Look at Go or Java virtual threads. Async I/O doesn't need function coloring.

Here is an example Zig code:

    defer stream.close(io);

    var read_buffer: [1024]u8 = undefined;
    var reader = stream.reader(io, &read_buffer);

    var write_buffer: [1024]u8 = undefined;
    var writer = stream.writer(io, &write_buffer);

    while (true) {
        const line = reader.interface.takeDelimiterInclusive('\n') catch |err| switch (err) {
            error.EndOfStream => break,
            else => return err,
        };
        try writer.interface.writeAll(line);
        try writer.interface.flush();
    }

The actual loop using reader/writer isn't aware of being used in async context at all. It can even live in a different library and it will work just fine.