Rust Deref Coercion 🦀 simplifying Borrowing and Dereferencing
Published: 2023-05-17 13:27:32

If you are a newbie or a experienced rust developer you must have come across a special term which represent a amazing features of which is `deref coercion` 

## Introduction:
Rust is a powerful and expressive programming language known for its strong emphasis on memory safety and ownership. One of the language's notable features is deref coercion, which allows Rust to automatically convert types through dereference operations. Deref coercion plays a vital role in simplifying borrowing and dereferencing, making code more concise and readable. In this article, we will explore the concept of deref coercion, its benefits, and how it contributes to Rust's safety guarantees.

## Understanding Deref and Borrowing:
Before diving into deref coercion, it's essential to understand the concepts of borrowing and dereferencing in Rust. 

Borrowing is a mechanism that allows temporary access to a value without taking ownership. A borrow can be of two types only

- mutable borrow `&val`
- non mutable borrow `&mut val`

In Rust, we can borrow a value using references denoted by the `&` symbol. 
```rust
let mut val = 32;
let b = &val; // create a borrow of `val`
```
On the other hand, dereferencing `*` refers to the process of accessing the value behind a reference or a pointer.
```rust
let b = &val; 
let mut k = *b; //original value read is "32" here 
``` 

## Deref Coercion

Deref coercion is Rust's ability to automatically apply dereference operations when calling methods or functions. 

It allows the programmer to write code that operates on a value directly, even if they only have a reference to it. 
Deref coercion is achieved through the `Deref` trait present std crate for the types which implement it 
```rust
fn call_me(val: &str) ... 

let mut boxed_type: Box<String> = Box::new("meow".to_string());

call_me(&boxed_type);
```
observe the `boxed_type` is a different type altogther yet we allowed to pass it as & to the method we are making the call for 

since `Box` type implement `Deref` trait which allows it to return the type of generic directly from the `deref_target` item type 

and since `&String` type implements the `Deref<Target = str>` we reach the `&str` directly

The compiler automatically inserts the necessary dereference operations, simplifying the code and reducing the need for explicit dereference syntax.

## Deref Trait and Operator Overloading

At the heart of deref coercion is the Deref trait. This trait is defined in the Rust standard library and allows types to customize the behavior of the dereference operator (*). By implementing the Deref trait for a type, we provide the necessary instructions on how to dereference instances of that type.

## Deref Coercion Rules:
Rust follows specific rules for deref coercion to ensure type safety. When the compiler encounters a type mismatch, it tries to find a series of Deref trait implementations that can bridge the gap between the expected and actual types. The rules for deref coercion are as follows:

If the type T implements the Deref trait to produce type U, and U implements the Deref trait to produce type V, Rust will automatically dereference T to get a value of type V when needed.

Deref coercion only applies to cases where the target type is a reference (&T). It does not work for owned types like Box<T> or Vec<T>. The goal is to avoid unnecessary cloning or moving of values.

## Benefits of Deref Coercion:
Deref coercion brings several benefits to Rust programming:

Improved Readability: By allowing implicit dereferencing, deref coercion simplifies code by reducing the need for explicit dereference operations. This leads to more readable and concise code.

Reduced Boilerplate: Deref coercion eliminates the need for manually writing dereference syntax when working with types that implement the Deref trait. This reduces unnecessary verbosity and boilerplate code.

Flexibility and Interoperability: Deref coercion enables seamless interoperability between types that can be dereferenced. It allows different types to be used interchangeably, as long as they implement the necessary Deref trait.

Safety and Type Checking: Rust's strong type system ensures that deref coercion follows a set of rules, preventing unsafe conversions. The compiler performs static type checking to ensure that the necessary Deref trait implementations exist and are valid.
 

## Conclusion:
Deref coercion is a powerful feature in Rust that simplifies borrowing and dereferencing. By leveraging the Deref trait and compiler rules, Rust can automatically apply dereference operations when needed, reducing code verbosity and improving code readability.