Best route for bathroom sink drainage?

[Sensors]

I'm rearranging my bathroom and trying to plan my drainage so I was wondering if it's okay to join the sink waste to the soil waste coming from the toilet (blue line below), since that's the nearest point of drainage. I'm creating a stud wall (90mm thick) so there's decent space within that to run drainage.

If so, is there anything else I need to consider to make sure there's no strange plumbing bevahiour?

I might be over thinking it, but I'm a bit worried that suction created from the toilet water flushing might do something weird like empty the U bend of the sink and allow some smells to creep out.

The alternative would be to run it behind the toilet and over to join the shower drain waste pipe (red line below).

 

[Hugh Jaleak]

Firstly, we have Basins in bathrooms, not sinks.

Secondly, your Basin waste will be fine connected into the WC outlet, Pan Connectors with a 32mm boss are made exactly for this purpose. To cover all bases, fit an Anti Vac trap to the basin, this will remove any possibility of the WC siphoning the basin trap when flushed.

 

[Sensors]

My lack of plumbing familiarity is showing!

Thanks for the info. I'll look into anti-vac traps, didn't know they were a thing either.

 

[Madrab]

Pan Connectors with a 32mm boss are made exactly for this purpose

I always thought they were a vent boss