| use crate::ArtificialField; |
| use crate::Overlap; |
| use crate::{AccessDepth, Deep, Shallow}; |
| use rustc_hir as hir; |
| use rustc_middle::mir::{Body, BorrowKind, Local, Place, PlaceElem, PlaceRef, ProjectionElem}; |
| use rustc_middle::ty::{self, TyCtxt}; |
| use std::cmp::max; |
| use std::iter; |
| |
| /// When checking if a place conflicts with another place, this enum is used to influence decisions |
| /// where a place might be equal or disjoint with another place, such as if `a[i] == a[j]`. |
| /// `PlaceConflictBias::Overlap` would bias toward assuming that `i` might equal `j` and that these |
| /// places overlap. `PlaceConflictBias::NoOverlap` assumes that for the purposes of the predicate |
| /// being run in the calling context, the conservative choice is to assume the compared indices |
| /// are disjoint (and therefore, do not overlap). |
| #[derive(Copy, Clone, Debug, Eq, PartialEq)] |
| pub(crate) enum PlaceConflictBias { |
| Overlap, |
| NoOverlap, |
| } |
| |
| /// Helper function for checking if places conflict with a mutable borrow and deep access depth. |
| /// This is used to check for places conflicting outside of the borrow checking code (such as in |
| /// dataflow). |
| pub(crate) fn places_conflict<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| body: &Body<'tcx>, |
| borrow_place: Place<'tcx>, |
| access_place: Place<'tcx>, |
| bias: PlaceConflictBias, |
| ) -> bool { |
| borrow_conflicts_with_place( |
| tcx, |
| body, |
| borrow_place, |
| BorrowKind::Mut { allow_two_phase_borrow: true }, |
| access_place.as_ref(), |
| AccessDepth::Deep, |
| bias, |
| ) |
| } |
| |
| /// Checks whether the `borrow_place` conflicts with the `access_place` given a borrow kind and |
| /// access depth. The `bias` parameter is used to determine how the unknowable (comparing runtime |
| /// array indices, for example) should be interpreted - this depends on what the caller wants in |
| /// order to make the conservative choice and preserve soundness. |
| #[instrument(level = "debug", skip(tcx, body))] |
| pub(super) fn borrow_conflicts_with_place<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| body: &Body<'tcx>, |
| borrow_place: Place<'tcx>, |
| borrow_kind: BorrowKind, |
| access_place: PlaceRef<'tcx>, |
| access: AccessDepth, |
| bias: PlaceConflictBias, |
| ) -> bool { |
| // This Local/Local case is handled by the more general code below, but |
| // it's so common that it's a speed win to check for it first. |
| if let Some(l1) = borrow_place.as_local() && let Some(l2) = access_place.as_local() { |
| return l1 == l2; |
| } |
| |
| place_components_conflict(tcx, body, borrow_place, borrow_kind, access_place, access, bias) |
| } |
| |
| fn place_components_conflict<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| body: &Body<'tcx>, |
| borrow_place: Place<'tcx>, |
| borrow_kind: BorrowKind, |
| access_place: PlaceRef<'tcx>, |
| access: AccessDepth, |
| bias: PlaceConflictBias, |
| ) -> bool { |
| // The borrowck rules for proving disjointness are applied from the "root" of the |
| // borrow forwards, iterating over "similar" projections in lockstep until |
| // we can prove overlap one way or another. Essentially, we treat `Overlap` as |
| // a monoid and report a conflict if the product ends up not being `Disjoint`. |
| // |
| // At each step, if we didn't run out of borrow or place, we know that our elements |
| // have the same type, and that they only overlap if they are the identical. |
| // |
| // For example, if we are comparing these: |
| // BORROW: (*x1[2].y).z.a |
| // ACCESS: (*x1[i].y).w.b |
| // |
| // Then our steps are: |
| // x1 | x1 -- places are the same |
| // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ) |
| // x1[2].y | x1[i].y -- equal or disjoint |
| // *x1[2].y | *x1[i].y -- equal or disjoint |
| // (*x1[2].y).z | (*x1[i].y).w -- we are disjoint and don't need to check more! |
| // |
| // Because `zip` does potentially bad things to the iterator inside, this loop |
| // also handles the case where the access might be a *prefix* of the borrow, e.g. |
| // |
| // BORROW: (*x1[2].y).z.a |
| // ACCESS: x1[i].y |
| // |
| // Then our steps are: |
| // x1 | x1 -- places are the same |
| // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ) |
| // x1[2].y | x1[i].y -- equal or disjoint |
| // |
| // -- here we run out of access - the borrow can access a part of it. If this |
| // is a full deep access, then we *know* the borrow conflicts with it. However, |
| // if the access is shallow, then we can proceed: |
| // |
| // x1[2].y | (*x1[i].y) -- a deref! the access can't get past this, so we |
| // are disjoint |
| // |
| // Our invariant is, that at each step of the iteration: |
| // - If we didn't run out of access to match, our borrow and access are comparable |
| // and either equal or disjoint. |
| // - If we did run out of access, the borrow can access a part of it. |
| |
| let borrow_local = borrow_place.local; |
| let access_local = access_place.local; |
| |
| match place_base_conflict(borrow_local, access_local) { |
| Overlap::Arbitrary => { |
| bug!("Two base can't return Arbitrary"); |
| } |
| Overlap::EqualOrDisjoint => { |
| // This is the recursive case - proceed to the next element. |
| } |
| Overlap::Disjoint => { |
| // We have proven the borrow disjoint - further |
| // projections will remain disjoint. |
| debug!("borrow_conflicts_with_place: disjoint"); |
| return false; |
| } |
| } |
| |
| // loop invariant: borrow_c is always either equal to access_c or disjoint from it. |
| for (i, (borrow_c, &access_c)) in |
| iter::zip(borrow_place.projection, access_place.projection).enumerate() |
| { |
| debug!(?borrow_c, ?access_c); |
| |
| let borrow_proj_base = &borrow_place.projection[..i]; |
| |
| // Borrow and access path both have more components. |
| // |
| // Examples: |
| // |
| // - borrow of `a.(...)`, access to `a.(...)` |
| // - borrow of `a.(...)`, access to `b.(...)` |
| // |
| // Here we only see the components we have checked so |
| // far (in our examples, just the first component). We |
| // check whether the components being borrowed vs |
| // accessed are disjoint (as in the second example, |
| // but not the first). |
| match place_projection_conflict( |
| tcx, |
| body, |
| borrow_local, |
| borrow_proj_base, |
| borrow_c, |
| access_c, |
| bias, |
| ) { |
| Overlap::Arbitrary => { |
| // We have encountered different fields of potentially |
| // the same union - the borrow now partially overlaps. |
| // |
| // There is no *easy* way of comparing the fields |
| // further on, because they might have different types |
| // (e.g., borrows of `u.a.0` and `u.b.y` where `.0` and |
| // `.y` come from different structs). |
| // |
| // We could try to do some things here - e.g., count |
| // dereferences - but that's probably not a good |
| // idea, at least for now, so just give up and |
| // report a conflict. This is unsafe code anyway so |
| // the user could always use raw pointers. |
| debug!("arbitrary -> conflict"); |
| return true; |
| } |
| Overlap::EqualOrDisjoint => { |
| // This is the recursive case - proceed to the next element. |
| } |
| Overlap::Disjoint => { |
| // We have proven the borrow disjoint - further |
| // projections will remain disjoint. |
| debug!("disjoint"); |
| return false; |
| } |
| } |
| } |
| |
| if borrow_place.projection.len() > access_place.projection.len() { |
| for (i, elem) in borrow_place.projection[access_place.projection.len()..].iter().enumerate() |
| { |
| // Borrow path is longer than the access path. Examples: |
| // |
| // - borrow of `a.b.c`, access to `a.b` |
| // |
| // Here, we know that the borrow can access a part of |
| // our place. This is a conflict if that is a part our |
| // access cares about. |
| |
| let proj_base = &borrow_place.projection[..access_place.projection.len() + i]; |
| let base_ty = Place::ty_from(borrow_local, proj_base, body, tcx).ty; |
| |
| match (elem, &base_ty.kind(), access) { |
| (_, _, Shallow(Some(ArtificialField::ArrayLength))) |
| | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => { |
| // The array length is like additional fields on the |
| // type; it does not overlap any existing data there. |
| // Furthermore, if cannot actually be a prefix of any |
| // borrowed place (at least in MIR as it is currently.) |
| // |
| // e.g., a (mutable) borrow of `a[5]` while we read the |
| // array length of `a`. |
| debug!("borrow_conflicts_with_place: implicit field"); |
| return false; |
| } |
| |
| (ProjectionElem::Deref, _, Shallow(None)) => { |
| // e.g., a borrow of `*x.y` while we shallowly access `x.y` or some |
| // prefix thereof - the shallow access can't touch anything behind |
| // the pointer. |
| debug!("borrow_conflicts_with_place: shallow access behind ptr"); |
| return false; |
| } |
| (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Not), _) => { |
| // Shouldn't be tracked |
| bug!("Tracking borrow behind shared reference."); |
| } |
| (ProjectionElem::Deref, ty::Ref(_, _, hir::Mutability::Mut), AccessDepth::Drop) => { |
| // Values behind a mutable reference are not access either by dropping a |
| // value, or by StorageDead |
| debug!("borrow_conflicts_with_place: drop access behind ptr"); |
| return false; |
| } |
| |
| (ProjectionElem::Field { .. }, ty::Adt(def, _), AccessDepth::Drop) => { |
| // Drop can read/write arbitrary projections, so places |
| // conflict regardless of further projections. |
| if def.has_dtor(tcx) { |
| return true; |
| } |
| } |
| |
| (ProjectionElem::Deref, _, Deep) |
| | (ProjectionElem::Deref, _, AccessDepth::Drop) |
| | (ProjectionElem::Field { .. }, _, _) |
| | (ProjectionElem::Index { .. }, _, _) |
| | (ProjectionElem::ConstantIndex { .. }, _, _) |
| | (ProjectionElem::Subslice { .. }, _, _) |
| | (ProjectionElem::OpaqueCast { .. }, _, _) |
| | (ProjectionElem::Downcast { .. }, _, _) => { |
| // Recursive case. This can still be disjoint on a |
| // further iteration if this a shallow access and |
| // there's a deref later on, e.g., a borrow |
| // of `*x.y` while accessing `x`. |
| } |
| } |
| } |
| } |
| |
| // Borrow path ran out but access path may not |
| // have. Examples: |
| // |
| // - borrow of `a.b`, access to `a.b.c` |
| // - borrow of `a.b`, access to `a.b` |
| // |
| // In the first example, where we didn't run out of |
| // access, the borrow can access all of our place, so we |
| // have a conflict. |
| // |
| // If the second example, where we did, then we still know |
| // that the borrow can access a *part* of our place that |
| // our access cares about, so we still have a conflict. |
| if borrow_kind == BorrowKind::Shallow |
| && borrow_place.projection.len() < access_place.projection.len() |
| { |
| debug!("borrow_conflicts_with_place: shallow borrow"); |
| false |
| } else { |
| debug!("borrow_conflicts_with_place: full borrow, CONFLICT"); |
| true |
| } |
| } |
| |
| // Given that the bases of `elem1` and `elem2` are always either equal |
| // or disjoint (and have the same type!), return the overlap situation |
| // between `elem1` and `elem2`. |
| fn place_base_conflict(l1: Local, l2: Local) -> Overlap { |
| if l1 == l2 { |
| // the same local - base case, equal |
| debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL"); |
| Overlap::EqualOrDisjoint |
| } else { |
| // different locals - base case, disjoint |
| debug!("place_element_conflict: DISJOINT-LOCAL"); |
| Overlap::Disjoint |
| } |
| } |
| |
| // Given that the bases of `elem1` and `elem2` are always either equal |
| // or disjoint (and have the same type!), return the overlap situation |
| // between `elem1` and `elem2`. |
| fn place_projection_conflict<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| body: &Body<'tcx>, |
| pi1_local: Local, |
| pi1_proj_base: &[PlaceElem<'tcx>], |
| pi1_elem: PlaceElem<'tcx>, |
| pi2_elem: PlaceElem<'tcx>, |
| bias: PlaceConflictBias, |
| ) -> Overlap { |
| match (pi1_elem, pi2_elem) { |
| (ProjectionElem::Deref, ProjectionElem::Deref) => { |
| // derefs (e.g., `*x` vs. `*x`) - recur. |
| debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF"); |
| Overlap::EqualOrDisjoint |
| } |
| (ProjectionElem::OpaqueCast(v1), ProjectionElem::OpaqueCast(v2)) => { |
| if v1 == v2 { |
| // same type - recur. |
| debug!("place_element_conflict: DISJOINT-OR-EQ-OPAQUE"); |
| Overlap::EqualOrDisjoint |
| } else { |
| // Different types. Disjoint! |
| debug!("place_element_conflict: DISJOINT-OPAQUE"); |
| Overlap::Disjoint |
| } |
| } |
| (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => { |
| if f1 == f2 { |
| // same field (e.g., `a.y` vs. `a.y`) - recur. |
| debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD"); |
| Overlap::EqualOrDisjoint |
| } else { |
| let ty = Place::ty_from(pi1_local, pi1_proj_base, body, tcx).ty; |
| if ty.is_union() { |
| // Different fields of a union, we are basically stuck. |
| debug!("place_element_conflict: STUCK-UNION"); |
| Overlap::Arbitrary |
| } else { |
| // Different fields of a struct (`a.x` vs. `a.y`). Disjoint! |
| debug!("place_element_conflict: DISJOINT-FIELD"); |
| Overlap::Disjoint |
| } |
| } |
| } |
| (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => { |
| // different variants are treated as having disjoint fields, |
| // even if they occupy the same "space", because it's |
| // impossible for 2 variants of the same enum to exist |
| // (and therefore, to be borrowed) at the same time. |
| // |
| // Note that this is different from unions - we *do* allow |
| // this code to compile: |
| // |
| // ``` |
| // fn foo(x: &mut Result<i32, i32>) { |
| // let mut v = None; |
| // if let Ok(ref mut a) = *x { |
| // v = Some(a); |
| // } |
| // // here, you would *think* that the |
| // // *entirety* of `x` would be borrowed, |
| // // but in fact only the `Ok` variant is, |
| // // so the `Err` variant is *entirely free*: |
| // if let Err(ref mut a) = *x { |
| // v = Some(a); |
| // } |
| // drop(v); |
| // } |
| // ``` |
| if v1 == v2 { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-FIELD"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::Index(..), |
| ProjectionElem::Index(..) |
| | ProjectionElem::ConstantIndex { .. } |
| | ProjectionElem::Subslice { .. }, |
| ) |
| | ( |
| ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. }, |
| ProjectionElem::Index(..), |
| ) => { |
| // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint |
| // (if the indexes differ) or equal (if they are the same). |
| match bias { |
| PlaceConflictBias::Overlap => { |
| // If we are biased towards overlapping, then this is the recursive |
| // case that gives "equal *or* disjoint" its meaning. |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX"); |
| Overlap::EqualOrDisjoint |
| } |
| PlaceConflictBias::NoOverlap => { |
| // If we are biased towards no overlapping, then this is disjoint. |
| debug!("place_element_conflict: DISJOINT-ARRAY-INDEX"); |
| Overlap::Disjoint |
| } |
| } |
| } |
| ( |
| ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: false }, |
| ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: false }, |
| ) |
| | ( |
| ProjectionElem::ConstantIndex { offset: o1, min_length: _, from_end: true }, |
| ProjectionElem::ConstantIndex { offset: o2, min_length: _, from_end: true }, |
| ) => { |
| if o1 == o2 { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::ConstantIndex { |
| offset: offset_from_begin, |
| min_length: min_length1, |
| from_end: false, |
| }, |
| ProjectionElem::ConstantIndex { |
| offset: offset_from_end, |
| min_length: min_length2, |
| from_end: true, |
| }, |
| ) |
| | ( |
| ProjectionElem::ConstantIndex { |
| offset: offset_from_end, |
| min_length: min_length1, |
| from_end: true, |
| }, |
| ProjectionElem::ConstantIndex { |
| offset: offset_from_begin, |
| min_length: min_length2, |
| from_end: false, |
| }, |
| ) => { |
| // both patterns matched so it must be at least the greater of the two |
| let min_length = max(min_length1, min_length2); |
| // `offset_from_end` can be in range `[1..min_length]`, 1 indicates the last |
| // element (like -1 in Python) and `min_length` the first. |
| // Therefore, `min_length - offset_from_end` gives the minimal possible |
| // offset from the beginning |
| if offset_from_begin >= min_length - offset_from_end { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-FE"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-FE"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }, |
| ProjectionElem::Subslice { from, to, from_end: false }, |
| ) |
| | ( |
| ProjectionElem::Subslice { from, to, from_end: false }, |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }, |
| ) => { |
| if (from..to).contains(&offset) { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }, |
| ProjectionElem::Subslice { from, .. }, |
| ) |
| | ( |
| ProjectionElem::Subslice { from, .. }, |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }, |
| ) => { |
| if offset >= from { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true }, |
| ProjectionElem::Subslice { to, from_end: true, .. }, |
| ) |
| | ( |
| ProjectionElem::Subslice { to, from_end: true, .. }, |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true }, |
| ) => { |
| if offset > to { |
| debug!( |
| "place_element_conflict: \ |
| DISJOINT-OR-EQ-SLICE-CONSTANT-INDEX-SUBSLICE-FE" |
| ); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-SLICE-CONSTANT-INDEX-SUBSLICE-FE"); |
| Overlap::Disjoint |
| } |
| } |
| ( |
| ProjectionElem::Subslice { from: f1, to: t1, from_end: false }, |
| ProjectionElem::Subslice { from: f2, to: t2, from_end: false }, |
| ) => { |
| if f2 >= t1 || f1 >= t2 { |
| debug!("place_element_conflict: DISJOINT-ARRAY-SUBSLICES"); |
| Overlap::Disjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES"); |
| Overlap::EqualOrDisjoint |
| } |
| } |
| (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-SLICE-SUBSLICES"); |
| Overlap::EqualOrDisjoint |
| } |
| ( |
| ProjectionElem::Deref |
| | ProjectionElem::Field(..) |
| | ProjectionElem::Index(..) |
| | ProjectionElem::ConstantIndex { .. } |
| | ProjectionElem::OpaqueCast { .. } |
| | ProjectionElem::Subslice { .. } |
| | ProjectionElem::Downcast(..), |
| _, |
| ) => bug!( |
| "mismatched projections in place_element_conflict: {:?} and {:?}", |
| pi1_elem, |
| pi2_elem |
| ), |
| } |
| } |
