How can I solve my "cannot borrow as mutable because it is also borrowed as immutable" problem

help
#1

Here is a minimal example of my problem: I have a stage (here in 1D for simplicity) and a struct that holds some parameters for that stage. The center() parameter here is a very simplified example. My restrictions are:

  • StageStats evaluates parameters for a given & Stage1D content (immutable reference)
  • the calculated values must be stored (cached) within a StageStats instance that evaluated them

Because I always need both Stage1D and StageStats in any given time, I contained one inside the other, and here is where the problem starts:

cannot borrow `self.my_stats` as mutable because it is also borrowed as immutable

I understand why my code can be dangerous and why Rust doesn't allow me to write that. My question is: what are my other options to implement that (besides moving StageStats out of Stage1D)

pub struct Stage1D {
    points: Vec<f32>,
    my_stats: StageStats,
}

pub struct StageStats { recent_center:f32 }

impl StageStats {
    pub fn center(&mut self, s:&Stage1D) -> f32 { 
        let sum : f32 = s.points.iter().sum();
        self.recent_center = sum / s.points.len() as f32;
        return self.recent_center;
    }
}

impl Stage1D {
    pub fn new() -> Stage1D { Stage1D{points: Vec::new(), my_stats: StageStats{recent_center:0.0}} }
    pub fn my_center(&mut self) -> f32 { self.my_stats.center(&self) }
}

fn main ()
{
    let _ = Stage1D::new();
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0502]: cannot borrow `self.my_stats` as mutable because it is also borrowed as immutable
  --> src/main.rs:18:42
   |
18 |     pub fn my_center(&mut self) -> f32 { self.my_stats.center(&self) }
   |                                          ^^^^^^^^^^^^^^------^-----^
   |                                          |             |      |
   |                                          |             |      immutable borrow occurs here
   |                                          |             immutable borrow later used by call
   |                                          mutable borrow occurs here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `playground` due to previous error
#2

I would have split Stage1D into two structs : data and stats.
So you can immutably borrow data part and pass it to stats.

pub struct Stage1D {
    data: StageData,
    my_stats: StageStats,
}

pub struct StageData {
    points: Vec<f32>,
}

pub struct StageStats { recent_center:f32 }

impl StageStats {
    pub fn center(&mut self, s:&StageData) -> f32 { 
        let sum : f32 = s.points.iter().sum();
        self.recent_center = sum / s.points.len() as f32;
        return self.recent_center;
    }
}

impl Stage1D {
    pub fn new() -> Stage1D { Stage1D{ data: StageData {points: Vec::new()}, my_stats: StageStats{recent_center:0.0}} }
    pub fn my_center(&mut self) -> f32 { self.my_stats.center(&self.data) }
}
#3

Maybe center() can be a method of stage1D?

#4

View structs are one way.

Here's an article on the topic with some other approaches.

#5

Oh no, the poor error formatting has really gone off the rails here...