Is this a reasonable workaround for the limitations of the borrow checker?

[SerhiiH]

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?

[a-rust-coder]

Why do you need to mut only specific fields? Can't you just use &mut self, or do you get an error?

Using different sructs for break down a big task is generally not the best idea, unless these types are useful in some other places. If they are only supposed to be used in my_method, this is probably not the best way and you should consider using &mut self everywhere.

Using different types and macros only to break down a method is not very readable nor maintainable

[parasyte]

What you've proposed is essentially the same as described in After NLL: Interprocedural conflicts · baby steps, within the "View structs as a general, but extreme solution" section. This post includes a few alternatives, such as "factoring as a possible fix" (may not always be applicable).

There is also a follow-up from February of this year: View types redux and abstract fields · baby steps (which links to a trail of other interesting ideas). This part of the thread doesn't have anything actionable for users right now. I mention it to show that it's a well known issue that language designers are keen to address.

[nerditation]

@SerhiiH

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,

in general, rust prefers smaller types, so you may consider refactoring the code to break down the struct definition itself instead.

@SerhiiH

Are there better alternatives I should consider?

you may want to check out the partial-borrow crate. it is for this particular use case, and it's more flexible.

here's what your example may look like using partial-borrow:

(note I use easy_ext::ext in this example for the method call syntax. if you are ok with normal function call syntax, no need for ext, see documentation of partial-borrow for details)

#[derive(PartialBorrow)]
struct MyStruct {
    a: A,
    b: B,
    c: C,
    x: X,
    y: Y,
}

impl MyStruct {
    pub fn my_method(&mut self) -> Result<(), Error> {
        // Submethod 1: needs mutable access to field_a, immutable to field_x
        // `split_off()` reborrows `self` into disjoint partial borrows:
        let (process_view, remaining) = self.split_off_mut();
        let result = process_view.process_data()?;
        
        // Submethod 2: needs mutable access to field_b, immutable to field_x
        let (update_view, remaining) = remaining.split_off_mut();
        update_view.update_data(&result)?;
        
        // Submethod 3: needs mutable access to field_c, immutable to field_y
        // this is the last partial borrow, don't need the remaining permissions, so I use `as_mut()
        remaining.as_mut().simulate_data()?;
    }   
}

#[ext]
impl partial!(MyStruct mut a, const x, ! *) {
    fn process_data(&mut self) -> Result<Data, Error> {
        todo!()
    }
}

#[ext]
impl partial!(MyStruct mut b, const x, ! *) {
    fn update_data(&mut self, data &Data) -> Result<(), Error> {
        todo!()
    }
}

#[ext]
impl partial!(MyStruct mut c, const y, ! *) {
    fn simulate_data(&mut self) -> Result<(), Error> {
        todo!()
    }
}