How to call method on struct that has mutable self in Rust

Question

I am having trouble with the following set up:

#[derive(Debug)]
struct SomeStruct {
    numbers: Vec<u32>,
}

impl SomeStruct {
    fn some_func(&mut self) { // `mut` causes the issue
        self.numbers.push(9); // Contrived but need to call self in here.
        println!("Hello from some func");
    }
}

pub fn main() {
    let struct1 = SomeStruct {
        numbers: vec![1, 2, 3],
    };
    let struct2 = SomeStruct {
        numbers: vec![99, 98, 97],
    };

    let mut vec = Vec::new();
    vec.push(&struct1);
    vec.push(&struct2);

    let first = vec.first_mut().unwrap();

    // cannot borrow `**first` as mutable, as it is behind a `&` reference
    // cannot borrow as mutable rustc(E0596)
    first.some_func();
}

There are many questions about mutable & borrowing but I still can't figure it out. Could someone please explain why this is wrong and how to fix it?

sebpuetz · Accepted Answer · 2021-05-13 17:28:11Z

3

The problem is that vec only has shared references, i.e. &SomeStruct instead of SomeStruct or &mut SomeStruct.

The call to some_func() can't succeed in this setup because it requires &mut self. However, there are ways to work around this:

let vec own SomeStruct, e.g.: vec![struct1, struct2]
Push &mut struct1 and &mut struct2 in the vec
use interior mutability, e.g. through RefCell which would allow you to modify data while holding a shared reference

The interior mutability solution would look something like this:

use std::cell::RefCell;

#[derive(Debug)]
struct SomeStruct {
    numbers: RefCell<Vec<u32>>,
}

impl SomeStruct {
    fn some_func(&self) {
        self.numbers.borrow_mut().push(9); // Contrived but need to call self in here.
        println!("Hello from some func");
    }
}

pub fn main() {
    let struct1 = SomeStruct {
        numbers: RefCell::new(vec![1, 2, 3]),
    };
    let struct2 = SomeStruct {
        numbers: RefCell::new(vec![99, 98, 97]),
    };

    let mut vec = Vec::new();
    vec.push(&struct1);
    vec.push(&struct2);

    let first = vec.first().unwrap();

    // cannot borrow `**first` as mutable, as it is behind a `&` reference
    // cannot borrow as mutable rustc(E0596)
    first.some_func();
}

answered May 13, 2021 at 17:28

sebpuetz

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3 Comments

nanobar Over a year ago

I want to keep that as refs as it's very perf critical code (think the other way has copy cost?). Wonder if borrow_mut has runtime cost, will look into it otherwise use mut on the structs which works too. Thanks a lot!

trent Over a year ago

@Dominic Your intuition about performance of Vec<&SomeStruct> vs. Vec<SomeStruct> is very likely wrong. Write things in the most obvious and maintainable way first, then measure before you start adding &s in places where they cause problems. Yes, by the way, borrow_mut() does have a (small) runtime cost.

trent Over a year ago

@Dominic If it's the size of the struct you're concerned about, consider putting it inside a Box (or Rc/Arc for sharing). Box<Vec<SomeStruct>> has exactly the same size and, once created, is as cheap to use as &Vec<SomeStruct>, but doesn't need to borrow. Either way, though (Box or &), you pay a stiff penalty for adding a layer of indirection, which is likely to cancel out any advantage you get from making the type smaller, unless you do a lot more passing-around than you do dereferencing. Only profiling can say for sure.

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