How to implement the C flexible array member pattern in Rust?

Question

I would like to implement this C code which uses a flexible array member (sometimes called the struct hack) in Rust:

struct test {
  int key;
  int dataSize;
  int data[];
};

struct test* t = malloc(sizeof(struct test) + sizeOfData)

The empty array at the end of structure allows you to allocate your meta fields and data all at once. Unfortunately, I can't figure out how to do such thing in Rust.

Answer 1

The Rust equivalent of this struct would use a slice:

struct test {
  key: i32,
  dataSize: i32,
  data: [i32],
}

however, these types are not really convenient to use at the moment as there is no safe way to construct them.

A more practical way to do this is to use a generic:

struct test<T: ?Sized> {
  key: i32,
  dataSize: i32,
  data: T,
}

and then to make sure that you only use test when T is an array or a slice (which you could only create through coercion).

See also:

• Dynamically Sized Types (DSTs) on the nomicon