What is the cost of async in Rust

The cost of async in Rust is zero at runtime for the abstraction itself, as Rust uses zero-cost abstractions where higher-level features compile to code as fast as manual implementations. The actual performance depends on the runtime (like tokio or async-std) and how you structure your tasks, not the async/await keywords.

use tokio;

#[tokio::main]
async fn main() {
    // This async function compiles to a state machine
    // with no overhead compared to manual callback code
    println!("Hello async!");
}

Async code in Rust doesn't slow your program down just by being there; it compiles into efficient machine code just like regular functions. Think of it like a well-organized kitchen where chefs (tasks) pause and resume work without wasting time waiting for ingredients, rather than a chef standing still. You only pay a performance cost if the underlying runtime is inefficient or if you create too many tiny tasks.