How to resolve basic borrow issues? - rust

Question

I'm pretty new to rust and I'm stuck on whats probably a very simple question about borrowing. I've simplified my code to as below:
pub struct MyStruct {
pub val: i32
}
impl MyStruct {
pub fn new(val: i32) -> Self {
MyStruct {
val
}
}
}
pub struct MyStructVec {
pub vec: Vec::<MyStruct>
}
impl MyStructVec {
pub fn new() -> Self {
MyStructVec {
vec: Vec::new()
}
}
pub fn insert(&mut self, mut my_struct: MyStruct) {
self.vec.push(my_struct);
}
fn run(&mut self) {
for s in self.vec.iter_mut() {
s.val = s.val + 1;
self.edit(s);
}
}
fn edit(&self, my_struct: &mut MyStruct) {
my_struct.val = my_struct.val * 2;
}
}
fn main() {
let mut my_struct1 = MyStruct::new(69);
let mut my_struct2 = MyStruct::new(420);
let mut my_struct_vec = MyStructVec::new();
my_struct_vec.insert(my_struct1);
my_struct_vec.insert(my_struct2);
my_struct_vec.run();
}
When I run this i get the following error:
error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
--> src/main.rs:33:13
|
31 | for s in self.vec.iter_mut() {
| -------------------
| |
| mutable borrow occurs here
| mutable borrow later used here
32 | s.val = s.val + 1;
33 | self.edit(s);
| ^^^^ immutable borrow occurs here
But if I edit my code like this:
fn run(&mut self) {
for s in self.vec.iter_mut() {
s.val = s.val + 1;
//self.edit(s);
s.val = s.val * 2;
}
}
Then it runs perfectly fine. This examples a simplified case but in my actual code I might just want to separate out my code into a separate function for easier readability like with the edit function. So what is the proper way to do this in Rust? Thanks very much!

Answer 1

I think the error is that for some reason you are making your MyStructVec modify MyStruct when it could be simple a method on MyStruct. You are messing around with more references that you need, and you are not really taking the ones you need. It seems fair that edit should be encapsulated in the object that is gonna modify:
impl MyStruct {
...
fn edit(&mut self) {
self.val = self.val * 2;
}
}
...
impl MyStructVec {
...
fn run(&mut self) {
for s in self.vec.iter_mut() {
s.val = s.val + 1;
s.edit();
}
}
}
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