Understanding Rust Error E0501: Mutable variable already captured by closure

Introduction to Rust Error E0501

In Rust, Error E0501 occurs when a mutable variable is used while it is still captured by a closure. The closure might have borrowed the variable, making it unavailable until the closure goes out of scope. In this article, we will explore different scenarios of this error and their corresponding solutions.

Example of Rust Error E0501

Consider the following erroneous code example:

```rust
#![allow(unused)]
fn main() {
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();
}
}
```

Fixing Rust Error E0501

There are a few ways to resolve this error. Let's explore each method.

1. Finish using the closure before using the captured variable

In this solution, simply call and use the closure before accessing the captured mutable variable. Borrow ends after the closure.

```rust
#![allow(unused)]
fn main() {
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!
}
}
```

2. Pass the variable as a parameter to the closure

Instead of capturing the mutable variable in a closure, pass it as a parameter. This way, the closure will not maintain a borrow on the variable.

```rust
#![allow(unused)]
fn main() {
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);
}
}
```

3. Define the closure later

Another solution is to define the closure after using the borrowed mutable variable. This will ensure that the variable is no longer captured before usage.

```rust
#![allow(unused)]
fn main() {
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();
}
}
```