Rust Design Patterns

https://rust-unofficial.github.io/patterns
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Rust Design Patterns is a comprehensive guide to design patterns, idioms, and anti-patterns in Rust,...
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### Rust Struct with Methods and Example

Source: https://rust-unofficial.github.io/patterns/idioms/rustdoc-init.html

Demonstrates a Rust struct 'Connection' with a 'send_request' method and its associated example, including necessary boilerplate for testing.

```rust
struct Connection {
    name: String,
    stream: TcpStream,
}

impl Connection {
    /// Sends a request over the connection.
    ///
    /// # Example
    /// ```no_run
    /// # // Boilerplate are required to get an example working.
    /// # let stream = TcpStream::connect("127.0.0.1:34254");
    /// # let connection = Connection { name: "foo".to_owned(), stream };
    /// # let request = Request::new("RequestId", RequestType::Get, "payload");
    /// let response = connection.send_request(request);
    /// assert!(response.is_ok());
    /// ```
    fn send_request(&self, request: Request) -> Result<Status, SendErr> {
        // ...
    }

    /// Oh no, all that boilerplate needs to be repeated here!
    fn check_status(&self) -> Status {
        // ...
    }
}

```

--------------------------------

### Rust Struct with Helper Function for Examples

Source: https://rust-unofficial.github.io/patterns/idioms/rustdoc-init.html

Illustrates a Rust struct 'Connection' with a 'send_request' method, using a wrapping helper function to simplify examples and avoid repetitive boilerplate.

```rust
struct Connection {
    name: String,
    stream: TcpStream,
}

impl Connection {
    /// Sends a request over the connection.
    ///
    /// # Example
    /// ```
    /// # fn call_send(connection: Connection, request: Request) {
    /// let response = connection.send_request(request);
    /// assert!(response.is_ok());
    /// # }
    /// ```
    fn send_request(&self, request: Request) -> Result<Status, SendErr> {
        // ...
    }
}

```

--------------------------------

### Rust Serialization and Deserialization Example

Source: https://rust-unofficial.github.io/patterns/print.html

Demonstrates serializing a Rust struct to JSON and then deserializing it back using generated visitor types and macros.

```rust
fn main() {
    let a = TestStruct { a: 5, b: "hello".to_string() };
    let a_data = a.serialize().to_json();
    println!("Our Test Struct as JSON: {a_data}");
    let b = TestStruct::deserialize(
        generated_visitor_for!(TestStruct)::from_json(a_data));
}
```

--------------------------------

### Concordance Serialization/Deserialization Example

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

A basic Rust implementation for serializing and deserializing a custom `Concordance` struct to and from a String. This serves as a practical example for the Iso pattern.

```rust
#![allow(unused)]
fn main() {
use std::collections::HashMap;

struct Concordance {
    keys: HashMap<String, usize>,
    value_table: Vec<(usize, usize)> M
}

struct ConcordanceSerde {}

impl ConcordanceSerde {
    fn serialize(value: Concordance) -> String {
        todo!()
    }
    // invalid concordances are empty
    fn deserialize(value: String) -> Concordance {
        todo!()
    }
}
}
```

--------------------------------

### JSON Deserialization Example

Source: https://rust-unofficial.github.io/patterns/print.html

Demonstrates how to deserialize a JSON string into a Rust struct using Serde's derive macro.

```APIDOC
## JSON Deserialization Example

### Description

This example shows how to use Serde's `Deserialize` derive macro to automatically generate the implementation for deserializing a JSON string into a Rust struct.

### Request Body Example (JSON)

```json
{
  "name": "Alice",
  "customer_id": "12345"
}
```

### Rust Struct Definition

```rust
use serde::Deserialize;

#[derive(Deserialize)]
struct IdRecord {
    name: String,
    customer_id: String,
}
```

### Usage

To deserialize the JSON above into an `IdRecord` struct, you would typically use a JSON deserializer provided by Serde:

```rust
use serde::Deserialize;

#[derive(Deserialize)]
struct IdRecord {
    name: String,
    customer_id: String,
}

fn main() {
    let json_data = r#"{
        "name": "Alice",
        "customer_id": "12345"
    }"#;

    // Using serde_json::from_str to deserialize
    let record: Result<IdRecord, serde_json::Error> = serde_json::from_str(json_data);

    match record {
        Ok(id_record) => {
            println!("Name: {}", id_record.name);
            println!("Customer ID: {}", id_record.customer_id);
        }
        Err(e) => {
            eprintln!("Deserialization error: {}", e);
        }
    }
}
```

### Response Example (Success)

If deserialization is successful, you will have an `IdRecord` instance:

```rust
// Inside the main function, after successful deserialization:
// Name: Alice
// Customer ID: 12345
```
```

--------------------------------

### Embedded HAL Pin Mode Example

Source: https://rust-unofficial.github.io/patterns/print.html

This example from the `embedded-hal` ecosystem shows how a pin's generic type determines its usable functions based on its mode.

```Rust
struct PA0<MODE>
```

--------------------------------

### Using Serde Derive and Visitor Macros

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

Example demonstrating the usage of `serialize` and `deserialize` methods on `TestStruct` after applying `#[derive(Serde)]` and using a hypothetical `generated_visitor_for!` macro.

```rust
fn main() {
    let a = TestStruct { a: 5, b: "hello".to_string() };
    let a_data = a.serialize().to_json();
    println!("Our Test Struct as JSON: {a_data}");
    let b = TestStruct::deserialize(
        generated_visitor_for!(TestStruct)::from_json(a_data));
}

```

--------------------------------

### Hyper Client API Example

Source: https://rust-unofficial.github.io/patterns/print.html

The `hyper` HTTP client library uses this pattern to provide different methods and trait implementations for clients with various connectors.

```Rust
struct Client<C>
```

--------------------------------

### Rust Implementation of FromStr and ToString

Source: https://rust-unofficial.github.io/patterns/print.html

Example implementation of FromStr and ToString for a custom struct in Rust. Note the use of `todo!()` indicating incomplete implementation.

```rust
use anyhow;

use std::str::FromStr;

struct TestStruct {
    a: usize,
    b: String,
}

impl FromStr for TestStruct {
    type Err = anyhow::Error;
    fn from_str(s: &str) -> Result<TestStruct, Self::Err> {
        todo!()
    }
}

impl ToString for TestStruct {
    fn to_string(&self) -> String {
        todo!()
    }
}

fn main() {
    let a = TestStruct {
        a: 5,
        b: "hello".to_string(),
    };
    println!("Our Test Struct as JSON: {}", a.to_string());
}
```

--------------------------------

### Implement Strategy Pattern in Rust

Source: https://rust-unofficial.github.io/patterns/patterns/behavioural/strategy.html

Demonstrates the Strategy pattern using Rust traits and structs. This example shows how to create interchangeable algorithms for report formatting (Text and Json) that can be selected at runtime.

```Rust
use std::collections::HashMap;

type Data = HashMap<String, u32>;

trait Formatter {
    fn format(&self, data: &Data, buf: &mut String);
}

struct Report;

impl Report {
    // Write should be used but we kept it as String to ignore error handling
    fn generate<T: Formatter>(g: T, s: &mut String) {
        // backend operations...
        let mut data = HashMap::new();
        data.insert("one".to_string(), 1);
        data.insert("two".to_string(), 2);
        // generate report
        g.format(&data, s);
    }
}

struct Text;
impl Formatter for Text {
    fn format(&self, data: &Data, buf: &mut String) {
        for (k, v) in data {
            let entry = format!( K " {v}\n");
            buf.push_str(&entry);
        }
    }
}

struct Json;
impl Formatter for Json {
    fn format(&self, data: &Data, buf: &mut String) {
        buf.push('[');
        for (k, v) in data.into_iter() {
            let entry = format!(r#"{{"{}":"{}"}}"#, k, v);
            buf.push_str(&entry);
            buf.push(',');
        }
        if !data.is_empty() {
            buf.pop(); // remove extra , at the end
        }
        buf.push(']');
    }
}

fn main() {
    let mut s = String::from("");
    Report::generate(Text, &mut s);
    assert!(s.contains("one 1"));
    assert!(s.contains("two 2"));

    s.clear(); // reuse the same buffer
    Report::generate(Json, &mut s);
    assert!(s.contains(r#"{{"one":"1"}}"#));
    assert!(s.contains(r#"{{"two":"2"}}"#));
}

```

--------------------------------

### Java Inheritance Example

Source: https://rust-unofficial.github.io/patterns/anti_patterns/deref.html

Illustrates a common inheritance pattern in Java where a subclass inherits methods from a superclass.

```java
class Foo {
    void m() { ... }
}

class Bar extends Foo {}

public static void main(String[] args) {
    Bar b = new Bar();
    b.m();
}
```

--------------------------------

### Adding Protocol-Specific Fields (NFS Example)

Source: https://rust-unofficial.github.io/patterns/functional/generics-type-classes.html

Extending the struct to include protocol-specific fields like `mount_point` for NFS. This necessitates runtime checks in methods like `mount_point()`.

```Rust
struct FileDownloadRequest {
    file_name: PathBuf,
    authentication: AuthInfo,
    mount_point: Option<PathBuf>,
}

impl FileDownloadRequest {
    // ... other methods ...

    /// Gets an NFS mount point if this is an NFS request. Otherwise,
    /// return None.
    pub fn mount_point(&self) -> Option<&Path> {
        self.mount_point.as_ref()
    }
}
```

--------------------------------

### Implementing FromStr and ToString for TestStruct

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

An example of implementing `FromStr` and `ToString` traits for a custom `TestStruct` in Rust. Note that `todo!()` is used as a placeholder for actual implementation.

```rust
use anyhow;

use std::str::FromStr;

struct TestStruct {
    a: usize,
    b: String,
}

impl FromStr for TestStruct {
    type Err = anyhow::Error;
    fn from_str(s: &str) -> Result<TestStruct, Self::Err> {
        todo!()
    }
}

impl ToString for TestStruct {
    fn to_string(&self) -> String {
        todo!()
    }
}

fn main() {
    let a = TestStruct {
        a: 5,
        b: "hello".to_string(),
    };
    println!("Our Test Struct as JSON: {}", a.to_string());
}

```

--------------------------------

### Rust: Helper function for Connection and Request

Source: https://rust-unofficial.github.io/patterns/print.html

Use a wrapping helper function to simplify creating a `Connection` and `Request`, reducing boilerplate code in examples. This is useful when `no_run` is not strictly required but compile-time checks are desired.

```rust
struct Connection {
    name: String,
    stream: TcpStream,
}

impl Connection {
    /// Sends a request over the connection.
    ///
    /// # Example
    /// ```
    /// # fn call_send(connection: Connection, request: Request) {
    /// let response = connection.send_request(request);
    /// assert!(response.is_ok());
    /// # }
    /// ```
    fn send_request(&self, request: Request) -> Result<Status, SendErr> {
        // ...
    }
}
```

--------------------------------

### Derive Macro for Deserialize

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

Example of using the `#[derive(Deserialize)]` macro to automatically implement the `Deserialize` trait for a struct.

```APIDOC
## `#[derive(Deserialize)]` Macro

### Description

The `#[derive(Deserialize)]` macro automatically generates an implementation of the `Deserialize` trait for a struct or enum, simplifying the process of making types deserializable.

### Method

`#[derive(Deserialize)]`

### Endpoint

N/A (Attribute macro)

### Parameters

Applied to a struct or enum definition.

### Request Body

N/A

### Response

N/A

### Code Example

```rust
use serde::Deserialize;

#[derive(Deserialize)]
struct IdRecord {
    name: String,
    customer_id: String,
}
```
```

--------------------------------

### Compile-Time Error Example

Source: https://rust-unofficial.github.io/patterns/functional/generics-type-classes.html

Attempting to call a protocol-specific method (e.g., `mount_point()`) on a request type that does not support it (e.g., `FileDownloadRequest<Bootp>`) results in a compile-time error.

```Rust
fn main() {
    let mut socket = crate::bootp::listen()?;
    while let Some(request) = socket.next_request()? {
        match request.mount_point().as_ref() {
            "/secure" => socket.send("Access denied"),
            _ => {} // continue on...
        }
        // Rest of the code here
    }
}
```

--------------------------------

### Rust Struct with Serde Derive and Visitor Generation

Source: https://rust-unofficial.github.io/patterns/print.html

Example of a Rust struct using a `Serde` derive macro for automatic trait implementation and a macro to generate an associated visitor type.

```rust
#[derive(Default, Serde)] // the "Serde" derive creates the trait impl block
struct TestStruct {
    a: usize,
    b: String,
}

// user writes this macro to generate an associated visitor type
generate_visitor!(TestStruct);
```

--------------------------------

### Mutex Guard Implementation in Rust

Source: https://rust-unofficial.github.io/patterns/patterns/behavioural/RAII.html

This example demonstrates a simplified Mutex guard, showcasing RAII for resource management. It ensures that a mutex is unlocked when the guard goes out of scope. Access to the guarded data is mediated through the guard.

```rust
use std::ops::Deref;

struct Foo {}

struct Mutex<T> {
    // We keep a reference to our data: T here.
    //..
}

struct MutexGuard<'a, T: 'a> {
    data: &'a T,
    //..
}

// Locking the mutex is explicit.
impl<T> Mutex<T> {
    fn lock(&self) -> MutexGuard<T> {
        // Lock the underlying OS mutex.
        //..

        // MutexGuard keeps a reference to self
        MutexGuard {
            data: self,
            //..
        }
    }
}

// Destructor for unlocking the mutex.
impl<'a, T> Drop for MutexGuard<'a, T> {
    fn drop(&mut self) {
        // Unlock the underlying OS mutex.
        //..
    }
}

// Implementing Deref means we can treat MutexGuard like a pointer to T.
impl<'a, T> Deref for MutexGuard<'a, T> {
    type Target = T;

    fn deref(&self) -> &T {
        self.data
    }
}

fn baz(x: Mutex<Foo>) {
    let xx = x.lock();
    xx.foo(); // foo is a method on Foo.
              // The borrow checker ensures we can't store a reference to the underlying
              // Foo which will outlive the guard xx.

    // x is unlocked when we exit this function and xx's destructor is executed.
}
```

--------------------------------

### Concrete Folder Implementation: Renamer

Source: https://rust-unofficial.github.io/patterns/patterns/creational/fold.html

An example implementation of the `Folder` trait that renames all `Name` nodes to 'foo'. It utilizes default implementations for other node types.

```rust
use fold::*;
use ast::*;

struct Renamer;
impl Folder for Renamer {
    fn fold_name(&mut self, n: Box<Name>) -> Box<Name> {
        Box::new(Name { value: "foo".to_owned() })
    }
}
```

--------------------------------

### Implement Custom Display for String using Newtype

Source: https://rust-unofficial.github.io/patterns/patterns/behavioural/newtype.html

Use a Newtype to create a custom `Display` implementation for `String`, for example, to mask sensitive data like passwords. This example demonstrates overriding the `Display` trait for a `String` wrapper.

```Rust
use std::fmt::Display;

// Create Newtype Password to override the Display trait for String
struct Password(String);

impl Display for Password {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "****************")
    }
}

fn main() {
    let unsecured_password: String = "ThisIsMyPassword".to_string();
    let secured_password: Password = Password(unsecured_password.clone());
    println!("unsecured_password: {unsecured_password}");
    println!("secured_password: {secured_password}");
}
```

--------------------------------

### Rust Serde API Overview

Source: https://rust-unofficial.github.io/patterns/print.html

Explains the core components and their interactions within the Serde API.

```APIDOC
## Serde API Concepts

### Description

The Serde API in Rust is designed for efficient and generic serialization and deserialization of data structures. It achieves this through a combination of traits and a visitor pattern.

### Key Components

1.  **`Deserialize` Trait**: Implemented by types that can be deserialized from a data format. This is often generated via a derive macro.
2.  **`Deserializer` Trait**: Handles the specifics of parsing a data format. It drives the deserialization process by calling methods on a `Visitor`.
3.  **`Visitor` Trait**: Defines how to construct a data structure from a generic data model. Each data structure implements a `Visitor` to specify how it should be built.

### Workflow

-   A `Deserializer` is created for a specific data format (e.g., JSON).
-   A `Visitor` is created, tailored to the target data structure.
-   The `Deserializer` traverses the data, calling appropriate `Visitor` methods (e.g., `visit_bool`, `visit_u64`, `visit_str`).
-   The `Visitor` reconstructs the target data structure based on the data encountered.

### Example Data Structure Transformation

For a simple structure, the expected visitor pattern might involve:

1.  Beginning a map.
2.  Visiting string keys.
3.  Visiting map values (which could be other maps, lists, or primitives).
4.  Ending maps and lists.
5.  Storing the reconstructed data into the target structure's fields.
```

--------------------------------

### Deriving and Using the Default Trait in Rust

Source: https://rust-unofficial.github.io/patterns/idioms/default.html

Demonstrates how to automatically derive the `Default` trait for a struct where all fields implement `Default`. Shows how to create a default instance and partially initialize it.

```rust
use std::{path::PathBuf, time::Duration};

// note that we can simply auto-derive Default here.
#[derive(Default, Debug, PartialEq)]
struct MyConfiguration {
    // Option defaults to None
    output: Option<PathBuf>,
    // Vecs default to empty vector
    search_path: Vec<PathBuf>,
    // Duration defaults to zero time
    timeout: Duration,
    // bool defaults to false
    check: bool,
}

impl MyConfiguration {
    // add setters here
}

fn main() {
    // construct a new instance with default values
    let mut conf = MyConfiguration::default();
    // do something with conf here
    conf.check = true;
    println!("conf = {conf:#?}");

    // partial initialization with default values, creates the same instance
    let conf1 = MyConfiguration {
        check: true,
        ..Default::default()
    };
    assert_eq!(conf, conf1);
}
```

--------------------------------

### Struct with Derive Deserialize

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

Example of deriving the Deserialize trait for a struct using a macro. This automatically generates the necessary implementation for deserializing the struct from data.

```rust
use serde::Deserialize;

#[derive(Deserialize)]
struct IdRecord {
    name: String,
    customer_id: String,
}
```

--------------------------------

### Define AST Nodes for Folding

Source: https://rust-unofficial.github.io/patterns/patterns/creational/fold.html

Defines the abstract syntax tree (AST) nodes used in the folding example. These structures represent the data that will be processed.

```rust
mod ast {
    pub enum Stmt {
        Expr(Box<Expr>),
        Let(Box<Name>, Box<Expr>),
    }

    pub struct Name {
        value: String,
    }

    pub enum Expr {
        IntLit(i64),
        Add(Box<Expr>, Box<Expr>),
        Sub(Box<Expr>, Box<Expr>),
    }
}
```

--------------------------------

### Command Pattern Implementation in Rust

Source: https://rust-unofficial.github.io/patterns/patterns/behavioural/command.html

This snippet implements the Command pattern using a trait for migration operations. It includes structs for `CreateTable` and `AddField`, a `Schema` struct to manage commands, and a `main` function to demonstrate their usage. Use this for scenarios requiring deferred execution or undoable operations.

```rust
__
pub trait Migration {
    fn execute(&self) -> &str;
    fn rollback(&self) -> &str;
}

pub struct CreateTable;
impl Migration for CreateTable {
    fn execute(&self) -> &str {
        "create table"
    }
    fn rollback(&self) -> &str {
        "drop table"
    }
}

pub struct AddField;
impl Migration for AddField {
    fn execute(&self) -> &str {
        "add field"
    }
    fn rollback(&self) -> &str {
        "remove field"
    }
}

struct Schema {
    commands: Vec<Box<dyn Migration>>,
}

impl Schema {
    fn new() -> Self {
        Self { commands: vec![] }
    }

    fn add_migration(&mut self, cmd: Box<dyn Migration>) {
        self.commands.push(cmd);
    }

    fn execute(&self) -> Vec<&str> {
        self.commands.iter().map(|cmd| cmd.execute()).collect()
    }
    fn rollback(&self) -> Vec<&str> {
        self.commands
            .iter()
            .rev() // reverse iterator's direction
            .map(|cmd| cmd.rollback())
            .collect()
    }
}

fn main() {
    let mut schema = Schema::new();

    let cmd = Box::new(CreateTable);
    schema.add_migration(cmd);
    let cmd = Box::new(AddField);
    schema.add_migration(cmd);

    assert_eq!(vec!["create table", "add field"], schema.execute());
    assert_eq!(vec!["remove field", "drop table"], schema.rollback());
}

```

--------------------------------

### Declarative Summation with Fold in Rust

Source: https://rust-unofficial.github.io/patterns/functional/paradigms.html

Employ this declarative method using `fold` for a more functional style of summing numbers. It composes addition functions with a range, starting from an initial value.

```rust
#![allow(unused)]
fn main() {
println!("{}", (1..11).fold(0, |a, b| a + b));
}
```

--------------------------------

### Chain Iterator with Option in Rust

Source: https://rust-unofficial.github.io/patterns/idioms/option-iter.html

Use `.chain()` to append an `Option`'s element to an existing iterator. Note that `turing.iter()` is used to get an iterator over the `Option`'s content.

```Rust
#![allow(unused)]
fn main() {
let turing = Some("Turing");
let logicians = vec!["Curry", "Kleene", "Markov"];

for logician in logicians.iter().chain(turing.iter()) {
    println!("{logician} is a logician");
}
}
```

--------------------------------

### Manual Facade Method Implementation in Rust

Source: https://rust-unofficial.github.io/patterns/anti_patterns/deref.html

An alternative to Deref polymorphism, this shows how to manually implement facade methods to delegate calls to a composed struct. This approach is more explicit and less surprising than using Deref.

```rust
impl Bar {
    fn m(&self) {
        self.f.m()
    }
}
```

--------------------------------

### Rust: Build with Deny Warnings Flag

Source: https://rust-unofficial.github.io/patterns/print.html

Use the RUSTFLAGS environment variable to set linting behavior at build time. This approach decouples build settings from the code itself.

```bash
RUSTFLAGS="-D warnings" cargo build
```

--------------------------------

### Using &str function with sentence splitting

Source: https://rust-unofficial.github.io/patterns/idioms/coercion-arguments.html

This example demonstrates using the `three_vowels` function (declared with `&str`) to check words within a sentence. It highlights the flexibility of `&str` over `&String` for such operations.

```Rust
fn three_vowels(word: &str) -> bool {
    let mut vowel_count = 0;
    for c in word.chars() {
        match c {
            'a' | 'e' | 'i' | 'o' | 'u' => {
                vowel_count += 1;
                if vowel_count >= 3 {
                    return true;
                }
            }
            _ => vowel_count = 0,
        }
    }
    false
}

fn main() {
    let sentence_string =
        "Once upon a time, there was a friendly curious crab named Ferris".to_string();
    for word in sentence_string.split(' ') {
        if three_vowels(word) {
            println!("{word} has three consecutive vowels!");
        }
    }
}
```

--------------------------------

### Implementing `Deref` for Smart Pointer Behavior

Source: https://rust-unofficial.github.io/patterns/print.html

The `Deref` trait allows types to be treated like smart pointers, enabling implicit dereferencing. This example shows `Deref` implementation for a `Vec<T>` to a slice `[T]`.

```rust
use std::ops::Deref;

struct Vec<T> {
    data: RawVec<T>,
    //..
}

impl<T> Deref for Vec<T> {
    type Target = [T];

    fn deref(&self) -> &[T] {
        //..
    }
}

```

--------------------------------

### Serde API Overview

Source: https://rust-unofficial.github.io/patterns/functional/optics.html

Conceptual overview of the Serde API structure with generic types for serialization and deserialization.

```APIDOC
## Serde API Concepts

### Normal Form

```rust
case class Serde[T, F] {
    serialize: T, F -> String
    deserialize: String, F -> Result[T, Error]
}
```

### Prism Concept

The `Visitor` type allows each data structure to define its own parsing logic, bridging the Serde API with specific data formats.

### Enhanced Normal Form

```rust
case class Serde[T] {
    serialize: F -> String
    deserialize F, String -> Result[T, Error]
}

case class VisitorForT {
    build: F, String -> Result[T, Error]
    decompose: F, T -> String
}

case class SerdeFormat[T, V] {
    toString: T, V -> String
    fromString: V, String -> Result[T, Error]
}
```

### Serde API Implementation Details

1.  **`Deserialize` / `Serialize` Traits**: Each type to be serialized implements these traits, equivalent to the `Serde` class.
2.  **`Visitor` Trait**: A type implementing the `Visitor` trait handles the logic to construct or deconstruct data between the data type and the Serde data model format. This is often generated by a derive macro.
3.  **`Deserializer` Trait**: This trait handles format-specific details, driven by the `Visitor`.
```

--------------------------------

### Dynamic Dispatch with Mutable References in Rust

Source: https://rust-unofficial.github.io/patterns/idioms/on-stack-dyn-dispatch.html

Demonstrates dynamic dispatch using mutable references for I/O operations, allowing either standard input or a file to be read. Requires Rust 1.79.0 or later for automatic lifetime extension.

```Rust
use std::io;
use std::fs;

fn main() -> Result<(), Box<dyn std::error::Error>> {
let arg = "-";

// We need to describe the type to get dynamic dispatch.
let readable: &mut dyn io::Read = if arg == "-" {
    &mut io::stdin()
} else {
    &mut fs::File::open(arg)?
};

// Read from `readable` here.

Ok(())
}

```

--------------------------------

### ToString for Cow<str>

Source: https://rust-unofficial.github.io/patterns/print.html

Illustrates the specialization of the `ToString` method for `Cow` when the type parameter is `str`.

```Rust
impl<'_> ToString for Cow<'_, str>
```

--------------------------------

### Visitor Pattern AST and Interpreter Implementation in Rust

Source: https://rust-unofficial.github.io/patterns/patterns/behavioural/visitor.html

Defines the abstract syntax tree (AST) for a simple expression language and an Interpreter visitor to evaluate these expressions. Requires the ast and visit modules.

```Rust
// The data we will visit
mod ast {
    pub enum Stmt {
        Expr(Expr),
        Let(Name, Expr),
    }

    pub struct Name {
        value: String,
    }

    pub enum Expr {
        IntLit(i64),
        Add(Box<Expr>, Box<Expr>),
        Sub(Box<Expr>, Box<Expr>),
    }
}

// The abstract visitor
mod visit {
    use ast::*;

    pub trait Visitor<T> {
        fn visit_name(&mut self, n: &Name) -> T;
        fn visit_stmt(&mut self, s: &Stmt) -> T;
        fn visit_expr(&mut self, e: &Expr) -> T;
    }
}

use ast::*;
use visit::*;

// An example concrete implementation - walks the AST interpreting it as code.
struct Interpreter;
impl Visitor<i64> for Interpreter {
    fn visit_name(&mut self, n: &Name) -> i64 {
        panic!()
    }
    fn visit_stmt(&mut self, s: &Stmt) -> i64 {
        match *s {
            Stmt::Expr(ref e) => self.visit_expr(e),
            Stmt::Let(..) => unimplemented!(),
        }
    }

    fn visit_expr(&mut self, e: &Expr) -> i64 {
        match *e {
            Expr::IntLit(n) => n,
            Expr::Add(ref lhs, ref rhs) => self.visit_expr(lhs) + self.visit_expr(rhs),
            Expr::Sub(ref lhs, ref rhs) => self.visit_expr(lhs) - self.visit_expr(rhs),
        }
    }
}
```

--------------------------------

### Concatenate Strings with format!

Source: https://rust-unofficial.github.io/patterns/idioms/concat-format.html

Use `format!` for convenient string building, especially with mixed literal and non-literal strings. For maximum efficiency, consider `push` operations on a pre-allocated mutable string.

```Rust
#![allow(unused)]
fn main() {
fn say_hello(name: &str) -> String {
    // We could construct the result string manually.
    // let mut result = "Hello ".to_owned();
    // result.push_str(name);
    // result.push('!');
    // result

    // But using format! is better.
    format!("Hello {name}!")
}
}
```

--------------------------------

### Illustrating Borrow Checker Issue with Large Struct

Source: https://rust-unofficial.github.io/patterns/print.html

This example demonstrates a scenario where a large struct causes borrow checker conflicts due to simultaneous mutable and immutable borrow attempts. It highlights the problem that struct decomposition aims to solve.

```rust
struct Database {
    connection_string: String,
    timeout: u32,
    pool_size: u32,
}

fn print_database(database: &Database) {
    println!("Connection string: {}", database.connection_string);
    println!("Timeout: {}", database.timeout);
    println!("Pool size: {}", database.pool_size);
}

fn main() {
    let mut db = Database {
        connection_string: "initial string".to_string(),
        timeout: 30,
        pool_size: 100,
    };

    let connection_string = &mut db.connection_string;
    print_database(&db);
    *connection_string = "new string".to_string();
}
```

```rust
let connection_string = &mut db.connection_string;
                        ------------------------- mutable borrow occurs here
print_database(&db);
               ^^^ immutable borrow occurs here
*connection_string = "new string".to_string();
------------------ mutable borrow later used here
```

--------------------------------

### Rust Unsafe myset_store Function

Source: https://rust-unofficial.github.io/patterns/patterns/ffi/wrappers.html

An example of an unsafe function that stores a value in a set while potentially iterating over it. This function demonstrates how mutable access to the set can violate Rust's aliasing rules if an iterator is active, leading to undefined behavior.

```rust
pub mod unsafe_module {

    // other module content

    pub fn myset_store(myset: *mut MySetWrapper, key: datum, value: datum) -> libc::c_int {
        // DO NOT USE THIS CODE. IT IS UNSAFE TO DEMONSTRATE A PROBLEM.

        let myset: &mut MySet = unsafe {
            // SAFETY: whoops, UB occurs in here!
            &mut (*myset).myset
        };

        /* ...check and cast key and value data... */

        match myset.store(casted_key, casted_value) {
            Ok(_) => 0,
            Err(e) => e.into(),
        }
    }
}
```

--------------------------------

### Rust Concordance Structure and SerDe Implementation

Source: https://rust-unofficial.github.io/patterns/print.html

Provides a basic Rust structure for Concordance and an outline for its serialization and deserialization methods. Note that the implementation details are marked as 'todo!()'.

```rust
#![allow(unused)]
fn main() {
use std::collections::HashMap;

struct Concordance {
    keys: HashMap<String, usize>,
    value_table: Vec<(usize, usize)>,
}

struct ConcordanceSerde {}

impl ConcordanceSerde {
    fn serialize(value: Concordance) -> String {
        todo!()
    }
    // invalid concordances are empty
    fn deserialize(value: String) -> Concordance {
        todo!()
    }
}
}
```

--------------------------------

### DBM API Definition in C

Source: https://rust-unofficial.github.io/patterns/patterns/ffi/export.html

This C code defines the DBM (Database Manager) API, which serves as an example of an object-based API for file-based databases. It includes types for opaque database handles and data elements, along with functions for managing the database.

```c
struct DBM;
typedef struct { void *dptr, size_t dsize } datum;

int     dbm_clearerr(DBM *);
void    dbm_close(DBM *);
int     dbm_delete(DBM *, datum);
int     dbm_error(DBM *);
datum   dbm_fetch(DBM *, datum);
datum   dbm_firstkey(DBM *);
datum   dbm_nextkey(DBM *);
DBM    *dbm_open(const char *, int, mode_t);
int     dbm_store(DBM *, datum, datum, int);


```

--------------------------------

### Using `#[non_exhaustive]` for Extensible Structs and Enums in Rust

Source: https://rust-unofficial.github.io/patterns/idioms/priv-extend.html

Demonstrates how to apply `#[non_exhaustive]` to structs and enums to allow adding new variants or fields in the future without breaking existing code. It shows how to handle these constructs in match statements using `..` and wildcard patterns.

```rust
#![allow(unused)]
fn main() {
mod a {
    // Public struct.
    #[non_exhaustive]
    pub struct S {
        pub foo: i32,
    }

    #[non_exhaustive]
    pub enum AdmitMoreVariants {
        VariantA,
        VariantB,
        #[non_exhaustive]
        VariantC {
            a: String,
        },
    }
}

fn print_matched_variants(s: a::S) {
    // Because S is `#[non_exhaustive]`, it cannot be named here and
    // we must use `..` in the pattern.
    let a::S { foo: _, .. } = s;

    let some_enum = a::AdmitMoreVariants::VariantA;
    match some_enum {
        a::AdmitMoreVariants::VariantA => println!("it's an A"),
        a::AdmitMoreVariants::VariantB => println!("it's a b"),

        // .. required because this variant is non-exhaustive as well
        a::AdmitMoreVariants::VariantC { a, .. } => println!("it's a c"),

        // The wildcard match is required because more variants may be
        // added in the future
        _ => println!("it's a new variant"),
    }
}
}
```
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