Generalizing over pointer mutability [duplicate]

Question

A few times, I've run into the scenario where an accessor method is needed for both mutable and immutable references.

For ~3 lines it isn't a problem to duplicate the logic, but when the logic gets more complex, it's not nice to copy-paste large blocks of code.

I'd like to be able to re-use the code for both.

Does Rust provide some way handle this better then copy-pasting code, or using unsafe casts?

e.g.:

impl MyStruct {
    pub fn get_foo(&self) -> &Bar {
        // ~20 lines of code
        // --- snip ---
        return bar;
    }
    pub fn get_foo_mut(&mut self) -> &mut Bar {
        // ~20 lines of code
        // (exactly matching previous code except `bar` is mutable)
        // --- snip ---
        return bar;
    }
}

---

Here is a more detailed excerpt of a code-base where an immutable return argument was cast to mutable to support both immutable and mutable versions of a function. This uses a wrapped pointer type (ConstP and MutP for immutable and mutable references), but the logic of the function should be clear.

pub fn face_vert_share_loop<V, F>(f: F, v: V) -> LoopConstP
    where V: Into<VertConstP>,
          F: Into<FaceConstP>
{
    into_expand!(f, v);

    let l_first = f.l_first.as_const();
    let mut l_iter = l_first;
    loop {
        if l_iter.v == v {
            return l_iter;
        }

        l_iter = l_iter.next.as_const();
        if l_iter == l_first {
            break;
        }
    }

    return null_const();
}
pub fn face_vert_share_loop_mut(f: FaceMutP, v: VertMutP) -> LoopMutP {
    let l = face_vert_share_loop(f, v);
    return unsafe {
        // Evil! but what are the alternatives?
        // Perform an unsafe `const` to `mut` cast :(
        // While in general this should be avoided,
        // its 'OK' in this case since input is also mutable.
        l.as_mut()
    };
}

Answer 1

(playground links to solutions using type parameters and associated types)

In this case &T and &mut T are just two different types. Code that is generic over different types (at both compile-time and run-time) is idiomatically written in Rust using traits. For example, given:

struct Foo { value: i32 }
struct Bar { foo: Foo }

suppose we want to provide Bar with a generic accessor for its Foo data member. The accessor should work on both &Bar and &mut Bar appropriately returning &Foo or &mut Foo. So we write a trait FooGetter

trait FooGetter {
    type Output;
    fn get(self) -> Self::Output;
}

whose job is to be generic over the particular type of Bar we have. Its Output type will depend on Bar since we want get to sometimes return &Foo and sometimes &mut Foo. Note also that it consumes self of type Self. Since we want get to be generic over &Bar and &mut Bar we need to implement FooGetter for both, so that Self has the appropriate types:

// FooGetter::Self == &Bar
impl<'a> FooGetter for &'a Bar {
    type Output = &'a Foo;
    fn get(self) -> Self::Output { & self.foo }
}

// FooGetter::Self == &mut Bar
impl<'a> FooGetter for &'a mut Bar {
    type Output = &'a mut Foo;
    fn get(mut self) -> Self::Output { &mut self.foo }
}

Now we can easily use .get() in generic code to obtain & or &mut references to Foo from a &Bar or a &mut Bar (by just requiring T: FooGetter). For example:

// exemplary generic function:
fn foo<T: FooGetter>(t: T) -> <T as FooGetter>::Output {
    t.get() 
}

fn main() {
    let x = Bar { foo: Foo {value: 2} };
    let mut y = Bar { foo: Foo {value: 2} };

    foo(&mut y).value = 3;
    println!("{} {}\n", foo(&x).value, foo(&mut y).value);
}

Note that you can also implement FooGetter for Bar, so that get is generic over &T,&mut T, and T itself (by moving it in). This is actually how the .iter() method is implemented in the standard library, and why it always does "the right thing" independently of the reference-ness of the argument its invoked on.