1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]
//! From the NLL RFC: "The deep [aka 'supporting'] prefixes for an
//! place are formed by stripping away fields and derefs, except that
//! we stop when we reach the deref of a shared reference. [...] "
//!
//! "Shallow prefixes are found by stripping away fields, but stop at
//! any dereference. So: writing a path like `a` is illegal if `a.b`
//! is borrowed. But: writing `a` is legal if `*a` is borrowed,
//! whether or not `a` is a shared or mutable reference. [...] "
use super::MirBorrowckCtxt;
use rustc_hir as hir;
use rustc_middle::mir::{Body, PlaceRef, ProjectionElem};
use rustc_middle::ty::{self, TyCtxt};
pub trait IsPrefixOf<'tcx> {
fn is_prefix_of(&self, other: PlaceRef<'tcx>) -> bool;
}
impl<'tcx> IsPrefixOf<'tcx> for PlaceRef<'tcx> {
fn is_prefix_of(&self, other: PlaceRef<'tcx>) -> bool {
self.local == other.local
&& self.projection.len() <= other.projection.len()
&& self.projection == &other.projection[..self.projection.len()]
}
}
pub(super) struct Prefixes<'cx, 'tcx> {
body: &'cx Body<'tcx>,
tcx: TyCtxt<'tcx>,
kind: PrefixSet,
next: Option<PlaceRef<'tcx>>,
}
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub(super) enum PrefixSet {
/// Doesn't stop until it returns the base case (a Local or
/// Static prefix).
All,
/// Stops at any dereference.
Shallow,
/// Stops at the deref of a shared reference.
Supporting,
}
impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
/// Returns an iterator over the prefixes of `place`
/// (inclusive) from longest to smallest, potentially
/// terminating the iteration early based on `kind`.
pub(super) fn prefixes(
&self,
place_ref: PlaceRef<'tcx>,
kind: PrefixSet,
) -> Prefixes<'cx, 'tcx> {
Prefixes { next: Some(place_ref), kind, body: self.body, tcx: self.infcx.tcx }
}
}
impl<'cx, 'tcx> Iterator for Prefixes<'cx, 'tcx> {
type Item = PlaceRef<'tcx>;
fn next(&mut self) -> Option<Self::Item> {
let mut cursor = self.next?;
// Post-processing `place`: Enqueue any remaining
// work. Also, `place` may not be a prefix itself, but
// may hold one further down (e.g., we never return
// downcasts here, but may return a base of a downcast).
'cursor: loop {
match cursor.last_projection() {
None => {
self.next = None;
return Some(cursor);
}
Some((cursor_base, elem)) => {
match elem {
ProjectionElem::Field(_ /*field*/, _ /*ty*/) => {
// FIXME: add union handling
self.next = Some(cursor_base);
return Some(cursor);
}
ProjectionElem::Downcast(..)
| ProjectionElem::Subslice { .. }
| ProjectionElem::OpaqueCast { .. }
| ProjectionElem::ConstantIndex { .. }
| ProjectionElem::Index(_) => {
cursor = cursor_base;
continue 'cursor;
}
ProjectionElem::Subtype(..) => {
panic!("Subtype projection is not allowed before borrow check")
}
ProjectionElem::Deref => {
// (handled below)
}
}
assert_eq!(elem, ProjectionElem::Deref);
match self.kind {
PrefixSet::Shallow => {
// Shallow prefixes are found by stripping away
// fields, but stop at *any* dereference.
// So we can just stop the traversal now.
self.next = None;
return Some(cursor);
}
PrefixSet::All => {
// All prefixes: just blindly enqueue the base
// of the projection.
self.next = Some(cursor_base);
return Some(cursor);
}
PrefixSet::Supporting => {
// Fall through!
}
}
assert_eq!(self.kind, PrefixSet::Supporting);
// Supporting prefixes: strip away fields and
// derefs, except we stop at the deref of a shared
// reference.
let ty = cursor_base.ty(self.body, self.tcx).ty;
match ty.kind() {
ty::RawPtr(_) | ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Not) => {
// don't continue traversing over derefs of raw pointers or shared
// borrows.
self.next = None;
return Some(cursor);
}
ty::Ref(_ /*rgn*/, _ /*ty*/, hir::Mutability::Mut) => {
self.next = Some(cursor_base);
return Some(cursor);
}
ty::Adt(..) if ty.is_box() => {
self.next = Some(cursor_base);
return Some(cursor);
}
_ => panic!("unknown type fed to Projection Deref."),
}
}
}
}
}
}