Is this reborrow? about nest borrow

help
1

this case

fn main() {
    let mut a = String::from("abc");
    let mut b = &mut a;// 'a
    let c = &mut b; // 'b
    
    *b = String::from("123");

    
    println!("{}", c);// 'a and 'b extend here
}

and the error is

image

why *b is borrowed?

2

c is an exclusive borrow of b, of type &'b mut &'a mut String, through which you can do any of these things:

  • modify the text "abc"
  • replace the String owned by a with another one
  • change what b points to

So, by the general principles of exclusive borrowing in Rust, you cannot use b while it is borrowed by c. (The fact that you then only use c for reading, not writing, is irrelevant.)

Is this reborrow?

I wouldn't say so, because c isn't pointing to any part of what b points to; it is simply a reference to b. But it's certainly a borrow derived from b in some way, which just like a reborrow constrains how you can use b (in this case, completely blocks using b).

3

It's a nested borrow, not a reborrow.

b can't be used while c is active, and that includes writing to *b.

If you just meant why is the error phrased that way, as far as I know, it's just repeating the place being written to. But regardless, every reference b is borrowing the pointed-to place, *b.

4

To understand lifetime, I recommend this article.

If you are a Chinese, this I wrote in Chinese may help.

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5

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