discourse/app/services/trust_level_granter.rb

class TrustLevelGranter

  def initialize(trust_level, user)
    @trust_level, @user = trust_level, user
  end

  def self.grant(trust_level, user)
    TrustLevelGranter.new(trust_level, user).grant
  end

  def grant
    if @user.trust_level < @trust_level
      @user.change_trust_level!(@trust_level)
      @user.save!
    end
  end
end