what happens to a rust mutable value after each assignment

Question

Here are two examples of dynamic mutable value.

pub fn main() {
    let mut x = String::from("Hello");
    println!("{}", x);

    x = format!("{x}, world!");
    println!("{}", x);
}

So as you can see, we have a mutable variable x. x gets borrowed by format! & turns to a new value that is then assigned back to x.

pub fn main() {
    let mut x = String::from("Hello, world!");
    println!("{}", x);

    x = String::from("Lorem, ipsum!");
    println!("{}", x);
}

My real problem is this code, where the value assigned to x has no connection with it's initial value. Variables can only take owner-ship over a single value, so how does Rust keep track of "Hello, world!" & "Lorem, ipsum!"? Exactly what happens to mutable values after assignments?

My guess is that it checks if the value getting assigned to depends on the first one, if yes, it won't drop it.

The original value of `x` gets dropped in both of these cases. The return value of `format!` does not hold a reference to its parameters, it creates a new `String`. — pigeonhands, Feb 05 '23 at 03:46
What happens on a case that would borrow , instead of cloning? — KianFakheriAghdam, Feb 05 '23 at 03:48
In the case that the parameters of `format!` are borrowed? It still constructs a new `String` value that does not hold those references. — pigeonhands, Feb 05 '23 at 03:49
@pigeonhands What about the case where `x = takes_ownership_and_gives_back(x)`, would it still drop? — KianFakheriAghdam, Feb 05 '23 at 04:00
No it would not be dropped. Values are dropped when they no longer have an owner. — pigeonhands, Feb 05 '23 at 04:02
[If the new value borrows from the old value, that's an error](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=21940847c78f416b088e22624d6527d9). — Chayim Friedman, Feb 05 '23 at 05:11

what happens to a rust mutable value after each assignment

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