cannot assign to `self.b` because it is borrowed `self.b` is assigned to here but it was already borrowed

Here is a minimal subset of my code. My Rust compiler reports "cannot assign to self.b because it is borrowed. self.b is assigned to here but it was already borrowed"

struct Test {
    a: String,
    b: String,
}

impl Test {
    fn borrow_func(&self) -> &String {
        &self.a
    }

    fn test_func(&mut self) -> usize {
        let a = self.borrow_func();
        self.b = "".to_string();
        a.len()
    }
}

When I inline the function borrow_func, the error is omitted. However, the inlined code lacks readability. How can I fix the bug while preserving the readability?

Here is the inlined code:

struct Test {
    a: String,
    b: String,
}

impl Test {
    fn test_func(&mut self) -> usize {
        let a = &self.a;
        self.b = "".to_string();
        a.len()
    }
}

The solution is just reorder the operations, The way you do the operations you have an inmmutable reference active and mutable reference active, so that's why the borrow checker throws the error about you have borrowed a variable.

I would recommend to study the rust lifetimes and trying to understand borrowing and ownership.

https://doc.rust-lang.org/book/ch04-00-understanding-ownership.html

struct Test {
    a: String,
    b: String,
}

impl Test {
    fn borrow_func(&self) -> &String {
        &self.a
    }

    fn test_func(&mut self) -> usize {
        self.b = "".to_string();
        let a = self.borrow_func();
        a.len()
    }
}

fn main() {
    let mut t: Test = Test {
        a: String::from("hola"),
        b: String::from("hello"),
    };

    println!("{}", t.test_func());
}

I have found that this code works

#[derive(Clone)]
struct Test {
    a: String,
    b: String,
}

impl Test {
    fn borrow_func(&self) -> &String {
        &self.a
    }

    fn test_func(&mut self) -> usize {
        let s = self.clone();
        let a = s.borrow_func();
        self.b = "".to_string();
        a.len()
    }
}

What is happening is that when self.borrow_func() is run it's passing self not &self because self is already a reference. To fix this we would instead do

self.clone().borrow_func();

But that gives us an error that it is a temporary value so to fix this we do

let s = self.clone();
s.borrow_func();