Is it possible to control the size of an array using the type parameter of a generic?

This ability is not available yet. RFC 2000 — const generics will support it when it is implemented and progress is tracked in issue #44580. Basic support is scheduled for Rust 1.51.

In nightly Rust, your example would look like:

// 1.51.0-nightly (2020-12-30)

struct Vec<T: Sized, const COUNT: usize> {
    a: [T; COUNT],
}

---

If you look at the design of Rust, you will notice that it started first by tackling the hardest problems (memory-safe, data-race free) but there are otherwise lots of areas where it is "incomplete" (compared to what could be achieved).

In particular, generic structures and functions are somewhat limited today:

• lack of Higher Kinded Types (HKT)
• lack of non-type parameters => arrays are special-cased, and implementing a trait for an array is a known issue, the work-around being to implement it for a few different dimensions
• lack of variadic parameters => tuples are special-cased, and implementing a trait for all tuples is similarly difficult

For the moment, those are not implemented, not because they are not desired but simply because time was lacking. The idea of Rust 1.0 was not to release a final product that would not evolve, but a stable base from which to start; some or maybe all will come.

While waiting for Rust to gain first-class support for this, there are crates that provide certain levels of this functionality, such as:

• typenum
• generic-array