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Fallout from allowing some mutation in guards.

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For some reason, allowing restricted mutation in match arms exposed an
obvious case where a unique borrow can indeed fail, namely something
like:

```rust
match b {
    ...
    ref mut r if { (|| { let bar = &mut *r; **bar = false; })(); false } => { &mut *r }
    //                             ~~~~~~~
    //                                |
    // This ends up holding a `&unique` borrow of `r`, but there ends up being an
    // implicit shared borrow in the guard thanks to rust-lang/rust#49870
    ...
}
```
This commit is contained in:
Felix S. Klock II 2018-05-07 15:58:09 +02:00
parent 5c30dc85c2
commit 638acd300f
1 changed files with 10 additions and 8 deletions
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18
src/librustc_mir/borrow_check/mod.rs
View File

@ -1697,14 +1697,16 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
);
let mut error_reported = false;
match kind {
Reservation(WriteKind::MutableBorrow(BorrowKind::Unique))
| Write(WriteKind::MutableBorrow(BorrowKind::Unique)) => {
if let Err(_place_err) = self.is_mutable(place, LocalMutationIsAllowed::Yes) {
span_bug!(span, "&unique borrow for {:?} should not fail", place);
}
}
Reservation(WriteKind::MutableBorrow(BorrowKind::Mut { .. }))
| Write(WriteKind::MutableBorrow(BorrowKind::Mut { .. })) => {
Reservation(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Unique))
| Reservation(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Mut { .. }))
| Write(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Unique))
| Write(WriteKind::MutableBorrow(borrow_kind @ BorrowKind::Mut { .. })) =>
{
let is_local_mutation_allowed = match borrow_kind {
BorrowKind::Unique => LocalMutationIsAllowed::Yes,
BorrowKind::Mut { .. } => is_local_mutation_allowed,
BorrowKind::Shared => unreachable!(),
};
match self.is_mutable(place, is_local_mutation_allowed) {
Ok(root_place) => self.add_used_mut(root_place, flow_state),
Err(place_err) => {