Explicit async IO is the building block which allows the things you mention to be efficient, and Rust's aim is being great at low-level code, e.g. implementing

Explicit async IO is the building block which allows the things you mention to be efficient, and Rust's aim is being great at low-level code, e.g. implementing the runtime systems for those languages. Another use of explicit async IO for Rust is building nice abstractions for other Rust code to use, and these abstractions may or may not be so explicit about the asynchronicity. Like with a lot of Rust development at the moment, these libraries are building blocks, they're not the end of the story.