Kill conflicting borrows of places with projections.

Resolves rust-lang#62007.

Due to a bug, the previous version of this check did not actually kill
any conflicting borrows unless the borrowed place had no projections.
Specifically, `entry_set` will always be empty when `statement_effect`
is called. It does not contain the set of borrows which are live at this
point in the program.

src/librustc_mir/dataflow/impls/borrows.rs

@@ -193,43 +193,38 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
  place: &Place<'tcx>
  ) {
  debug!("kill_borrows_on_place: place={:?}", place);
- // Handle the `Place::Local(..)` case first and exit early.
- if let Place::Base(PlaceBase::Local(local)) = place {
- if let Some(borrow_indices) = self.borrow_set.local_map.get(&local) {
- debug!("kill_borrows_on_place: borrow_indices={:?}", borrow_indices);
- sets.kill_all(borrow_indices);
+
+ if let Some(local) = place.base_local() {
+ let other_borrows_of_local = self
+ .borrow_set
+ .local_map
+ .get(&local)
+ .into_iter()
+ .flat_map(|bs| bs.into_iter());
+
+ // If the borrowed place is a local with no projections, all other borrows of this
+ // local must conflict. This is purely an optimization so we don't have to call
+ // `places_conflict` for every borrow.
+ if let Place::Base(PlaceBase::Local(_)) = place {
+ sets.kill_all(other_borrows_of_local);
  return;
  }
- }
-
- // Otherwise, look at all borrows that are live and if they conflict with the assignment
- // into our place then we can kill them.
- let mut borrows = sets.on_entry.clone();
- let _ = borrows.union(sets.gen_set);
- for borrow_index in borrows.iter() {
- let borrow_data = &self.borrows()[borrow_index];
- debug!(
- "kill_borrows_on_place: borrow_index={:?} borrow_data={:?}",
- borrow_index, borrow_data,
- );
 
  // By passing `PlaceConflictBias::NoOverlap`, we conservatively assume that any given
  // pair of array indices are unequal, so that when `places_conflict` returns true, we
  // will be assured that two places being compared definitely denotes the same sets of
  // locations.
- if places_conflict::places_conflict(
- self.tcx,
- self.body,
- &borrow_data.borrowed_place,
- place,
- places_conflict::PlaceConflictBias::NoOverlap,
- ) {
- debug!(
- "kill_borrows_on_place: (kill) borrow_index={:?} borrow_data={:?}",
- borrow_index, borrow_data,
- );
- sets.kill(borrow_index);
- }
+ let definitely_conflicting_borrows = other_borrows_of_local
+ .filter(|&&i| {
+ places_conflict::places_conflict(
+ self.tcx,
+ self.body,
+ &self.borrow_set.borrows[i].borrowed_place,
+ place,
+ places_conflict::PlaceConflictBias::NoOverlap)
+ });
+
+ sets.kill_all(definitely_conflicting_borrows);
  }
  }
 }

src/test/run-pass/borrowck/borrowck-borrow-of-projection-kills-other-borrows-issue-62007.rs

@@ -0,0 +1,21 @@
+// run-pass
+#![allow(dead_code)]
+
+struct List<T> {
+ value: T,
+ next: Option<Box<List<T>>>,
+}
+
+fn to_refs<T>(mut list: (&mut List<T>,)) -> Vec<&mut T> {
+ let mut result = vec![];
+ loop {
+ result.push(&mut (list.0).value);
+ if let Some(n) = (list.0).next.as_mut() {
+ list.0 = n;
+ } else {
+ return result;
+ }
+ }
+}
+
+fn main() {}