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|
//! 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::ConstantIndex { .. }
| ProjectionElem::Index(_) => {
cursor = cursor_base;
continue 'cursor;
}
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."),
}
}
}
}
}
}
|
