Runtimes

Start with the idea

The language defines Future and .await; polling, timers, and I/O readiness come from a runtime you choose, such as tokio or async-std. That keeps async usable on an embedded target with no allocator, at the cost of one decision you have to make per project.

Rust's language and standard library define Future and .await, but a program needs an executor to poll futures and often a runtime to provide timers, sockets, and task spawning. Tokio is a common choice, but an async function is not inherently tied to one runtime. A library should be clear about whether it uses runtime-specific I/O types or exposes runtime-neutral futures. The decision affects how easily callers can integrate it.

The runtime schedules ready tasks; it cannot make blocking code magically nonblocking. A synchronous file operation or long CPU loop inside an async task can occupy an executor worker and delay unrelated tasks.