On the whole primitive types and references thing, I find it helps me to remember that a reference/pointer (subtle but importance difference between rust and C where rust has more guarantees around a pointer to make it a "reference) is also basically just a number like a "primitive" i32 etc. And references/pointers obviously (?) have to get passed by value or copied (in order to "survive" their original stack frame right?), so passing any primitive by value or copying it really isn't different from passing it by reference, apart from when you're running a borrow checker for managing memory of course.

This will make more sense once we (this community) get to the 4th week/session and properly get into ownership, but lemme try to explain anyways:

A variable without a & in front is moved into the [function's] scope.

Upon exiting a scope, Rust automatically drops/de-allocates any variables that were owned by/moved into said scope.

This is why you need to either

1. pass by ref / have the scope borrow a reference to the variable, or

2. have your function return the variable/object/memory handle that was moved into it

when you want a variable/some data to "out-live" being passed as argument to a function call.

Most often you will use 1), but there are some cases where it can be much nicer to move things "into" a function call and store what you "get back out" (i.e. the return value). Using a "[Type-]State" pattern/approach is a good example of such a case (here's a better explanation than I can give in a lemmy comment).

Example:

struct Unauthenticated;
struct Authenticated { name: String };

impl Unauthenticated {
    fn login(self, username: String) -> Authenticated {
        Authenticated { name: username }
    }
}

pub fn main() {
    let un_authed_user = Unauthenticated::new();
    let authed_user = un_authed_user.login("Alice"); // `un_authed_user` has effectively been moved into `authed_user`
}

Here, we as programmers don't need to worry about making sure un_authed_user gets cleaned up before the program exits, and we don't need to worry about data that could have been stored inside un_authed_user being freed too early (and thus not being available to authed_user).

Admittedly, this is a contrived example that doesn't really need to worry about ownership, it's just the bare minimum to illustrate the idea of moving data into and then back out of a function scope. I don't know of a small enough "real-world" example to give here instead.