### Async Trait Not Dyn Compatible Example

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

Demonstrates the error encountered when trying to use a trait with an async function as a dynamic trait object.

```rust
pub trait Trait {
    async fn f(&self);
}

pub fn make() -> Box<dyn Trait> {
    unimplemented!()
}
```

```console
error[E0038]: the trait `Trait` is not dyn compatible
 --> src/main.rs:5:22
  |
5 | pub fn make() -> Box<dyn Trait> {
|                      ^^^^^^^^^ `Trait` is not dyn compatible
|
note: for a trait to be dyn compatible it needs to allow building a vtable
      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
 --> src/main.rs:2:14
  |
1 | pub trait Trait {
|           ----- this trait is not dyn compatible...
2 |     async fn f(&self);
|              ^ ...because method `f` is `async`
  = help: consider moving `f` to another trait
```

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

### Define Async Trait with async-trait

Source: https://context7.com/dtolnay/async-trait/llms.txt

Annotate trait definitions and impl blocks with `#[async_trait]` to enable async methods for dynamic dispatch. This example demonstrates a basic database trait and its implementation.

```rust
use async_trait::async_trait;

#[async_trait]
trait Database {
    async fn connect(&self) -> Result<(), String>;
    async fn query(&self, sql: &str) -> Vec<String>;
    async fn disconnect(&mut self);
}

struct PostgresDb {
    connection_string: String,
}

#[async_trait]
impl Database for PostgresDb {
    async fn connect(&self) -> Result<(), String> {
        println!("Connecting to {}", self.connection_string);
        Ok(())
    }

    async fn query(&self, sql: &str) -> Vec<String> {
        println!("Executing: {}", sql);
        vec!["row1".to_string(), "row2".to_string()]
    }

    async fn disconnect(&mut self) {
        println!("Disconnecting...");
    }
}

#[tokio::main]
async fn main() {
    let mut db = PostgresDb {
        connection_string: "postgres://localhost/mydb".to_string(),
    };

    db.connect().await.unwrap();
    let results = db.query("SELECT * FROM users").await;
    println!("Got {} results", results.len());
    db.disconnect().await;
}
```

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

### Dynamic Dispatch with Async Trait Objects

Source: https://context7.com/dtolnay/async-trait/llms.txt

Use `async_trait` to create trait objects (`dyn Trait`) with async methods, enabling runtime polymorphism. This example processes messages using a collection of different `MessageHandler` implementations.

```rust
use async_trait::async_trait;
use std::sync::Arc;

#[async_trait]
trait MessageHandler: Send + Sync {
    async fn handle(&self, message: &str) -> String;
}

struct EchoHandler;

#[async_trait]
impl MessageHandler for EchoHandler {
    async fn handle(&self, message: &str) -> String {
        format!("Echo: {}", message)
    }
}

struct UppercaseHandler;

#[async_trait]
impl MessageHandler for UppercaseHandler {
    async fn handle(&self, message: &str) -> String {
        message.to_uppercase()
    }
}

async fn process_messages(
    handlers: &[Arc<dyn MessageHandler>],
    message: &str,
) -> Vec<String> {
    let mut results = Vec::new();
    for handler in handlers {
        results.push(handler.handle(message).await);
    }
    results
}

#[tokio::main]
async fn main() {
    let handlers: Vec<Arc<dyn MessageHandler>> = vec![
        Arc::new(EchoHandler),
        Arc::new(UppercaseHandler),
    ];

    let results = process_messages(&handlers, "hello").await;
    // results: ["Echo: hello", "HELLO"]
    for result in results {
        println!("{}", result);
    }
}
```

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

### Fixing Elided Lifetimes in Async Functions

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

These examples show the correct ways to define an async function within a trait using `#[async_trait]` when dealing with elided lifetimes, either by naming the lifetime explicitly or using the `'_` placeholder.

```rust
#[async_trait]
trait Test {
    // either
    async fn test<'e>(elided: Elided<'e>) {}
    // or
    async fn test(elided: Elided<'_>) {}
}
```

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

### Non-Send Futures with ?Send Attribute

Source: https://context7.com/dtolnay/async-trait/llms.txt

Use `#[async_trait(?Send)]` when futures do not need to be `Send`, such as when using single-threaded executors or when dealing with non-`Send` types across await points. This example uses `Rc` and `RefCell` which are not `Send`.

```rust
use async_trait::async_trait;
use std::rc::Rc;
use std::cell::RefCell;

// Rc is not Send, so we need ?Send
#[async_trait(?Send)]
trait LocalCache {
    async fn get(&self, key: &str) -> Option<String>;
    async fn set(&self, key: &str, value: String);
}

struct RcCache {
    data: Rc<RefCell<std::collections::HashMap<String, String>>>,
}

#[async_trait(?Send)]
impl LocalCache for RcCache {
    async fn get(&self, key: &str) -> Option<String> {
        self.data.borrow().get(key).cloned()
    }

    async fn set(&self, key: &str, value: String) {
        self.data.borrow_mut().insert(key.to_string(), value);
    }
}

// Use with a single-threaded runtime
fn main() {
    let rt = tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .unwrap();

    rt.block_on(async {
        let cache = RcCache {
            data: Rc::new(RefCell::new(std::collections::HashMap::new())),
        };

        cache.set("name", "Alice".to_string()).await;
        let name = cache.get("name").await;
        println!("Name: {:?}", name); // Name: Some("Alice")
    });
}
```

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

### Implementing Async Traits with async-trait

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

Shows how to use the `#[async_trait]` macro on traits and their implementations to enable async functions and dynamic dispatch.

```rust
use async_trait::async_trait;

#[async_trait]
trait Advertisement {
    async fn run(&self);
}

struct Modal;

#[async_trait]
impl Advertisement for Modal {
    async fn run(&self) {
        self.render_fullscreen().await;
        for _ in 0..4u16 {
            remind_user_to_join_mailing_list().await;
        }
        self.hide_for_now().await;
    }
}

struct AutoplayingVideo {
    media_url: String,
}

#[async_trait]
impl Advertisement for AutoplayingVideo {
    async fn run(&self) {
        let stream = connect(&self.media_url).await;
        stream.play().await;

        // Video probably persuaded user to join our mailing list!
        Modal.run().await;
    }
}
```

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

### Mixing Async and Non-Async Methods in a Trait

Source: https://context7.com/dtolnay/async-trait/llms.txt

Demonstrates how to define a trait with both synchronous and asynchronous methods using the `async-trait` macro. Only async methods are transformed.

```Rust
use async_trait::async_trait;

#[async_trait]
trait Service {
    // Synchronous method - not transformed
    fn name(&self) -> &str;

    // Synchronous method with default implementation
    fn version(&self) -> u32 {
        1
    }

    // Async methods - transformed by macro
    async fn start(&self) -> Result<(), String>;
    async fn health_check(&self) -> bool;
}

struct ApiService {
    service_name: String,
}

#[async_trait]
impl Service for ApiService {
    fn name(&self) -> &str {
        &self.service_name
    }

    async fn start(&self) -> Result<(), String> {
        println!("Starting {}...", self.service_name);
        Ok(())
    }

    async fn health_check(&self) -> bool {
        true
    }
}

#[tokio::main]
async fn main() {
    let service = ApiService {
        service_name: "UserAPI".to_string(),
    };

    println!("Service: {} v{}", service.name(), service.version());
    service.start().await.unwrap();
    println!("Healthy: {}", service.health_check().await);
}
```

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

### Expanded Async Trait Implementation

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

Illustrates the internal transformation performed by the `#[async_trait]` macro, converting async methods into futures.

```rust
impl Advertisement for AutoplayingVideo {
    fn run<'async_trait>(
        &'async_trait self,
    ) -> Pin<Box<dyn std::future::Future<Output = ()> + Send + 'async_trait>>
    where
        Self: Sync + 'async_trait,
    {
        Box::pin(async move {
            /* the original method body */
        })
    }
}
```

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

### Async Trait with Generics and Associated Types

Source: https://context7.com/dtolnay/async-trait/llms.txt

Illustrates an async trait that uses generic type parameters, associated types, and supports common traits like Send and Sync. This is useful for creating flexible data repositories.

```rust
use async_trait::async_trait;

#[async_trait]
trait Repository<T: Send + Sync> {
    type Error: Send;

    async fn find_by_id(&self, id: u64) -> Result<Option<T>, Self::Error>;
    async fn save(&self, entity: &T) -> Result<(), Self::Error>;
    async fn delete(&self, id: u64) -> Result<bool, Self::Error>;
}

#[derive(Clone, Debug)]
struct User {
    id: u64,
    name: String,
}

struct InMemoryUserRepo {
    users: std::sync::RwLock<std::collections::HashMap<u64, User>>,
}

#[async_trait]
impl Repository<User> for InMemoryUserRepo {
    type Error = String;

    async fn find_by_id(&self, id: u64) -> Result<Option<User>, Self::Error> {
        Ok(self.users.read().unwrap().get(&id).cloned())
    }

    async fn save(&self, entity: &User) -> Result<(), Self::Error> {
        self.users.write().unwrap().insert(entity.id, entity.clone());
        Ok(())
    }

    async fn delete(&self, id: u64) -> Result<bool, Self::Error> {
        Ok(self.users.write().unwrap().remove(&id).is_some())
    }
}

#[tokio::main]
async fn main() {
    let repo = InMemoryUserRepo {
        users: std::sync::RwLock::new(std::collections::HashMap::new()),
    };

    let user = User { id: 1, name: "Alice".to_string() };
    repo.save(&user).await.unwrap();

    let found = repo.find_by_id(1).await.unwrap();
    println!("Found: {:?}", found); // Found: Some(User { id: 1, name: "Alice" })

    repo.delete(1).await.unwrap();
    let found = repo.find_by_id(1).await.unwrap();
    println!("After delete: {:?}", found); // After delete: None
}
```

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

### Async Trait with Default Retry Implementation

Source: https://context7.com/dtolnay/async-trait/llms.txt

Demonstrates a default implementation for a retry mechanism in an async trait. This default can be used by implementors or overridden for custom behavior.

```rust
use async_trait::async_trait;

#[async_trait]
trait Retryable {
    async fn attempt(&self) -> Result<String, String>;

    // Default implementation with retry logic
    async fn with_retry(&self, max_attempts: u32) -> Result<String, String> {
        let mut last_error = String::new();
        for attempt in 1..=max_attempts {
            match self.attempt().await {
                Ok(result) => return Ok(result),
                Err(e) => {
                    last_error = e;
                    println!("Attempt {} failed, retrying...", attempt);
                }
            }
        }
        Err(format!("All {} attempts failed: {}", max_attempts, last_error))
    }
}

struct UnreliableService {
    fail_count: std::sync::atomic::AtomicU32,
}

#[async_trait]
impl Retryable for UnreliableService {
    async fn attempt(&self) -> Result<String, String> {
        let count = self.fail_count.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
        if count < 2 {
            Err("Service unavailable".to_string())
        } else {
            Ok("Success!".to_string())
        }
    }
    // Uses default with_retry implementation
}

#[tokio::main]
async fn main() {
    let service = UnreliableService {
        fail_count: std::sync::atomic::AtomicU32::new(0),
    };

    let result = service.with_retry(5).await;
    println!("Result: {:?}", result); // Result: Ok("Success!")
}
```

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

### Non-Threadsafe Futures with async-trait

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

Demonstrates how to use `#[async_trait(?Send)]` to prevent `Send` and `Sync` bounds on futures when they are not required.

```rust
#[async_trait]
// ... trait definition ...

#[async_trait(?Send)]
// ... impl block ...

```

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

### Async Trait with Arc Self Receiver

Source: https://context7.com/dtolnay/async-trait/llms.txt

Shows how to use `Arc<Self>` as a receiver for async trait methods, facilitating shared ownership and safe concurrent access to trait objects.

```rust
use async_trait::async_trait;
use std::sync::Arc;

#[async_trait]
trait Worker: Send + Sync {
    async fn run(self: Arc<Self>);
    async fn status(&self) -> String;
}

struct BackgroundWorker {
    name: String,
}

#[async_trait]
impl Worker for BackgroundWorker {
    async fn run(self: Arc<Self>) {
        println!("{} starting...", self.name);
        // Simulate work
        tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
        println!("{} completed!", self.name);
    }

    async fn status(&self) -> String {
        format!("Worker '{}' is ready", self.name)
    }
}

#[tokio::main]
async fn main() {
    let worker: Arc<dyn Worker> = Arc::new(BackgroundWorker {
        name: "DataProcessor".to_string(),
    });

    println!("{}", worker.status().await);

    // Clone Arc for spawning
    let worker_clone = Arc::clone(&worker);
    tokio::spawn(async move {
        worker_clone.run().await;
    }).await.unwrap();
}
```

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

### Compiler Error for Implicit Elided Lifetime

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

The compiler output shows the specific error E0726 for an implicit elided lifetime in an async function, suggesting how to fix it by indicating the anonymous lifetime.

```console
error[E0726]: implicit elided lifetime not allowed here
 --> src/main.rs:9:29
  |
9 |
  |                             ^^^^^^- help: indicate the anonymous lifetime: `<'_>`
```

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

### Async Function with Implicit Elided Lifetime Error

Source: https://github.com/dtolnay/async-trait/blob/master/README.md

This code demonstrates an error that occurs when an async function within a trait using `#[async_trait]` attempts to use an implicitly elided lifetime, which is not allowed.

```rust
type Elided<'a> = &'a usize;

#[async_trait]
trait Test {
    async fn test(not_okay: Elided, okay: &usize) {}
}
```

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