Debloat your async Rust

The problem: async is powerful — and sneaky

Async Rust is praised for letting code wait without burning CPU or threads. But it has been reported that as projects grow, those neat async functions can multiply into a forest of compiler-generated statemachines — bigger binaries, more RAM, and extra CPU work. Frustrating? Absolutely. The emotional sting comes when you realize the abstractions you relied on for clean code are quietly costing you resources. Who likes hidden bills?

Where the bloat hides

It has been reported that common culprits include traits and generic abstractions that sometimes need async behavior and sometimes don't. For example, a Loader trait that can load config from disk or return a constant value will still force the compiler to produce a statemachine even for the trivial, non-blocking implementation. Similarly, embedded HAL traits like embedded-hal-async can cause a driver to create an extra statemachine simply by forwarding to another async transaction — more layers than necessary. The result is extra code size and runtime overhead for no real benefit.

Simple fixes and a lint on the way

There are straightforward ways out. It has been reported that using Option parameters, returning ready() futures for immediate values, or changing an async fn to return an underlying future instead of awaiting and re-wrapping can eliminate needless state machines. The post also notes — and it has been reported that a colleague named Wouter opened a PR — for a new Clippy lint to catch some of these patterns. In short: tell the compiler what you actually want, and it won't overdeliver.

Why this matters

Rust developers love abstractions. They also love small, predictable binaries — especially in embedded and resource-constrained domains. This is a practical reminder: async is not magic; it's a trade-off. Watch your traits, watch your forwards, and you’ll avoid ending up with more statemachines than sense. Who knew performance hygiene could feel like spring cleaning?

Sources: tweedegolf.nl, Lobsters