Isn't that trivial? Just use Rust but reject any occurrence of `async` or `await` in your code or dependencies. Rust doesn't even force the use of async code for certain features in its standard library.
And use which first class libs? IO related libs are almost all based on Tokio. I mean I'm happy that at least there's some consent regarding the runtime. But you can't undo the decisions for going async (instead of green threads) that easily.