Take the following code as an example.

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
    pub val: i32,
    pub next: Option<Box<ListNode>>,
}

// This one is fine
fn test1(mut node: &mut Box<ListNode>) {
    while let Some(next) = &mut node.next {
        node = next;
    }
}

// This one won't compile!
fn test2(mut node: &mut Box<ListNode>) {
    while let Some(next) = &mut node.next {
        if true {
            node = next;
        }
    }
}

The compiler thinks test1 is fine but treats test2 as illegal because it "cannot borrow node.next as mutable more than once at a time". Why does adding an if statement cause such a drastic difference? Why isn’t the same error reason applied to test1? In my view, test1 is already illegal because next and node are both mutable reference to the same object.

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