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A value was used after it was mutably borrowed.
Erroneous code example:
```compile_fail,E0503
fn main() {
let mut value = 3;
// Create a mutable borrow of `value`.
let borrow = &mut value;
let _sum = value + 1; // error: cannot use `value` because
// it was mutably borrowed
println!("{}", borrow);
}
```
In this example, `value` is mutably borrowed by `borrow` and cannot be
used to calculate `sum`. This is not possible because this would violate
Rust's mutability rules.
You can fix this error by finishing using the borrow before the next use of
the value:
```
fn main() {
let mut value = 3;
let borrow = &mut value;
println!("{}", borrow);
// The block has ended and with it the borrow.
// You can now use `value` again.
let _sum = value + 1;
}
```
Or by cloning `value` before borrowing it:
```
fn main() {
let mut value = 3;
// We clone `value`, creating a copy.
let value_cloned = value.clone();
// The mutable borrow is a reference to `value` and
// not to `value_cloned`...
let borrow = &mut value;
// ... which means we can still use `value_cloned`,
let _sum = value_cloned + 1;
// even though the borrow only ends here.
println!("{}", borrow);
}
```
For more information on Rust's ownership system, take a look at the
[References & Borrowing][references-and-borrowing] section of the Book.
[references-and-borrowing]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
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