src/librustc_mir/borrow_check/place_ext.rs

   1 use rustc::hir;
   2 use rustc::mir::ProjectionElem;
   3 use rustc::mir::{Mir, Place, PlaceBase, Mutability, Static, StaticKind};
   4 use rustc::ty::{self, TyCtxt};
   5 use crate::borrow_check::borrow_set::LocalsStateAtExit;
   6
   7 /// Extension methods for the `Place` type.
   8 crate trait PlaceExt<'tcx> {
   9     /// Returns `true` if we can safely ignore borrows of this place.
  10     /// This is true whenever there is no action that the user can do
  11     /// to the place `self` that would invalidate the borrow. This is true
  12     /// for borrows of raw pointer dereferents as well as shared references.
  13     fn ignore_borrow(
  14         &self,
  15         tcx: TyCtxt<'_, '_, 'tcx>,
  16         mir: &Mir<'tcx>,
  17         locals_state_at_exit: &LocalsStateAtExit,
  18         ) -> bool;
  19 }
  20
  21 impl<'tcx> PlaceExt<'tcx> for Place<'tcx> {
  22     fn ignore_borrow(
  23         &self,
  24         tcx: TyCtxt<'_, '_, 'tcx>,
  25         mir: &Mir<'tcx>,
  26         locals_state_at_exit: &LocalsStateAtExit,
  27     ) -> bool {
  28         match self {
  29             // If a local variable is immutable, then we only need to track borrows to guard
  30             // against two kinds of errors:
  31             // * The variable being dropped while still borrowed (e.g., because the fn returns
  32             //   a reference to a local variable)
  33             // * The variable being moved while still borrowed
  34             //
  35             // In particular, the variable cannot be mutated -- the "access checks" will fail --
  36             // so we don't have to worry about mutation while borrowed.
  37             Place::Base(PlaceBase::Local(index)) => {
  38                 match locals_state_at_exit {
  39                     LocalsStateAtExit::AllAreInvalidated => false,
  40                     LocalsStateAtExit::SomeAreInvalidated { has_storage_dead_or_moved } => {
  41                         let ignore = !has_storage_dead_or_moved.contains(*index) &&
  42                             mir.local_decls[*index].mutability == Mutability::Not;
  43                         debug!("ignore_borrow: local {:?} => {:?}", index, ignore);
  44                         ignore
  45                     }
  46                 }
  47             }
  48             Place::Base(PlaceBase::Static(box Static{ kind: StaticKind::Promoted(_), .. })) =>
  49                 false,
  50             Place::Base(PlaceBase::Static(box Static{ kind: StaticKind::Static(def_id), .. })) => {
  51                 tcx.is_mutable_static(*def_id)
  52             }
  53             Place::Projection(proj) => match proj.elem {
  54                 ProjectionElem::Field(..)
  55                 | ProjectionElem::Downcast(..)
  56                 | ProjectionElem::Subslice { .. }
  57                 | ProjectionElem::ConstantIndex { .. }
  58                 | ProjectionElem::Index(_) => proj.base.ignore_borrow(
  59                     tcx, mir, locals_state_at_exit),
  60
  61                 ProjectionElem::Deref => {
  62                     let ty = proj.base.ty(mir, tcx).ty;
  63                     match ty.sty {
  64                         // For both derefs of raw pointers and `&T`
  65                         // references, the original path is `Copy` and
  66                         // therefore not significant.  In particular,
  67                         // there is nothing the user can do to the
  68                         // original path that would invalidate the
  69                         // newly created reference -- and if there
  70                         // were, then the user could have copied the
  71                         // original path into a new variable and
  72                         // borrowed *that* one, leaving the original
  73                         // path unborrowed.
  74                         ty::RawPtr(..) | ty::Ref(_, _, hir::MutImmutable) => true,
  75                         _ => proj.base.ignore_borrow(tcx, mir, locals_state_at_exit),
  76                     }
  77                 }
  78             },
  79         }
  80     }
  81 }