compiler/rustc_borrowck/src/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, 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.last_projection() {
  71                 None => {
  72                     self.next = None;
  73                     return Some(cursor);
  74                 }
  75                 Some((cursor_base, elem)) => {
  76                     match elem {
  77                         ProjectionElem::Field(_ /*field*/, _ /*ty*/) => {
  78                             // FIXME: add union handling
  79                             self.next = Some(cursor_base);
  80                             return Some(cursor);
  81                         }
  82                         ProjectionElem::Downcast(..)
  83                         | ProjectionElem::Subslice { .. }
  84                         | ProjectionElem::ConstantIndex { .. }
  85                         | ProjectionElem::Index(_) => {
  86                             cursor = cursor_base;
  87                             continue 'cursor;
  88                         }
  89                         ProjectionElem::Deref => {
  90                             // (handled below)
  91                         }
  92                     }
  93
  94                     assert_eq!(elem, ProjectionElem::Deref);
  95
  96                     match self.kind {
  97                         PrefixSet::Shallow => {
  98                             // Shallow prefixes are found by stripping away
  99                             // fields, but stop at *any* dereference.
 100                             // So we can just stop the traversal now.
 101                             self.next = None;
 102                             return Some(cursor);
 103                         }
 104                         PrefixSet::All => {
 105                             // All prefixes: just blindly enqueue the base
 106                             // of the projection.
 107                             self.next = Some(cursor_base);
 108                             return Some(cursor);
 109                         }
 110                         PrefixSet::Supporting => {
 111                             // Fall through!
 112                         }
 113                     }
 114
 115                     assert_eq!(self.kind, PrefixSet::Supporting);
 116                     // Supporting prefixes: strip away fields and
 117                     // derefs, except we stop at the deref of a shared
 118                     // reference.
 119
 120                     let ty = cursor_base.ty(self.body, self.tcx).ty;
 121                     match ty.kind() {
 122                         ty::RawPtr(_) | ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Not) => {
 123                             // don't continue traversing over derefs of raw pointers or shared
 124                             // borrows.
 125                             self.next = None;
 126                             return Some(cursor);
 127                         }
 128
 129                         ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Mut) => {
 130                             self.next = Some(cursor_base);
 131                             return Some(cursor);
 132                         }
 133
 134                         ty::Adt(..) if ty.is_box() => {
 135                             self.next = Some(cursor_base);
 136                             return Some(cursor);
 137                         }
 138
 139                         _ => panic!("unknown type fed to Projection Deref."),
 140                     }
 141                 }
 142             }
 143         }
 144     }
 145 }