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**:
```rust
// ✅ 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**:
```rust
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**:
```rust
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: