOrDisjoint
519 (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => {
520 debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-SUBSLICES");
521 Overlap::EqualOrDisjoint
524 ProjectionElem::Deref
525 | ProjectionElem::Field(..)
526 | ProjectionElem::Index(..)
527 | ProjectionElem::ConstantIndex { .. }
528 | ProjectionElem::Subslice { .. }
529 | ProjectionElem::Downcast(..),
532 "mismatched projections in place_element_conflict: {:?} and {:?}",