compiler/rustc_mir/src/borrow_check/prefixes.rs

   1 //! From the NLL RFC: "The deep [aka 'supporting'] prefixes for an
   2 //! place are formed by stripping away fields and derefs, except that
   3 //! we stop when we reach the deref of a shared reference. [...] "
   4 //!
   5 //! "Shallow prefixes are found by stripping away fields, but stop at
   6 //! any dereference. So: writing a path like `a` is illegal if `a.b`
   7 //! is borrowed. But: writing `a` is legal if `*a` is borrowed,
   8 //! whether or not `a` is a shared or mutable reference. [...] "
   9
  10 use super::MirBorrowckCtxt;
  11
  12 use rustc_hir as hir;
  13 use rustc_middle::mir::{Body, Place, PlaceRef, ProjectionElem};
  14 use rustc_middle::ty::{self, TyCtxt};
  15
  16 pub trait IsPrefixOf<'tcx> {
  17     fn is_prefix_of(&self, other: PlaceRef<'tcx>) -> bool;
  18 }
  19
  20 impl<'tcx> IsPrefixOf<'tcx> for PlaceRef<'tcx> {
  21     fn is_prefix_of(&self, other: PlaceRef<'tcx>) -> bool {
  22         self.local == other.local
  23             && self.projection.len() <= other.projection.len()
  24             && self.projection == &other.projection[..self.projection.len()]
  25     }
  26 }
  27
  28 pub(super) struct Prefixes<'cx, 'tcx> {
  29     body: &'cx Body<'tcx>,
  30     tcx: TyCtxt<'tcx>,
  31     kind: PrefixSet,
  32     next: Option<PlaceRef<'tcx>>,
  33 }
  34
  35 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  36 pub(super) enum PrefixSet {
  37     /// Doesn't stop until it returns the base case (a Local or
  38     /// Static prefix).
  39     All,
  40     /// Stops at any dereference.
  41     Shallow,
  42     /// Stops at the deref of a shared reference.
  43     Supporting,
  44 }
  45
  46 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
  47     /// Returns an iterator over the prefixes of `place`
  48     /// (inclusive) from longest to smallest, potentially
  49     /// terminating the iteration early based on `kind`.
  50     pub(super) fn prefixes(
  51         &self,
  52         place_ref: PlaceRef<'tcx>,
  53         kind: PrefixSet,
  54     ) -> Prefixes<'cx, 'tcx> {
  55         Prefixes { next: Some(place_ref), kind, body: self.body, tcx: self.infcx.tcx }
  56     }
  57 }
  58
  59 impl<'cx, 'tcx> Iterator for Prefixes<'cx, 'tcx> {
  60     type Item = PlaceRef<'tcx>;
  61     fn next(&mut self) -> Option<Self::Item> {
  62         let mut cursor = self.next?;
  63
  64         // Post-processing `place`: Enqueue any remaining
  65         // work. Also, `place` may not be a prefix itself, but
  66         // may hold one further down (e.g., we never return
  67         // downcasts here, but may return a base of a downcast).
  68
  69         'cursor: loop {
  70             match &cursor {
  71                 PlaceRef { local: _, projection: [] } => {
  72                     self.next = None;
  73                     return Some(cursor);
  74                 }
  75                 PlaceRef { local: _, projection: [proj_base @ .., elem] } => {
  76                     match elem {
  77                         ProjectionElem::Field(_ /*field*/, _ /*ty*/) => {
  78                             // FIXME: add union handling
  79                             self.next =
  80                                 Some(PlaceRef { local: cursor.local, projection: proj_base });
  81                             return Some(cursor);
  82                         }
  83                         ProjectionElem::Downcast(..)
  84                         | ProjectionElem::Subslice { .. }
  85                         | ProjectionElem::ConstantIndex { .. }
  86                         | ProjectionElem::Index(_) => {
  87                             cursor = PlaceRef { local: cursor.local, projection: proj_base };
  88                             continue 'cursor;
  89                         }
  90                         ProjectionElem::Deref => {
  91                             // (handled below)
  92                         }
  93                     }
  94
  95                     assert_eq!(*elem, ProjectionElem::Deref);
  96
  97                     match self.kind {
  98                         PrefixSet::Shallow => {
  99                             // Shallow prefixes are found by stripping away
 100                             // fields, but stop at *any* dereference.
 101                             // So we can just stop the traversal now.
 102                             self.next = None;
 103                             return Some(cursor);
 104                         }
 105                         PrefixSet::All => {
 106                             // All prefixes: just blindly enqueue the base
 107                             // of the projection.
 108                             self.next =
 109                                 Some(PlaceRef { local: cursor.local, projection: proj_base });
 110                             return Some(cursor);
 111                         }
 112                         PrefixSet::Supporting => {
 113                             // Fall through!
 114                         }
 115                     }
 116
 117                     assert_eq!(self.kind, PrefixSet::Supporting);
 118                     // Supporting prefixes: strip away fields and
 119                     // derefs, except we stop at the deref of a shared
 120                     // reference.
 121
 122                     let ty = Place::ty_from(cursor.local, proj_base, self.body, self.tcx).ty;
 123                     match ty.kind() {
 124                         ty::RawPtr(_) | ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Not) => {
 125                             // don't continue traversing over derefs of raw pointers or shared
 126                             // borrows.
 127                             self.next = None;
 128                             return Some(cursor);
 129                         }
 130
 131                         ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Mut) => {
 132                             self.next =
 133                                 Some(PlaceRef { local: cursor.local, projection: proj_base });
 134                             return Some(cursor);
 135                         }
 136
 137                         ty::Adt(..) if ty.is_box() => {
 138                             self.next =
 139                                 Some(PlaceRef { local: cursor.local, projection: proj_base });
 140                             return Some(cursor);
 141                         }
 142
 143                         _ => panic!("unknown type fed to Projection Deref."),
 144                     }
 145                 }
 146             }
 147         }
 148     }
 149 }