src/librustc_mir/borrow_check/place_ext.rs

   1 use crate::borrow_check::borrow_set::LocalsStateAtExit;
   2 use rustc_hir as hir;
   3 use rustc_middle::mir::ProjectionElem;
   4 use rustc_middle::mir::{Body, Mutability, Place};
   5 use rustc_middle::ty::{self, TyCtxt};
   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         body: &Body<'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         body: &Body<'tcx>,
  26         locals_state_at_exit: &LocalsStateAtExit,
  27     ) -> bool {
  28         // If a local variable is immutable, then we only need to track borrows to guard
  29         // against two kinds of errors:
  30         // * The variable being dropped while still borrowed (e.g., because the fn returns
  31         //   a reference to a local variable)
  32         // * The variable being moved while still borrowed
  33         //
  34         // In particular, the variable cannot be mutated -- the "access checks" will fail --
  35         // so we don't have to worry about mutation while borrowed.
  36         if let LocalsStateAtExit::SomeAreInvalidated { has_storage_dead_or_moved } =
  37             locals_state_at_exit
  38         {
  39             let ignore = !has_storage_dead_or_moved.contains(self.local)
  40                 && body.local_decls[self.local].mutability == Mutability::Not;
  41             debug!("ignore_borrow: local {:?} => {:?}", self.local, ignore);
  42             if ignore {
  43                 return true;
  44             }
  45         }
  46
  47         for (i, elem) in self.projection.iter().enumerate() {
  48             let proj_base = &self.projection[..i];
  49
  50             if elem == ProjectionElem::Deref {
  51                 let ty = Place::ty_from(self.local, proj_base, body, tcx).ty;
  52                 match ty.kind {
  53                     ty::Ref(_, _, hir::Mutability::Not) if i == 0 => {
  54                         // For references to thread-local statics, we do need
  55                         // to track the borrow.
  56                         if body.local_decls[self.local].is_ref_to_thread_local() {
  57                             continue;
  58                         }
  59                         return true;
  60                     }
  61                     ty::RawPtr(..) | ty::Ref(_, _, hir::Mutability::Not) => {
  62                         // For both derefs of raw pointers and `&T`
  63                         // references, the original path is `Copy` and
  64                         // therefore not significant.  In particular,
  65                         // there is nothing the user can do to the
  66                         // original path that would invalidate the
  67                         // newly created reference -- and if there
  68                         // were, then the user could have copied the
  69                         // original path into a new variable and
  70                         // borrowed *that* one, leaving the original
  71                         // path unborrowed.
  72                         return true;
  73                     }
  74                     _ => {}
  75                 }
  76             }
  77         }
  78
  79         false
  80     }
  81 }