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|
A mutable variable is used but it is already captured by a closure.
Erroneous code example:
```compile_fail,E0501
fn inside_closure(x: &mut i32) {
// Actions which require unique access
}
fn outside_closure(x: &mut i32) {
// Actions which require unique access
}
fn foo(a: &mut i32) {
let mut bar = || {
inside_closure(a)
};
outside_closure(a); // error: cannot borrow `*a` as mutable because previous
// closure requires unique access.
bar();
}
```
This error indicates that a mutable variable is used while it is still captured
by a closure. Because the closure has borrowed the variable, it is not available
until the closure goes out of scope.
Note that a capture will either move or borrow a variable, but in this
situation, the closure is borrowing the variable. Take a look at the chapter
on [Capturing][capturing] in Rust By Example for more information.
[capturing]: https://doc.rust-lang.org/stable/rust-by-example/fn/closures/capture.html
To fix this error, you can finish using the closure before using the captured
variable:
```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}
fn foo(a: &mut i32) {
let mut bar = || {
inside_closure(a)
};
bar();
// borrow on `a` ends.
outside_closure(a); // ok!
}
```
Or you can pass the variable as a parameter to the closure:
```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}
fn foo(a: &mut i32) {
let mut bar = |s: &mut i32| {
inside_closure(s)
};
outside_closure(a);
bar(a);
}
```
It may be possible to define the closure later:
```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}
fn foo(a: &mut i32) {
outside_closure(a);
let mut bar = || {
inside_closure(a)
};
bar();
}
```
|
