AsyncDrop Pattern Guide

The AsyncDrop pattern enables async cleanup for types that hold resources requiring asynchronous teardown (network connections, file handles, background tasks, etc.).

Core Concept

Rust's Drop trait is synchronous, but sometimes cleanup needs to be async. The AsyncDrop pattern solves this by:

1. Wrapping values in AsyncDropGuard<T>
2. Requiring explicit async_drop().await calls
3. Panicking if cleanup is forgotten

Quick Reference

// Creating
let mut guard = AsyncDropGuard::new(my_value);

// Using (transparent via Deref)
guard.do_something();

// Cleanup (REQUIRED before dropping)
guard.async_drop().await?;

The AsyncDrop Trait

#[async_trait]
pub trait AsyncDrop {
    type Error: Debug;
    async fn async_drop_impl(&mut self) -> Result<(), Self::Error>;
}

Essential Rules

| Rule | Description | |------|-------------| | Always call async_drop() | Every AsyncDropGuard must have async_drop() called | | Factory methods return guards | fn new() -> AsyncDropGuard<Self>, never plain Self | | Types with guard members impl AsyncDrop | Delegate to member async_drops | | Use the macro when possible | with_async_drop_2! handles cleanup automatically | | Panics are exceptions | It's OK to skip async_drop on panic paths |

The with_async_drop_2! Macro

Automatically calls async_drop() on scope exit:

let resource = get_resource().await?;
with_async_drop_2!(resource, {
    // Use resource here
    resource.do_work().await?;
    Ok(result)
})

Additional References

• patterns.md - Implementation patterns and examples
• gotchas.md - Common mistakes and how to avoid them
• helpers.md - Helper types (AsyncDropArc, AsyncDropHashMap, etc.)

Location

Implementation: crates/utils/src/async_drop/