如何在Swift中的变异结构上正确创建一个惰性派生属性？

I'm making a mutating struct with a really expensive-to-compute derived value. So what I want to do is to compute this derived value lazily and store the result, until the struct gets mutated again, at which point the derived value is no longer valid and needs to be recomputed.I'm making a mutating struct with a really expe