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Add test that our handling of projections hasn't gone too far:

overwriting one field should not allow reborrow of an unrelated field.
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pnkfelix committed Jun 21, 2019
1 parent 241ceed commit 3b8ef010309f732d4ab04e63ba454383b69db2de
@@ -0,0 +1,25 @@
// Double-check we didn't go too far with our resolution to issue
// #62007: assigning over a field projection (`list.1 = n;` in this
// case) should kill only borrows of `list.1`; `list.0` can *not*
// necessarily be borrowed on the next iteration through the loop.

#![allow(dead_code)]

struct List<T> {
value: T,
next: Option<Box<List<T>>>,
}

fn to_refs<'a, T>(mut list: (&'a mut List<T>, &'a mut List<T>)) -> Vec<&'a mut T> {
let mut result = vec![];
loop {
result.push(&mut (list.0).value); //~ ERROR cannot borrow `list.0.value` as mutable
if let Some(n) = (list.0).next.as_mut() { //~ ERROR cannot borrow `list.0.next` as mutable
list.1 = n;
} else {
return result;
}
}
}

fn main() {}
@@ -0,0 +1,27 @@
error[E0499]: cannot borrow `list.0.value` as mutable more than once at a time
--> $DIR/issue-62007-assign-differing-fields.rs:16:21
|
LL | fn to_refs<'a, T>(mut list: (&'a mut List<T>, &'a mut List<T>)) -> Vec<&'a mut T> {
| -- lifetime `'a` defined here
...
LL | result.push(&mut (list.0).value);
| ^^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
...
LL | return result;
| ------ returning this value requires that `list.0.value` is borrowed for `'a`

error[E0499]: cannot borrow `list.0.next` as mutable more than once at a time
--> $DIR/issue-62007-assign-differing-fields.rs:17:26
|
LL | fn to_refs<'a, T>(mut list: (&'a mut List<T>, &'a mut List<T>)) -> Vec<&'a mut T> {
| -- lifetime `'a` defined here
...
LL | if let Some(n) = (list.0).next.as_mut() {
| ^^^^^^^^^^^^^---------
| |
| mutable borrow starts here in previous iteration of loop
| argument requires that `list.0.next` is borrowed for `'a`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0499`.

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