In rust, they separate the concept of a future from how it's actually run. Tokio is one way to run a bunch of futures, and it has many different options.

For instance, I use the CurrentThread runtime in tokio, because I'm using the rust code as a plugin to Postgres, and it accesses non-thread-safe APIs.

What you are asking for is essentially for the futures runtime to be hidden from you. That's fine for some languages that already have a big runtime and don't need the flexibility to do things differently, but doesn't work for rust.