I've never seen this but I've tried a similar fruit before! I don't remember the species, but it was also in the genus Carissa! In Mexico, they simply call it 'carissa'. The fruit is sweet but sour as well and actually, when it's ripe it looks exactly the berries in your photos! They do not turn purple. Other differences from your variety are that this plant grows into a small shrub and the fruits are kiwi sized. It is native to southern coastal Africa, if I remember correctly. It is a good fruit for people with diabetes. This family of fruits must be a medicinal powerhouse!