Rust 1.39.0 Release: async/await stabilization

Rust 1.39.0 stabilized the `async/await` syntax, allowing developers to write asynchronous code that looks and feels like synchronous code, greatly improving ergonomics.

Significance

This was the foundational event that made `async-std` possible. Without `async/await` syntax, the core problem `async-std` aimed to solve – providing an ergonomic async runtime – could not have been addressed effectively.