You will need to pass that for synchronous IO as well. All IO in the standard library is moving to the Io interface. Sync and async.

If I want to call a function that does asynchronous IO, I'll use:

   foo(io, ...);

If I want to call one that does synchronous IO, I'll write:

    foo(io, ...);

If I want to express that either one of the above can be run asynchronously if possible, I'll write:

    io.async(foo, .{ io, ... });

If I want to express that it must be run concurrently, then I'll write:

    try io.concurrent(foo, .{ io, ... });

Nowhere in the above do I distinguish whether or not foo does synchronous or asynchronous IO. I only mark that it does IO, by passing in a parameter of type std.Io.