async-drop

Development

Guide to the AsyncDrop pattern for async cleanup in Rust. Use when working with AsyncDropGuard, implementing AsyncDrop trait, or handling async resource cleanup.

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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:

Wrapping values in AsyncDropGuard<T>
Requiring explicit async_drop().await calls
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/