Hoping for clarity on how Rust works in these situations - eviltoast

I’m slowly starting Rust for Rustaceans, and it’s already poking holes in my understanding of Rust. Here’s a couple initial questions I have:

A shared reference, &T is , as the name implies, a pointer that may be shared. Any number of references may exist to the same value, and each shared reference is Copy, so you can trivially make more of them

I don’t understand why a shared reference has to implement copy. In fact, isn’t this not true just by the fact that references work for Strings and Strings size can’t be known at compile time?

  1. I’m having trouble with the idea of assigning a new value to a mutable reference.

let mut x = Box::new(42); *x = 84;

Why in this example, is the assignment dereferenced. Why not just do x=84? is it dereferenced specifically because is Boxed on the heap?

  • KillTheMule@programming.dev
    4·
    1 year ago

    In fact, isn’t this not true just by the fact that references work for Strings and Strings size can’t be known at compile time?

    I don’t understand this. Shared references to String are Copy, too. This doesn’t have to do anything with sizes. Rather, it’s implemented in the compiler, because it’s sound to have it and a huge QoL improvement over the alternative… just the same reason why e.g. usize is Copy, really.

    is it dereferenced specifically because is Boxed on the heap?

    No, it’s not really related to the heap. Box implements DerefMut, which is in-depth explained here.