| // Copyright 2018 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| use borrow_check::ArtificialField; |
| use borrow_check::Overlap; |
| use borrow_check::{Deep, Shallow, AccessDepth}; |
| use rustc::hir; |
| use rustc::mir::{BorrowKind, Mir, Place}; |
| use rustc::mir::{Projection, ProjectionElem}; |
| use rustc::ty::{self, TyCtxt}; |
| use std::cmp::max; |
| |
| pub(super) fn borrow_conflicts_with_place<'gcx, 'tcx>( |
| tcx: TyCtxt<'_, 'gcx, 'tcx>, |
| mir: &Mir<'tcx>, |
| borrow_place: &Place<'tcx>, |
| borrow_kind: BorrowKind, |
| access_place: &Place<'tcx>, |
| access: AccessDepth, |
| ) -> bool { |
| debug!( |
| "borrow_conflicts_with_place({:?},{:?},{:?})", |
| borrow_place, access_place, access |
| ); |
| |
| // 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 Place::Local(l1) = borrow_place { |
| if let Place::Local(l2) = access_place { |
| return l1 == l2; |
| } |
| } |
| |
| unroll_place(borrow_place, None, |borrow_components| { |
| unroll_place(access_place, None, |access_components| { |
| place_components_conflict( |
| tcx, |
| mir, |
| borrow_components, |
| borrow_kind, |
| access_components, |
| access |
| ) |
| }) |
| }) |
| } |
| |
| fn place_components_conflict<'gcx, 'tcx>( |
| tcx: TyCtxt<'_, 'gcx, 'tcx>, |
| mir: &Mir<'tcx>, |
| mut borrow_components: PlaceComponentsIter<'_, 'tcx>, |
| borrow_kind: BorrowKind, |
| mut access_components: PlaceComponentsIter<'_, 'tcx>, |
| access: AccessDepth, |
| ) -> 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. |
| loop { |
| // loop invariant: borrow_c is always either equal to access_c or disjoint from it. |
| if let Some(borrow_c) = borrow_components.next() { |
| debug!("borrow_conflicts_with_place: borrow_c = {:?}", borrow_c); |
| |
| if let Some(access_c) = access_components.next() { |
| debug!("borrow_conflicts_with_place: access_c = {:?}", access_c); |
| |
| // 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_element_conflict(tcx, mir, borrow_c, access_c) { |
| 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!("borrow_conflicts_with_place: 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!("borrow_conflicts_with_place: disjoint"); |
| return false; |
| } |
| } |
| } else { |
| // 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 (base, elem) = match borrow_c { |
| Place::Projection(box Projection { base, elem }) => (base, elem), |
| _ => bug!("place has no base?"), |
| }; |
| let base_ty = base.ty(mir, tcx).to_ty(tcx); |
| |
| match (elem, &base_ty.sty, access) { |
| (_, _, Shallow(Some(ArtificialField::Discriminant))) |
| | (_, _, Shallow(Some(ArtificialField::ArrayLength))) |
| | (_, _, Shallow(Some(ArtificialField::ShallowBorrow))) => { |
| // The discriminant and array length are like |
| // additional fields on the type; they do not |
| // overlap any existing data there. Furthermore, |
| // they 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::MutImmutable), _) => { |
| // Shouldn't be tracked |
| bug!("Tracking borrow behind shared reference."); |
| } |
| (ProjectionElem::Deref, ty::Ref(_, _, hir::MutMutable), AccessDepth::Drop) => { |
| // Values behind a mutatble 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::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`. |
| } |
| } |
| } |
| } else { |
| // 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 && access_components.next().is_some() { |
| debug!("borrow_conflicts_with_place: shallow borrow"); |
| return false; |
| } else { |
| debug!("borrow_conflicts_with_place: full borrow, CONFLICT"); |
| return true; |
| } |
| } |
| } |
| } |
| |
| /// A linked list of places running up the stack; begins with the |
| /// innermost place and extends to projections (e.g., `a.b` would have |
| /// the place `a` with a "next" pointer to `a.b`). Created by |
| /// `unroll_place`. |
| /// |
| /// NB: This particular impl strategy is not the most obvious. It was |
| /// chosen because it makes a measurable difference to NLL |
| /// performance, as this code (`borrow_conflicts_with_place`) is somewhat hot. |
| struct PlaceComponents<'p, 'tcx: 'p> { |
| component: &'p Place<'tcx>, |
| next: Option<&'p PlaceComponents<'p, 'tcx>>, |
| } |
| |
| impl<'p, 'tcx> PlaceComponents<'p, 'tcx> { |
| /// Converts a list of `Place` components into an iterator; this |
| /// iterator yields up a never-ending stream of `Option<&Place>`. |
| /// These begin with the "innermst" place and then with each |
| /// projection therefrom. So given a place like `a.b.c` it would |
| /// yield up: |
| /// |
| /// ```notrust |
| /// Some(`a`), Some(`a.b`), Some(`a.b.c`), None, None, ... |
| /// ``` |
| fn iter(&self) -> PlaceComponentsIter<'_, 'tcx> { |
| PlaceComponentsIter { value: Some(self) } |
| } |
| } |
| |
| /// Iterator over components; see `PlaceComponents::iter` for more |
| /// information. |
| /// |
| /// NB: This is not a *true* Rust iterator -- the code above just |
| /// manually invokes `next`. This is because we (sometimes) want to |
| /// keep executing even after `None` has been returned. |
| struct PlaceComponentsIter<'p, 'tcx: 'p> { |
| value: Option<&'p PlaceComponents<'p, 'tcx>>, |
| } |
| |
| impl<'p, 'tcx> PlaceComponentsIter<'p, 'tcx> { |
| fn next(&mut self) -> Option<&'p Place<'tcx>> { |
| if let Some(&PlaceComponents { component, next }) = self.value { |
| self.value = next; |
| Some(component) |
| } else { |
| None |
| } |
| } |
| } |
| |
| /// Recursively "unroll" a place into a `PlaceComponents` list, |
| /// invoking `op` with a `PlaceComponentsIter`. |
| fn unroll_place<'tcx, R>( |
| place: &Place<'tcx>, |
| next: Option<&PlaceComponents<'_, 'tcx>>, |
| op: impl FnOnce(PlaceComponentsIter<'_, 'tcx>) -> R, |
| ) -> R { |
| match place { |
| Place::Projection(interior) => unroll_place( |
| &interior.base, |
| Some(&PlaceComponents { |
| component: place, |
| next, |
| }), |
| op, |
| ), |
| |
| Place::Promoted(_) | |
| Place::Local(_) | Place::Static(_) => { |
| let list = PlaceComponents { |
| component: place, |
| next, |
| }; |
| op(list.iter()) |
| } |
| } |
| } |
| |
| // 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_element_conflict<'a, 'gcx: 'tcx, 'tcx>( |
| tcx: TyCtxt<'a, 'gcx, 'tcx>, |
| mir: &Mir<'tcx>, |
| elem1: &Place<'tcx>, |
| elem2: &Place<'tcx>, |
| ) -> Overlap { |
| match (elem1, elem2) { |
| (Place::Local(l1), Place::Local(l2)) => { |
| 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 |
| } |
| } |
| (Place::Static(static1), Place::Static(static2)) => { |
| if static1.def_id != static2.def_id { |
| debug!("place_element_conflict: DISJOINT-STATIC"); |
| Overlap::Disjoint |
| } else if tcx.is_static(static1.def_id) == Some(hir::Mutability::MutMutable) { |
| // We ignore mutable statics - they can only be unsafe code. |
| debug!("place_element_conflict: IGNORE-STATIC-MUT"); |
| Overlap::Disjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-STATIC"); |
| Overlap::EqualOrDisjoint |
| } |
| } |
| (Place::Promoted(p1), Place::Promoted(p2)) => { |
| if p1.0 == p2.0 { |
| if let ty::Array(_, size) = p1.1.sty { |
| if size.unwrap_usize(tcx) == 0 { |
| // Ignore conflicts with promoted [T; 0]. |
| debug!("place_element_conflict: IGNORE-LEN-0-PROMOTED"); |
| return Overlap::Disjoint; |
| } |
| } |
| // the same promoted - base case, equal |
| debug!("place_element_conflict: DISJOINT-OR-EQ-PROMOTED"); |
| Overlap::EqualOrDisjoint |
| } else { |
| // different promoteds - base case, disjoint |
| debug!("place_element_conflict: DISJOINT-PROMOTED"); |
| Overlap::Disjoint |
| } |
| } |
| (Place::Local(_), Place::Promoted(_)) | (Place::Promoted(_), Place::Local(_)) | |
| (Place::Promoted(_), Place::Static(_)) | (Place::Static(_), Place::Promoted(_)) | |
| (Place::Local(_), Place::Static(_)) | (Place::Static(_), Place::Local(_)) => { |
| debug!("place_element_conflict: DISJOINT-STATIC-LOCAL-PROMOTED"); |
| Overlap::Disjoint |
| } |
| (Place::Projection(pi1), Place::Projection(pi2)) => { |
| 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::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 = pi1.base.ty(mir, tcx).to_ty(tcx); |
| match ty.sty { |
| ty::Adt(def, _) if def.is_union() => { |
| // Different fields of a union, we are basically stuck. |
| debug!("place_element_conflict: STUCK-UNION"); |
| Overlap::Arbitrary |
| } |
| _ => { |
| // 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::Index(..), ProjectionElem::ConstantIndex { .. }) |
| | (ProjectionElem::Index(..), ProjectionElem::Subslice { .. }) |
| | (ProjectionElem::ConstantIndex { .. }, ProjectionElem::Index(..)) |
| | (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), so this |
| // is the recursive case that gives "equal *or* disjoint" its meaning. |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-INDEX"); |
| Overlap::EqualOrDisjoint |
| } |
| (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, .. }) |
| | (ProjectionElem::Subslice {from, .. }, |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }) => { |
| if offset >= from { |
| 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: true }, |
| ProjectionElem::Subslice {from: _, to }) |
| | (ProjectionElem::Subslice {from: _, to }, |
| ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true }) => { |
| if offset > to { |
| debug!("place_element_conflict: \ |
| DISJOINT-OR-EQ-ARRAY-CONSTANT-INDEX-SUBSLICE-FE"); |
| Overlap::EqualOrDisjoint |
| } else { |
| debug!("place_element_conflict: DISJOINT-ARRAY-CONSTANT-INDEX-SUBSLICE-FE"); |
| Overlap::Disjoint |
| } |
| } |
| (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => { |
| debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY-SUBSLICES"); |
| Overlap::EqualOrDisjoint |
| } |
| (ProjectionElem::Deref, _) |
| | (ProjectionElem::Field(..), _) |
| | (ProjectionElem::Index(..), _) |
| | (ProjectionElem::ConstantIndex { .. }, _) |
| | (ProjectionElem::Subslice { .. }, _) |
| | (ProjectionElem::Downcast(..), _) => bug!( |
| "mismatched projections in place_element_conflict: {:?} and {:?}", |
| elem1, |
| elem2 |
| ), |
| } |
| } |
| (Place::Projection(_), _) | (_, Place::Projection(_)) => bug!( |
| "unexpected elements in place_element_conflict: {:?} and {:?}", |
| elem1, |
| elem2 |
| ), |
| } |
| } |
