Thanks!

Seems I gotta dig into the borrow checker before thinking too much about this!

Otherwise, compiling your code snippet is a nice illustration of how helpful the compiler tries to be ... lots of tips in the output there! To anyone else, just try running rustc on this, with the second println! uncommented and see the output, which is half error half linting.

For variables that are passed by reference, the ‘&’ is about whether the ownership of that memory address is transferred or not.

Yea. So if the second println! were uncommented, how could we compile this? From what you've said, I'd guess that & means "borrow" (ie, not "move" ownership).

So if we alter abc to take a &String type and not String, and therefore only "borrow" the variable, and then pass in &v and not v to pass in a "borrowed" variable, it should compile.

fn abc(v: &String) {
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');
    abc(&v);

    println!("v is {}", v);
}

It seems to!

Of course, as the compiler suggests, we could instead just pass in v.clone() which presumably creates a new variable and effectively "passes by value".

Digging in a bit more, what happens if abc (tries to) mutate the variable?

We can add v.push('X') to abc and see if we get different printouts. As the compiler would tell us, we would need to make the argument v mutable for this to work.

fn abc(mut v: String) {

    v.push('X');
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');
    abc(v.clone());

    println!("v is {}", v);
}

// OUTPUT:
// v is abcX
// v is abc

I'm not clear on why I don't have to declare that the v.clone() is mutable in anyway though.

What about trying the same with a "borrowed' variable?

Well we need mutable borrowed variables, so necessary adjustments to the types of abc and its call in main. And adding an additional mutation of v in main after abc is called, and we get two different println outputs, with each mutation applying to the same variable.

fn abc(v: &mut String) {

    v.push('X');
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');

    abc(&mut v);

    v.push('Y');

    println!("v is {}", v);
}

// OUTPUT
// v is abcX
// v is abcXY

The ‘&’ sign doesn’t mean ‘pass by reference’ in the same way as it does in C. Anything with a size that’s fixed at compile time is typically passed by value, whereas variables who’s size might change are passed by reference.

Are you sure? I noticed the rust book said this:

A reference is like a pointer in that it’s an address we can follow to access the data stored at that address; that data is owned by some other variable. Unlike a pointer, a reference is guaranteed to point to a valid value of a particular type for the life of that reference.