patterns/idioms/pass-var-to-closure.md

# Pass variables to closure

## Description

By default, closures capture their environment by borrowing. Or you can use
`move`-closure to move whole environment. However, often you want to move just
some variables to closure, give it copy of some data, pass it by reference, or
perform some other transformation.

Use variable rebinding in separate scope for that.

## Example

Use

```rust
use std::rc::Rc;

let num1 = Rc::new(1);
let num2 = Rc::new(2);
let num3 = Rc::new(3);
let closure = {
    // `num1` is moved
    let num2 = num2.clone();  // `num2` is cloned
    let num3 = num3.as_ref();  // `num3` is borrowed
    move || {
        *num1 + *num2 + *num3;
    }
};
```

instead of

```rust
use std::rc::Rc;

let num1 = Rc::new(1);
let num2 = Rc::new(2);
let num3 = Rc::new(3);

let num2_cloned = num2.clone();
let num3_borrowed = num3.as_ref();
let closure = move || {
    *num1 + *num2_cloned + *num3_borrowed;
};
```

## Advantages

Copied data are grouped together with closure definition, so their purpose is
more clear and they will be dropped immediately even if they are not consumed
by closure.

Closure uses same variable names as surrounding code whether data are copied or
moved.

## Disadvantages

Additional indentation of closure body.