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

Answer 1

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();
}