Hi everyone,

I have a large struct that contains various fields, and I want to break down a complex method into smaller, focused submethods. However, each submethod needs different mutability access to different fields, and the borrow checker can't see that these operations are semi-independent:

impl MyStruct {
    pub fn my_method(&mut self) -> Result<(), Error> {
        // I want to break this down into submethods, but the borrow checker
        // can't see that each submethod only needs different parts of self:
        
        // Submethod 1: needs mutable access to field_a, immutable to field_x
        // let result = self.process_data()?;
        
        // Submethod 2: needs mutable access to field_b, immutable to field_x
        // self.update_data(&result)?;
        
        // Submethod 3: needs mutable access to field_c, immutable to field_y
        // self.simulate_data()?;
        
        // The borrow checker sees this as one big method that needs &mut self,
        // but can't distinguish that each submethod only borrows specific fields
    }
    
    // These won't work because they all need &mut self:
    fn process_data(&mut self) -> Result<Data, Error> { /* ... */ }
    fn update_data(&mut self, data: &Data) -> Result<(), Error> { /* ... */ }
    fn simulate_data(&mut self) -> Result<(), Error> { /* ... */ }
}

I created view structs that each borrow only the fields they need with the appropriate mutability:

struct ProcessView<'a, 'b> {
    field_a: &'a mut FieldA,  // mutable
    field_x: &'b FieldX,      // immutable
}

struct UpdateView<'a, 'b> {
    field_b: &'a mut FieldB,  // mutable
    field_x: &'b FieldX,      // same field_x, but immutable here
}

struct SimulateView<'a, 'b> {
    field_c: &'a mut FieldC,  // mutable
    field_y: &'b FieldY,      // immutable
}

And I use macros to create these views:

macro_rules! process_view {
    ($self:expr) => {{
        ProcessView {
            field_a: &mut $self.field_a,
            field_x: &$self.field_x,
        }
    }};
}

// Usage within my_method:
impl MyStruct {
    pub fn my_method(&mut self) -> Result<(), Error> {
        // Now I can break it down using views, and the same field can be
        // accessed with different mutability in different views:
        let result = process_view!(self).process_data()?;      // field_x is immutable
        update_view!(self).update_data(&result)?;              // field_x is immutable again
        simulate_view!(self).simulate_data()?;                 // field_y is immutable
        
        Ok(())
    }
}

I'm wondering:

1. Is this a reasonable approach given Rust's current limitations?
2. What are the downsides to doing this?
3. Are there better alternatives I should consider?

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