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. [...] "
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. [...] "
10 use super::MirBorrowckCtxt;
13 use rustc_middle::mir::{Body, Place, PlaceRef, ProjectionElem};
14 use rustc_middle::ty::{self, TyCtxt};
16 pub trait IsPrefixOf<'tcx> {
17 fn is_prefix_of(&self, other: PlaceRef<'tcx>) -> bool;
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()]
28 pub(super) struct Prefixes<'cx, 'tcx> {
29 body: &'cx Body<'tcx>,
32 next: Option<PlaceRef<'tcx>>,
35 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
37 pub(super) enum PrefixSet {
38 /// Doesn't stop until it returns the base case (a Local or
41 /// Stops at any dereference.
43 /// Stops at the deref of a shared reference.
47 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
48 /// Returns an iterator over the prefixes of `place`
49 /// (inclusive) from longest to smallest, potentially
50 /// terminating the iteration early based on `kind`.
51 pub(super) fn prefixes(
53 place_ref: PlaceRef<'tcx>,
55 ) -> Prefixes<'cx, 'tcx> {
56 Prefixes { next: Some(place_ref), kind, body: self.body, tcx: self.infcx.tcx }
60 impl<'cx, 'tcx> Iterator for Prefixes<'cx, 'tcx> {
61 type Item = PlaceRef<'tcx>;
62 fn next(&mut self) -> Option<Self::Item> {
63 let mut cursor = self.next?;
65 // Post-processing `place`: Enqueue any remaining
66 // work. Also, `place` may not be a prefix itself, but
67 // may hold one further down (e.g., we never return
68 // downcasts here, but may return a base of a downcast).
72 PlaceRef { local: _, projection: [] } => {
76 PlaceRef { local: _, projection: [proj_base @ .., elem] } => {
78 ProjectionElem::Field(_ /*field*/, _ /*ty*/) => {
79 // FIXME: add union handling
81 Some(PlaceRef { local: cursor.local, projection: proj_base });
84 ProjectionElem::Downcast(..)
85 | ProjectionElem::Subslice { .. }
86 | ProjectionElem::ConstantIndex { .. }
87 | ProjectionElem::Index(_) => {
88 cursor = PlaceRef { local: cursor.local, projection: proj_base };
91 ProjectionElem::Deref => {
96 assert_eq!(*elem, ProjectionElem::Deref);
99 PrefixSet::Shallow => {
100 // Shallow prefixes are found by stripping away
101 // fields, but stop at *any* dereference.
102 // So we can just stop the traversal now.
107 // All prefixes: just blindly enqueue the base
108 // of the projection.
110 Some(PlaceRef { local: cursor.local, projection: proj_base });
113 PrefixSet::Supporting => {
118 assert_eq!(self.kind, PrefixSet::Supporting);
119 // Supporting prefixes: strip away fields and
120 // derefs, except we stop at the deref of a shared
123 let ty = Place::ty_from(cursor.local, proj_base, self.body, self.tcx).ty;
125 ty::RawPtr(_) | ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Not) => {
126 // don't continue traversing over derefs of raw pointers or shared
132 ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Mut) => {
134 Some(PlaceRef { local: cursor.local, projection: proj_base });
138 ty::Adt(..) if ty.is_box() => {
140 Some(PlaceRef { local: cursor.local, projection: proj_base });
144 _ => panic!("unknown type fed to Projection Deref."),