I'm in odoo 12.0, I created a new database with only the project module.
Then I created a record rule with :
Name = user : update own task only
Access Rights : Apply for All
Rule definition : [('user_id','=',user.id)]
Groups : Project / User
Then I created a new internal user with only that group, and it can see all tasks.
I wanted that users from the Project / User group can only see their own assisgned tasks.