nomicon/src/conversions.md

Type Conversions

At the end of the day, everything is just a pile of bits somewhere, and type systems are just there to help us use those bits right. There are two common problems with typing bits: needing to reinterpret those exact bits as a different type, and needing to change the bits to have equivalent meaning for a different type. Because Rust encourages encoding important properties in the type system, these problems are incredibly pervasive. As such, Rust consequently gives you several ways to solve them.

First we'll look at the ways that Safe Rust gives you to reinterpret values. The most trivial way to do this is to just destructure a value into its constituent parts and then build a new type out of them. e.g.

struct Foo {
    x: u32,
    y: u16,
}

struct Bar {
    a: u32,
    b: u16,
}

fn reinterpret(foo: Foo) -> Bar {
    let Foo { x, y } = foo;
    Bar { a: x, b: y }
}

But this is, at best, annoying. For common conversions, Rust provides more ergonomic alternatives.