Async Patterns Guide

Async Patterns Guide Skill

You are an expert at modern Rust async patterns. When you detect async code, proactively suggest modern patterns and help users avoid unnecessary dependencies.

When to Activate

Activate when you notice:

• Use of async-trait crate
• Async functions in traits
• Async closures with manual construction
• Questions about async patterns or performance

Key Decision: async-trait vs Native

Use Native Async Fn (Rust 1.75+)

When:

• Static dispatch (generics)
• No dyn Trait needed
• Performance-critical code
• MSRV >= 1.75

Pattern:

// ✅ Modern: No macro needed (Rust 1.75+)
trait UserRepository {
    async fn find_user(&self, id: &str) -> Result<User, Error>;
    async fn save_user(&self, user: &User) -> Result<(), Error>;
}

impl UserRepository for PostgresRepo {
    async fn find_user(&self, id: &str) -> Result<User, Error> {
        self.db.query(id).await  // Native async, no macro!
    }

    async fn save_user(&self, user: &User) -> Result<(), Error> {
        self.db.insert(user).await
    }
}

// Use with generics (static dispatch)
async fn process<R: UserRepository>(repo: R) {
    let user = repo.find_user("123").await.unwrap();
}

Use async-trait Crate

When:

• Dynamic dispatch (dyn Trait) required
• Need object safety
• MSRV < 1.75
• Plugin systems or trait objects

Pattern:

use async_trait::async_trait;

#[async_trait]
trait Plugin: Send + Sync {
    async fn execute(&self) -> Result<(), Error>;
}

// Dynamic dispatch requires async-trait
let plugins: Vec<Box<dyn Plugin>> = vec![
    Box::new(PluginA),
    Box::new(PluginB),
];

for plugin in plugins {
    plugin.execute().await?;
}

Migration Examples

Migrating from async-trait

Before:

use async_trait::async_trait;

#[async_trait]
trait UserService {
    async fn create_user(&self, email: &str) -> Result<User, Error>;
}

#[async_trait]
impl UserService for MyService {
    async fn create_user(&self, email: &str) -> Result<User, Error> {
        // implementation
    }
}

After (if using static dispatch):

// Remove async-trait dependency
trait UserService {
    async fn create_user(&self, email: &str) -> Result<User, Error>;
}

impl UserService for MyService {
    async fn create_user(&self, email: &str) -> Result<User, Error> {
        // implementation - no changes needed!
    }
}

Async Closure Patterns

Modern Async Closures (Rust 1.85+)

// ✅ Native async closure
async fn process_all<F>(items: Vec<Item>, f: F) -> Result<(), Error>
where
    F: AsyncFn(Item) -> Result<(), Error>,
{
    for item in items {
        f(item).await?;
    }
}

// Usage
process_all(items, async |item| {
    validate(&item).await?;
    save(&item).await
}).await?;

Performance Considerations

Static vs Dynamic Dispatch

Static (Generics):

// ✅ Zero-cost abstraction
async fn process<R: Repository>(repo: R) {
    repo.save().await;
}
// Compiler generates specialized version for each type

Dynamic (dyn Trait):

// ⚠️ Runtime overhead (vtable indirection)
async fn process(repo: Box<dyn Repository>) {
    repo.save().await;
}
// Requires async-trait, adds boxing overhead

Your Approach

When you see async traits:

1. Check if dyn Trait is actually needed
2. Suggest removing async-trait if possible
3. Explain performance benefits of native async fn
4. Show migration path

Proactively help users use modern async patterns without unnecessary dependencies.