Euclid, Rust, and Vector2D

Part of my learning a new programming language is to read through an established code library on GitHub. I'm currently studying euclid — a basic linear algebra library written in Rust for the Servo web browser engine. Here's a snippet from the euclid::vector module as I currently understand it: 

#[repr(C)]
pub struct Vector2D<T, U> {
    pub x: T,
    pub y: T,
    #[doc(hidden)]
    pub _unit: PhantomData<U>
}
Two-dimensional vector implemented in Rust.

  1. Vector2D is a two-dimensional vector implemented as a struct. A Rust struct is a custom data type that can hold multiple related and named values. This particular implementation contains three properties.

  2. Property x is the horizontal coordinate. Property y is the vertical coordinate. x and y are defined by the generic T, which allows them to contain any data type as long as they share the same data type. Generic T is further defined by trait implementations so that T can only contain numeric primitives (u8, i32, f64, etc).

  3. Property _unit implemented by generic U typically contains an empty struct whose sole purpose is to differentiate vector spaces — Vector2D<f64, Canvas> will not mix with Vector2D<f64, DOM>. Wrapping _unit in a PhantomData marker allows _unit to be an empty data type. The Rust compiler would otherwise believe _unit to be an error.

  4. All Rust modules, functions, and data structures default to private, meaning external modules cannot access them. The pub keyword inverts this.

  5. The inner workings of the Rust attribute #[repr(C)] are currently beyond me. The documentation claims this metadata allows Vector2D to interact with code written in other languages as long as those languages implement C scalars.