src/librustc_mir/borrow_check/places_conflict.rs

   1 use crate::borrow_check::ArtificialField;
   2 use crate::borrow_check::Overlap;
   3 use crate::borrow_check::{Deep, Shallow, AccessDepth};
   4 use rustc::hir;
   5 use rustc::mir::{
   6     Body, BorrowKind, Place, PlaceBase, PlaceElem, PlaceRef, ProjectionElem, StaticKind,
   7 };
   8 use rustc::ty::{self, TyCtxt};
   9 use std::cmp::max;
  10
  11 /// When checking if a place conflicts with another place, this enum is used to influence decisions
  12 /// where a place might be equal or disjoint with another place, such as if `a[i] == a[j]`.
  13 /// `PlaceConflictBias::Overlap` would bias toward assuming that `i` might equal `j` and that these
  14 /// places overlap. `PlaceConflictBias::NoOverlap` assumes that for the purposes of the predicate
  15 /// being run in the calling context, the conservative choice is to assume the compared indices
  16 /// are disjoint (and therefore, do not overlap).
  17 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
  18 crate enum PlaceConflictBias {
  19     Overlap,
  20     NoOverlap,
  21 }
  22
  23 /// Helper function for checking if places conflict with a mutable borrow and deep access depth.
  24 /// This is used to check for places conflicting outside of the borrow checking code (such as in
  25 /// dataflow).
  26 crate fn places_conflict<'tcx>(
  27     tcx: TyCtxt<'tcx>,
  28     param_env: ty::ParamEnv<'tcx>,
  29     body: &Body<'tcx>,
  30     borrow_place: &Place<'tcx>,
  31     access_place: &Place<'tcx>,
  32     bias: PlaceConflictBias,
  33 ) -> bool {
  34     borrow_conflicts_with_place(
  35         tcx,
  36         param_env,
  37         body,
  38         borrow_place,
  39         BorrowKind::Mut { allow_two_phase_borrow: true },
  40         access_place.as_ref(),
  41         AccessDepth::Deep,
  42         bias,
  43     )
  44 }
  45
  46 /// Checks whether the `borrow_place` conflicts with the `access_place` given a borrow kind and
  47 /// access depth. The `bias` parameter is used to determine how the unknowable (comparing runtime
  48 /// array indices, for example) should be interpreted - this depends on what the caller wants in
  49 /// order to make the conservative choice and preserve soundness.
  50 pub(super) fn borrow_conflicts_with_place<'tcx>(
  51     tcx: TyCtxt<'tcx>,
  52     param_env: ty::ParamEnv<'tcx>,
  53     body: &Body<'tcx>,
  54     borrow_place: &Place<'tcx>,
  55     borrow_kind: BorrowKind,
  56     access_place: PlaceRef<'_, 'tcx>,
  57     access: AccessDepth,
  58     bias: PlaceConflictBias,
  59 ) -> bool {
  60     debug!(
  61         "borrow_conflicts_with_place({:?}, {:?}, {:?}, {:?})",
  62         borrow_place, access_place, access, bias,
  63     );
  64
  65     // This Local/Local case is handled by the more general code below, but
  66     // it's so common that it's a speed win to check for it first.
  67     if let Some(l1) = borrow_place.as_local() {
  68         if let Some(l2) = access_place.as_local() {
  69             return l1 == l2;
  70         }
  71     }
  72
  73     place_components_conflict(
  74         tcx,
  75         param_env,
  76         body,
  77         borrow_place,
  78         borrow_kind,
  79         access_place,
  80         access,
  81         bias,
  82     )
  83 }
  84
  85 fn place_components_conflict<'tcx>(
  86     tcx: TyCtxt<'tcx>,
  87     param_env: ty::ParamEnv<'tcx>,
  88     body: &Body<'tcx>,
  89     borrow_place: &Place<'tcx>,
  90     borrow_kind: BorrowKind,
  91     access_place: PlaceRef<'_, 'tcx>,
  92     access: AccessDepth,
  93     bias: PlaceConflictBias,
  94 ) -> bool {
  95     // The borrowck rules for proving disjointness are applied from the "root" of the
  96     // borrow forwards, iterating over "similar" projections in lockstep until
  97     // we can prove overlap one way or another. Essentially, we treat `Overlap` as
  98     // a monoid and report a conflict if the product ends up not being `Disjoint`.
  99     //
 100     // At each step, if we didn't run out of borrow or place, we know that our elements
 101     // have the same type, and that they only overlap if they are the identical.
 102     //
 103     // For example, if we are comparing these:
 104     // BORROW:  (*x1[2].y).z.a
 105     // ACCESS:  (*x1[i].y).w.b
 106     //
 107     // Then our steps are:
 108     //       x1         |   x1          -- places are the same
 109     //       x1[2]      |   x1[i]       -- equal or disjoint (disjoint if indexes differ)
 110     //       x1[2].y    |   x1[i].y     -- equal or disjoint
 111     //      *x1[2].y    |  *x1[i].y     -- equal or disjoint
 112     //     (*x1[2].y).z | (*x1[i].y).w  -- we are disjoint and don't need to check more!
 113     //
 114     // Because `zip` does potentially bad things to the iterator inside, this loop
 115     // also handles the case where the access might be a *prefix* of the borrow, e.g.
 116     //
 117     // BORROW:  (*x1[2].y).z.a
 118     // ACCESS:  x1[i].y
 119     //
 120     // Then our steps are:
 121     //       x1         |   x1          -- places are the same
 122     //       x1[2]      |   x1[i]       -- equal or disjoint (disjoint if indexes differ)
 123     //       x1[2].y    |   x1[i].y     -- equal or disjoint
 124     //
 125     // -- here we run out of access - the borrow can access a part of it. If this
 126     // is a full deep access, then we *know* the borrow conflicts with it. However,
 127     // if the access is shallow, then we can proceed:
 128     //
 129     //       x1[2].y    | (*x1[i].y)    -- a deref! the access can't get past this, so we
 130     //                                     are disjoint
 131     //
 132     // Our invariant is, that at each step of the iteration:
 133     //  - If we didn't run out of access to match, our borrow and access are comparable
 134     //    and either equal or disjoint.
 135     //  - If we did run out of access, the borrow can access a part of it.
 136
 137     let borrow_base = &borrow_place.base;
 138     let access_base = access_place.base;
 139
 140     match place_base_conflict(tcx, param_env, borrow_base, access_base) {
 141         Overlap::Arbitrary => {
 142             bug!("Two base can't return Arbitrary");
 143         }
 144         Overlap::EqualOrDisjoint => {
 145             // This is the recursive case - proceed to the next element.
 146         }
 147         Overlap::Disjoint => {
 148             // We have proven the borrow disjoint - further
 149             // projections will remain disjoint.
 150             debug!("borrow_conflicts_with_place: disjoint");
 151             return false;
 152         }
 153     }
 154
 155     // loop invariant: borrow_c is always either equal to access_c or disjoint from it.
 156     for (i, (borrow_c, access_c)) in
 157         borrow_place.projection.iter().zip(access_place.projection.iter()).enumerate()
 158     {
 159         debug!("borrow_conflicts_with_place: borrow_c = {:?}", borrow_c);
 160         let borrow_proj_base = &borrow_place.projection[..i];
 161
 162         debug!("borrow_conflicts_with_place: access_c = {:?}", access_c);
 163
 164         // Borrow and access path both have more components.
 165         //
 166         // Examples:
 167         //
 168         // - borrow of `a.(...)`, access to `a.(...)`
 169         // - borrow of `a.(...)`, access to `b.(...)`
 170         //
 171         // Here we only see the components we have checked so
 172         // far (in our examples, just the first component). We
 173         // check whether the components being borrowed vs
 174         // accessed are disjoint (as in the second example,
 175         // but not the first).
 176         match place_projection_conflict(
 177             tcx,
 178             body,
 179             borrow_base,
 180             borrow_proj_base,
 181             borrow_c,
 182             access_c,
 183             bias,
 184         ) {
 185             Overlap::Arbitrary => {
 186                 // We have encountered different fields of potentially
 187                 // the same union - the borrow now partially overlaps.
 188                 //
 189                 // There is no *easy* way of comparing the fields
 190                 // further on, because they might have different types
 191                 // (e.g., borrows of `u.a.0` and `u.b.y` where `.0` and
 192                 // `.y` come from different structs).
 193                 //
 194                 // We could try to do some things here - e.g., count
 195                 // dereferences - but that's probably not a good
 196                 // idea, at least for now, so just give up and
 197                 // report a conflict. This is unsafe code anyway so
 198                 // the user could always use raw pointers.
 199                 debug!("borrow_conflicts_with_place: arbitrary -> conflict");
 200                 return true;
 201             }
 202             Overlap::EqualOrDisjoint => {
 203                 // This is the recursive case - proceed to the next element.
 204             }
 205             Overlap::Disjoint => {
 206                 // We have proven the borrow disjoint - further
 207                 // projections will remain disjoint.
 208                 debug!("borrow_conflicts_with_place: disjoint");
 209                 return false;
 210             }
 211         }
 212     }
 213
 214     if borrow_place.projection.len() > access_place.projection.len() {
 215         for (i, elem) in borrow_place.projection[access_place.projection.len()..].iter().enumerate()
 216         {
 217             // Borrow path is longer than the access path. Examples:
 218             //
 219             // - borrow of `a.b.c`, access to `a.b`
 220             //
 221             // Here, we know that the borrow can access a part of
 222             // our place. This is a conflict if that is a part our
 223             // access cares about.
 224
 225             let proj_base = &borrow_place.projection[..access_place.projection.len() + i];
 226             let base_ty = Place::ty_from(borrow_base, proj_base, body, tcx).ty;
 227
 228             match (elem, &base_ty.kind, access) {
 229                 (_, _, Shallow(Some(ArtificialField::ArrayLength)))
 230                 | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => {
 231                     // The array length is like  additional fields on the
 232                     // type; it does not overlap any existing data there.
 233                     // Furthermore, if cannot actually be a prefix of any
 234                     // borrowed place (at least in MIR as it is currently.)
 235                     //
 236                     // e.g., a (mutable) borrow of `a[5]` while we read the
 237                     // array length of `a`.
 238                     debug!("borrow_conflicts_with_place: implicit field");
 239                     return false;
 240                 }
 241
 242                 (ProjectionElem::Deref, _, Shallow(None)) => {
 243                     // e.g., a borrow of `*x.y` while we shallowly access `x.y` or some
 244                     // prefix thereof - the shallow access can't touch anything behind
 245                     // the pointer.
 246                     debug!("borrow_conflicts_with_place: shallow access behind ptr");
 247                     return false;
 248                 }
 249                 (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Immutable), _) => {
 250                     // Shouldn't be tracked
 251                     bug!("Tracking borrow behind shared reference.");
 252                 }
 253                 (ProjectionElem::Deref,
 254                  ty::Ref(_, _, hir::Mutability::Mutable),
 255                  AccessDepth::Drop) => {
 256                     // Values behind a mutable reference are not access either by dropping a
 257                     // value, or by StorageDead
 258                     debug!("borrow_conflicts_with_place: drop access behind ptr");
 259                     return false;
 260                 }
 261
 262                 (ProjectionElem::Field { .. }, ty::Adt(def, _), AccessDepth::Drop) => {
 263                     // Drop can read/write arbitrary projections, so places
 264                     // conflict regardless of further projections.
 265                     if def.has_dtor(tcx) {
 266                         return true;
 267                     }
 268                 }
 269
 270                 (ProjectionElem::Deref, _, Deep)
 271                 | (ProjectionElem::Deref, _, AccessDepth::Drop)
 272                 | (ProjectionElem::Field { .. }, _, _)
 273                 | (ProjectionElem::Index { .. }, _, _)
 274                 | (ProjectionElem::ConstantIndex { .. }, _, _)
 275                 | (ProjectionElem::Subslice { .. }, _, _)
 276                 | (ProjectionElem::Downcast { .. }, _, _) => {
 277                     // Recursive case. This can still be disjoint on a
 278                     // further iteration if this a shallow access and
 279                     // there's a deref later on, e.g., a borrow
 280                     // of `*x.y` while accessing `x`.
 281                 }
 282             }
 283         }
 284     }
 285
 286     // Borrow path ran out but access path may not
 287     // have. Examples:
 288     //
 289     // - borrow of `a.b`, access to `a.b.c`
 290     // - borrow of `a.b`, access to `a.b`
 291     //
 292     // In the first example, where we didn't run out of
 293     // access, the borrow can access all of our place, so we
 294     // have a conflict.
 295     //
 296     // If the second example, where we did, then we still know
 297     // that the borrow can access a *part* of our place that
 298     // our access cares about, so we still have a conflict.
 299     if borrow_kind == BorrowKind::Shallow
 300         && borrow_place.projection.len() < access_place.projection.len()
 301     {
 302         debug!("borrow_conflicts_with_place: shallow borrow");
 303         false
 304     } else {
 305         debug!("borrow_conflicts_with_place: full borrow, CONFLICT");
 306         true
 307     }
 308 }
 309
 310 // Given that the bases of `elem1` and `elem2` are always either equal
 311 // or disjoint (and have the same type!), return the overlap situation
 312 // between `elem1` and `elem2`.
 313 fn place_base_conflict<'tcx>(
 314     tcx: TyCtxt<'tcx>,
 315     param_env: ty::ParamEnv<'tcx>,
 316     elem1: &PlaceBase<'tcx>,
 317     elem2: &PlaceBase<'tcx>,
 318 ) -> Overlap {
 319     match (elem1, elem2) {
 320         (PlaceBase::Local(l1), PlaceBase::Local(l2)) => {
 321             if l1 == l2 {
 322                 // the same local - base case, equal
 323                 debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL");
 324                 Overlap::EqualOrDisjoint
 325             } else {
 326                 // different locals - base case, disjoint
 327                 debug!("place_element_conflict: DISJOINT-LOCAL");
 328                 Overlap::Disjoint
 329             }
 330         }
 331         (PlaceBase::Static(s1), PlaceBase::Static(s2)) => {
 332             match (&s1.kind, &s2.kind) {
 333                 (StaticKind::Static, StaticKind::Static) => {
 334                     if s1.def_id != s2.def_id {
 335                         debug!("place_element_conflict: DISJOINT-STATIC");
 336                         Overlap::Disjoint
 337                     } else if tcx.is_mutable_static(s1.def_id) {
 338                         // We ignore mutable statics - they can only be unsafe code.
 339                         debug!("place_element_conflict: IGNORE-STATIC-MUT");
 340                         Overlap::Disjoint
 341                     } else {
 342                         debug!("place_element_conflict: DISJOINT-OR-EQ-STATIC");
 343                         Overlap::EqualOrDisjoint
 344                     }
 345                 },
 346                 (StaticKind::Promoted(promoted_1, _), StaticKind::Promoted(promoted_2, _)) => {
 347                     if promoted_1 == promoted_2 {
 348                         if let ty::Array(_, len) = s1.ty.kind {
 349                             if let Some(0) = len.try_eval_usize(tcx, param_env) {
 350                                 // Ignore conflicts with promoted [T; 0].
 351                                 debug!("place_element_conflict: IGNORE-LEN-0-PROMOTED");
 352                                 return Overlap::Disjoint;
 353                             }
 354                         }
 355                         // the same promoted - base case, equal
 356                         debug!("place_element_conflict: DISJOINT-OR-EQ-PROMOTED");
 357                         Overlap::EqualOrDisjoint
 358                     } else {
 359                         // different promoteds - base case, disjoint
 360                         debug!("place_element_conflict: DISJOINT-PROMOTED");
 361                         Overlap::Disjoint
 362                     }
 363                 },
 364                 (_, _) => {
 365                     debug!("place_element_conflict: DISJOINT-STATIC-PROMOTED");
 366                     Overlap::Disjoint
 367                 }
 368             }
 369         }
 370         (PlaceBase::Local(_), PlaceBase::Static(_)) |
 371         (PlaceBase::Static(_), PlaceBase::Local(_)) => {
 372             debug!("place_element_conflict: DISJOINT-STATIC-LOCAL-PROMOTED");
 373             Overlap::Disjoint
 374         }
 375     }
 376 }
 377
 378 // Given that the bases of `elem1` and `elem2` are always either equal
 379 // or disjoint (and have the same type!), return the overlap situation
 380 // between `elem1` and `elem2`.
 381 fn place_projection_conflict<'tcx>(
 382     tcx: TyCtxt<'tcx>,
 383     body: &Body<'tcx>,
 384     pi1_base: &PlaceBase<'tcx>,
 385     pi1_proj_base: &[PlaceElem<'tcx>],
 386     pi1_elem: &PlaceElem<'tcx>,
 387     pi2_elem: &PlaceElem<'tcx>,
 388     bias: PlaceConflictBias,
 389 ) -> Overlap {
 390     match (pi1_elem, pi2_elem) {
 391         (ProjectionElem::Deref, ProjectionElem::Deref) => {
 392             // derefs (e.g., `*x` vs. `*x`) - recur.
 393             debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF");
 394             Overlap::EqualOrDisjoint
 395         }
 396         (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => {
 397             if f1 == f2 {
 398                 // same field (e.g., `a.y` vs. `a.y`) - recur.
 399                 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
 400                 Overlap::EqualOrDisjoint
 401             } else {
 402                 let ty = Place::ty_from(pi1_base, pi1_proj_base, body, tcx).ty;
 403                 match ty.kind {
 404                     ty::Adt(def, _) if def.is_union() => {
 405                         // Different fields of a union, we are basically stuck.
 406                         debug!("place_element_conflict: STUCK-UNION");
 407                         Overlap::Arbitrary
 408                     }
 409                     _ => {
 410                         // Different fields of a struct (`a.x` vs. `a.y`). Disjoint!
 411                         debug!("place_element_conflict: DISJOINT-FIELD");
 412                         Overlap::Disjoint
 413                     }
 414                 }
 415             }
 416         }
 417         (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => {
 418             // different variants are treated as having disjoint fields,
 419             // even if they occupy the same "space", because it's
 420             // impossible for 2 variants of the same enum to exist
 421             // (and therefore, to be borrowed) at the same time.
 422             //
 423             // Note that this is different from unions - we *do* allow
 424             // this code to compile:
 425             //
 426             // ```
 427             // fn foo(x: &mut Result<i32, i32>) {
 428             //     let mut v = None;
 429             //     if let Ok(ref mut a) = *x {
 430             //         v = Some(a);
 431             //     }
 432             //     // here, you would *think* that the
 433             //     // *entirety* of `x` would be borrowed,
 434             //     // but in fact only the `Ok` variant is,
 435             //     // so the `Err` variant is *entirely free*:
 436             //     if let Err(ref mut a) = *x {
 437             //         v = Some(a);
 438             //     }
 439             //     drop(v);
 440             // }
 441             // ```
 442             if v1 == v2 {
 443                 debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD");
 444                 Overlap::EqualOrDisjoint
 445             } else {
 446                 debug!("place_element_conflict: DISJOINT-FIELD");
 447                 Overlap::Disjoint
 448             }
 449         }
 450         (ProjectionElem::Index(..), ProjectionElem::Index(..))
 451         | (ProjectionElem::Index(..), ProjectionElem::ConstantIndex { .. })
 452         | (ProjectionElem::Index(..), ProjectionElem::Subslice { .. })
 453         | (ProjectionElem::ConstantIndex { .. }, ProjectionElem::Index(..))
 454         | (ProjectionElem::Subslice { .. }, ProjectionElem::Index(..)) => {
 455             // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint
 456             // (if the indexes differ) or equal (if they are the same).
 457             match bias {
 458                 PlaceConflictBias::Overlap => {
 459                     // If we are biased towards overlapping, then this is the recursive
 460                     // case that gives "equal *or* disjoint" its meaning.
 461                     debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX");
 462                     Overlap::EqualOrDisjoint
 463                 }
 464                 PlaceConflictBias::NoOverlap => {
 465                     // If we are biased towards no overlapping, then this is disjoint.
 466                     debug!("place_element_conflict: DISJOINT-ARRAY-INDEX");
 467                     Overlap::Disjoint
 468                 }
 469             }
 470         }
 471         (ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: false },
 472             ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: false })
 473         | (ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: true },
 474             ProjectionElem::ConstantIndex {
 475                 offset: o2, min_length: _, from_end: true }) => {
 476             if o1 == o2 {
 477                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX");
 478                 Overlap::EqualOrDisjoint
 479             } else {
 480                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX");
 481                 Overlap::Disjoint
 482             }
 483         }
 484         (ProjectionElem::ConstantIndex {
 485             offset: offset_from_begin, min_length: min_length1, from_end: false },
 486             ProjectionElem::ConstantIndex {
 487                 offset: offset_from_end, min_length: min_length2, from_end: true })
 488         | (ProjectionElem::ConstantIndex {
 489             offset: offset_from_end, min_length: min_length1, from_end: true },
 490            ProjectionElem::ConstantIndex {
 491                offset: offset_from_begin, min_length: min_length2, from_end: false }) => {
 492             // both patterns matched so it must be at least the greater of the two
 493             let min_length = max(min_length1, min_length2);
 494             // `offset_from_end` can be in range `[1..min_length]`, 1 indicates the last
 495             // element (like -1 in Python) and `min_length` the first.
 496             // Therefore, `min_length - offset_from_end` gives the minimal possible
 497             // offset from the beginning
 498             if *offset_from_begin >= *min_length - *offset_from_end {
 499                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-FE");
 500                 Overlap::EqualOrDisjoint
 501             } else {
 502                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-FE");
 503                 Overlap::Disjoint
 504             }
 505         }
 506         (
 507             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 508             ProjectionElem::Subslice { from, to, from_end: false }
 509         )
 510         | (
 511             ProjectionElem::Subslice { from, to, from_end: false },
 512             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }
 513         ) => {
 514             if (from..to).contains(&offset) {
 515                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE");
 516                 Overlap::EqualOrDisjoint
 517             } else {
 518                 debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE");
 519                 Overlap::Disjoint
 520             }
 521         }
 522         (ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false },
 523          ProjectionElem::Subslice {from, .. })
 524         | (ProjectionElem::Subslice {from, .. },
 525             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }) => {
 526             if offset >= from {
 527                 debug!(
 528                     "place_element_conflict: DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE");
 529                 Overlap::EqualOrDisjoint
 530             } else {
 531                 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE");
 532                 Overlap::Disjoint
 533             }
 534         }
 535         (ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true },
 536          ProjectionElem::Subslice { to, from_end: true, .. })
 537         | (ProjectionElem::Subslice { to, from_end: true, .. },
 538             ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true }) => {
 539             if offset > to {
 540                 debug!("place_element_conflict: \
 541                        DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE-FE");
 542                 Overlap::EqualOrDisjoint
 543             } else {
 544                 debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE-FE");
 545                 Overlap::Disjoint
 546             }
 547         }
 548         (
 549             ProjectionElem::Subslice { from: f1, to: t1, from_end: false },
 550             ProjectionElem::Subslice { from: f2, to: t2, from_end: false }
 551         ) => {
 552             if f2 >= t1 || f1 >= t2 {
 553                 debug!("place_element_conflict: DISJOINT-ARRAY-SUBSLICES");
 554                 Overlap::Disjoint
 555             } else {
 556                 debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES");
 557                 Overlap::EqualOrDisjoint
 558             }
 559         }
 560         (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => {
 561             debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-SUBSLICES");
 562              Overlap::EqualOrDisjoint
 563         }
 564         (ProjectionElem::Deref, _)
 565         | (ProjectionElem::Field(..), _)
 566         | (ProjectionElem::Index(..), _)
 567         | (ProjectionElem::ConstantIndex { .. }, _)
 568         | (ProjectionElem::Subslice { .. }, _)
 569         | (ProjectionElem::Downcast(..), _) => bug!(
 570             "mismatched projections in place_element_conflict: {:?} and {:?}",
 571             pi1_elem,
 572             pi2_elem
 573         ),
 574     }
 575 }