compiler/rustc_borrowck/src/places_conflict.rs

   1 use crate::ArtificialField;
   2 use crate::Overlap;
   3 use crate::{AccessDepth, Deep, Shallow};
   4 use rustc_hir as hir;
   5 use rustc_middle::mir::{Body, BorrowKind, Local, Place, PlaceElem, PlaceRef, ProjectionElem};
   6 use rustc_middle::ty::{self, TyCtxt};
   7 use std::cmp::max;
   8 use std::iter;
   9
  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 {
  18     Overlap,
  19     NoOverlap,
  20 }
  21
  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
  24 /// dataflow).
  25 pub(crate) fn places_conflict<'tcx>(
  26     tcx: TyCtxt<'tcx>,
  27     body: &Body<'tcx>,
  28     borrow_place: Place<'tcx>,
  29     access_place: Place<'tcx>,
  30     bias: PlaceConflictBias,
  31 ) -> bool {
  32     borrow_conflicts_with_place(
  33         tcx,
  34         body,
  35         borrow_place,
  36         BorrowKind::Mut { allow_two_phase_borrow: true },
  37         access_place.as_ref(),
  38         AccessDepth::Deep,
  39         bias,
  40     )
  41 }
  42
  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>(
  48     tcx: TyCtxt<'tcx>,
  49     body: &Body<'tcx>,
  50     borrow_place: Place<'tcx>,
  51     borrow_kind: BorrowKind,
  52     access_place: PlaceRef<'tcx>,
  53     access: AccessDepth,
  54     bias: PlaceConflictBias,
  55 ) -> bool {
  56     debug!(
  57         "borrow_conflicts_with_place({:?}, {:?}, {:?}, {:?})",
  58         borrow_place, access_place, access, bias,
  59     );
  60
  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() {
  64         return l1 == l2;
  65     }
  66
  67     place_components_conflict(tcx, body, borrow_place, borrow_kind, access_place, access, bias)
  68 }
  69
  70 fn place_components_conflict<'tcx>(
  71     tcx: TyCtxt<'tcx>,
  72     body: &Body<'tcx>,
  73     borrow_place: Place<'tcx>,
  74     borrow_kind: BorrowKind,
  75     access_place: PlaceRef<'tcx>,
  76     access: AccessDepth,
  77     bias: PlaceConflictBias,
  78 ) -> bool {
  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`.
  83     //
  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.
  86     //
  87     // For example, if we are comparing these:
  88     // BORROW:  (*x1[2].y).z.a
  89     // ACCESS:  (*x1[i].y).w.b
  90     //
  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!
  97     //
  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.
 100     //
 101     // BORROW:  (*x1[2].y).z.a
 102     // ACCESS:  x1[i].y
 103     //
 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
 108     //
 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:
 112     //
 113     //       x1[2].y    | (*x1[i].y)    -- a deref! the access can't get past this, so we
 114     //                                     are disjoint
 115     //
 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.
 120
 121     let borrow_local = borrow_place.local;
 122     let access_local = access_place.local;
 123
 124     match place_base_conflict(borrow_local, access_local) {
 125         Overlap::Arbitrary => {
 126             bug!("Two base can't return Arbitrary");
 127         }
 128         Overlap::EqualOrDisjoint => {
 129             // This is the recursive case - proceed to the next element.
 130         }
 131         Overlap::Disjoint => {
 132             // We have proven the borrow disjoint - further
 133             // projections will remain disjoint.
 134             debug!("borrow_conflicts_with_place: disjoint");
 135             return false;
 136         }
 137     }
 138
 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()
 142     {
 143         debug!("borrow_conflicts_with_place: borrow_c = {:?}", borrow_c);
 144         let borrow_proj_base = &borrow_place.projection[..i];
 145
 146         debug!("borrow_conflicts_with_place: access_c = {:?}", access_c);
 147
 148         // Borrow and access path both have more components.
 149         //
 150         // Examples:
 151         //
 152         // - borrow of `a.(...)`, access to `a.(...)`
 153         // - borrow of `a.(...)`, access to `b.(...)`
 154         //
 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(
 161             tcx,
 162             body,
 163             borrow_local,
 164             borrow_proj_base,
 165             borrow_c,
 166             access_c,
 167             bias,
 168         ) {
 169             Overlap::Arbitrary => {
 170                 // We have encountered different fields of potentially
 171                 // the same union - the borrow now partially overlaps.
 172                 //
 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).
 177                 //
 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");
 184                 return true;
 185             }
 186             Overlap::EqualOrDisjoint => {
 187                 // This is the recursive case - proceed to the next element.
 188             }
 189             Overlap::Disjoint => {
 190                 // We have proven the borrow disjoint - further
 191                 // projections will remain disjoint.
 192                 debug!("borrow_conflicts_with_place: disjoint");
 193                 return false;
 194             }
 195         }
 196     }
 197
 198     if borrow_place.projection.len() > access_place.projection.len() {
 199         for (i, elem) in borrow_place.projection[access_place.projection.len()..].iter().enumerate()
 200         {
 201             // Borrow path is longer than the access path. Examples:
 202             //
 203             // - borrow of `a.b.c`, access to `a.b`
 204             //
 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.
 208
 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;
 211
 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.)
 219                     //
 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");
 223                     return false;
 224                 }
 225
 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
 229                     // the pointer.
 230                     debug!("borrow_conflicts_with_place: shallow access behind ptr");
 231                     return false;
 232                 }
 233                 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Not), _) => {
 234                     // Shouldn't be tracked
 235                     bug!("Tracking borrow behind shared reference.");
 236                 }
 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");
 241                     return false;
 242                 }
 243
 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) {
 248                         return true;
 249                     }
 250                 }
 251
 252                 (ProjectionElem::Deref, _, Deep)
 253                 | (ProjectionElem::Deref, _, AccessDepth::Drop)
 254                 | (ProjectionElem::Field { .. }, _, _)
 255                 | (ProjectionElem::Index { .. }, _, _)
 256                 | (ProjectionElem::ConstantIndex { .. }, _, _)
 257                 | (ProjectionElem::Subslice { .. }, _, _)
 258                 | (ProjectionElem::OpaqueCast { .. }, _, _)
 259                 | (ProjectionElem::Downcast { .. }, _, _) => {
 260                     // Recursive case. This can still be disjoint on a
 261                     // further iteration if this a shallow access and
 262                     // there's a deref later on, e.g., a borrow
 263                     // of `*x.y` while accessing `x`.
 264                 }
 265             }
 266         }
 267     }
 268
 269     // Borrow path ran out but access path may not
 270     // have. Examples:
 271     //
 272     // - borrow of `a.b`, access to `a.b.c`
 273     // - borrow of `a.b`, access to `a.b`
 274     //
 275     // In the first example, where we didn't run out of
 276     // access, the borrow can access all of our place, so we
 277     // have a conflict.
 278     //
 279     // If the second example, where we did, then we still know
 280     // that the borrow can access a *part* of our place that
 281     // our access cares about, so we still have a conflict.
 282     if borrow_kind == BorrowKind::Shallow
 283         && borrow_place.projection.len() < access_place.projection.len()
 284     {
 285         debug!("borrow_conflicts_with_place: shallow borrow");
 286         false
 287     } else {
 288         debug!("borrow_conflicts_with_place: full borrow, CONFLICT");
 289         true
 290     }
 291 }
 292
 293 // Given that the bases of `elem1` and `elem2` are always either equal
 294 // or disjoint (and have the same type!), return the overlap situation
 295 // between `elem1` and `elem2`.
 296 fn place_base_conflict(l1: Local, l2: Local) -> Overlap {
 297     if l1 == l2 {
 298         // the same local - base case, equal
 299         debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL");
 300         Overlap::EqualOrDisjoint
 301     } else {
 302         // different locals - base case, disjoint
 303         debug!("place_element_conflict: DISJOINT-LOCAL");
 304         Overlap::Disjoint
 305     }
 306 }
 307
 308 // Given that the bases of `elem1` and `elem2` are always either equal
 309 // or disjoint (and have the same type!), return the overlap situation
 310 // between `elem1` and `elem2`.
 311 fn place_projection_conflict<'tcx>(
 312     tcx: TyCtxt<'tcx>,
 313     body: &Body<'tcx>,
 314     pi1_local: Local,
 315     pi1_proj_base: &[PlaceElem<'tcx>],
 316     pi1_elem: PlaceElem<'tcx>,
 317     pi2_elem: PlaceElem<'tcx>,
 318     bias: PlaceConflictBias,
 319 ) -> Overlap {
 320     match (pi1_elem, pi2_elem) {
 321         (ProjectionElem::Deref, ProjectionElem::Deref) => {
 322             // derefs (e.g., `*x` vs. `*x`) - recur.
 323             debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF");
 324             Overlap::EqualOrDisjoint
 325         }
 326         (ProjectionElem::OpaqueCast(v1), ProjectionElem::OpaqueCast(v2)) => {
 327             if v1 == v2 {
 328                 // same type - recur.
 329                 debug!("place_element_conflict: DISJOINT-OR-EQ-OPAQUE");
 330                 Overlap::EqualOrDisjoint
 331             } else {
 332                 // Different types. Disjoint!
 333                 debug!("place_element_conflict: DISJOINT-OPAQUE");
 334                 Overlap::Disjoint
 335             }
 336         }
 337         (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => {
 338             if f1 == f2 {
 339                 // same field (e.g., `a.y` vs. `a.y`) - recur.
 340                 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
 341                 Overlap::EqualOrDisjoint
 342             } else {
 343                 let ty = Place::ty_from(pi1_local, pi1_proj_base, body, tcx).ty;
 344                 if ty.is_union() {
 345                     // Different fields of a union, we are basically stuck.
 346                     debug!("place_element_conflict: STUCK-UNION");
 347                     Overlap::Arbitrary
 348                 } else {
 349                     // Different fields of a struct (`a.x` vs. `a.y`). Disjoint!
 350                     debug!("place_element_conflict: DISJOINT-FIELD");
 351                     Overlap::Disjoint
 352                 }
 353             }
 354         }
 355         (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => {
 356             // different variants are treated as having disjoint fields,
 357             // even if they occupy the same "space", because it's
 358             // impossible for 2 variants of the same enum to exist
 359             // (and therefore, to be borrowed) at the same time.
 360             //
 361             // Note that this is different from unions - we *do* allow
 362             // this code to compile:
 363             //
 364             // ```
 365             // fn foo(x: &mut Result<i32, i32>) {
 366             //     let mut v = None;
 367             //     if let Ok(ref mut a) = *x {
 368             //         v = Some(a);
 369             //     }
 370             //     // here, you would *think* that the
 371             //     // *entirety* of `x` would be borrowed,
 372             //     // but in fact only the `Ok` variant is,
 373             //     // so the `Err` variant is *entirely free*:
 374             //     if let Err(ref mut a) = *x {
 375             //         v = Some(a);
 376             //     }
 377             //     drop(v);
 378             // }
 379             // ```
 380             if v1 == v2 {
 381                 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
 382                 Overlap::EqualOrDisjoint
 383             } else {
 384                 debug!("place_element_conflict: DISJOINT-FIELD");
 385                 Overlap::Disjoint
 386             }
 387         }
 388         (
 389             ProjectionElem::Index(..),
 390             ProjectionElem::Index(..)
 391             | ProjectionElem::ConstantIndex { .. }
 392             | ProjectionElem::Subslice { .. },
 393         )
 394         | (
 395             ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. },
 396             ProjectionElem::Index(..),
 397         ) => {
 398             // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint
 399             // (if the indexes differ) or equal (if they are the same).
 400             match bias {
 401                 PlaceConflictBias::Overlap => {
 402                     // If we are biased towards overlapping, then this is the recursive
 403                     // case that gives "equal *or* disjoint" its meaning.
 404                     debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX");
 405                     Overlap::EqualOrDisjoint
 406                 }
 407                 PlaceConflictBias::NoOverlap => {
 408                     // If we are biased towards no overlapping, then this is disjoint.
 409                     debug!("place_element_conflict: DISJOINT-ARRAY-INDEX");
 410                     Overlap::Disjoint
 411                 }
 412             }
 413         }
 414         (
 415             ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: false },
 416             ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: false },
 417         )
 418         | (
 419             ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: true },
 420             ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: true },
 421         ) => {
 422             if o1 == o2 {
 423                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX");
 424                 Overlap::EqualOrDisjoint
 425             } else {
 426                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX");
 427                 Overlap::Disjoint
 428             }
 429         }
 430         (
 431             ProjectionElem::ConstantIndex {
 432                 offset: offset_from_begin,
 433                 min_length: min_length1,
 434                 from_end: false,
 435             },
 436             ProjectionElem::ConstantIndex {
 437                 offset: offset_from_end,
 438                 min_length: min_length2,
 439                 from_end: true,
 440             },
 441         )
 442         | (
 443             ProjectionElem::ConstantIndex {
 444                 offset: offset_from_end,
 445                 min_length: min_length1,
 446                 from_end: true,
 447             },
 448             ProjectionElem::ConstantIndex {
 449                 offset: offset_from_begin,
 450                 min_length: min_length2,
 451                 from_end: false,
 452             },
 453         ) => {
 454             // both patterns matched so it must be at least the greater of the two
 455             let min_length = max(min_length1, min_length2);
 456             // `offset_from_end` can be in range `[1..min_length]`, 1 indicates the last
 457             // element (like -1 in Python) and `min_length` the first.
 458             // Therefore, `min_length - offset_from_end` gives the minimal possible
 459             // offset from the beginning
 460             if offset_from_begin >= min_length - offset_from_end {
 461                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-FE");
 462                 Overlap::EqualOrDisjoint
 463             } else {
 464                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-FE");
 465                 Overlap::Disjoint
 466             }
 467         }
 468         (
 469             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 470             ProjectionElem::Subslice { from, to, from_end: false },
 471         )
 472         | (
 473             ProjectionElem::Subslice { from, to, from_end: false },
 474             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 475         ) => {
 476             if (from..to).contains(&offset) {
 477                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE");
 478                 Overlap::EqualOrDisjoint
 479             } else {
 480                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE");
 481                 Overlap::Disjoint
 482             }
 483         }
 484         (
 485             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 486             ProjectionElem::Subslice { from, .. },
 487         )
 488         | (
 489             ProjectionElem::Subslice { from, .. },
 490             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 491         ) => {
 492             if offset >= from {
 493                 debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE");
 494                 Overlap::EqualOrDisjoint
 495             } else {
 496                 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE");
 497                 Overlap::Disjoint
 498             }
 499         }
 500         (
 501             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
 502             ProjectionElem::Subslice { to, from_end: true, .. },
 503         )
 504         | (
 505             ProjectionElem::Subslice { to, from_end: true, .. },
 506             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
 507         ) => {
 508             if offset > to {
 509                 debug!(
 510                     "place_element_conflict: \
 511                        DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE-FE"
 512                 );
 513                 Overlap::EqualOrDisjoint
 514             } else {
 515                 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE-FE");
 516                 Overlap::Disjoint
 517             }
 518         }
 519         (
 520             ProjectionElem::Subslice { from: f1, to: t1, from_end: false },
 521             ProjectionElem::Subslice { from: f2, to: t2, from_end: false },
 522         ) => {
 523             if f2 >= t1 || f1 >= t2 {
 524                 debug!("place_element_conflict: DISJOINT-ARRAY-SUBSLICES");
 525                 Overlap::Disjoint
 526             } else {
 527                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES");
 528                 Overlap::EqualOrDisjoint
 529             }
 530         }
 531         (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => {
 532             debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-SUBSLICES");
 533             Overlap::EqualOrDisjoint
 534         }
 535         (
 536             ProjectionElem::Deref
 537             | ProjectionElem::Field(..)
 538             | ProjectionElem::Index(..)
 539             | ProjectionElem::ConstantIndex { .. }
 540             | ProjectionElem::OpaqueCast { .. }
 541             | ProjectionElem::Subslice { .. }
 542             | ProjectionElem::Downcast(..),
 543             _,
 544         ) => bug!(
 545             "mismatched projections in place_element_conflict: {:?} and {:?}",
 546             pi1_elem,
 547             pi2_elem
 548         ),
 549     }
 550 }