It depends what kind of diabetes you mean. Type 1 diabetes typically presents in childhood/adolescence. It is an auto-immune disease with some genetic risk factors but as far as I know diet is not a risk factor for it.

Type 2 diabetes typically presents in adults. Although there are some genetic risk factors for it, environmental effects are also important - being overweight is a notable example.

As far as carbohydrates are concerned, eating things which contain a lot of glucose or sucrose as well as foods which contain processed carbohydrates (eg, white bread, white rice) are risk factors. This is because their carbohydrates are rapidly converted into glucose during digestion (they are high GI foods). The rapid spike of glucose leads to what is called insulin resistance which can progress to Type 2 diabetes.

In contrast, eating foods which contain unprocessed carbohydrates (eg, wholemeal bread, brown rice and wholemeal pasta) is fine in moderation. That is because their glucose is released relatively slowly during digestion (they are low GI foods) which means that the risk of insulin resistance is reduced.

In addition, foods which contain unprocessed carbohydrate are good sources of fibre, which is protective against bowel cancer.