Support DerefMut for mocks

[bradforj287]

I ran into a bit of a snag testing code involving a lot of smart pointers.

I wanted to do something like this:

1. create the mock in an Arc
2. set some expectations on the mock
3. pass a pointer to the mock to the code under test.
4. trigger the code utilizing the mock
5. checkpoint()
6. repeat, go back to step 2

I couldn't accomplish this because this code won't compile:
This code:

    let mut mock = Arc::new(MockMyService::new());
    mock.expect_get_current()
        .times(1)
        .returning(|input| {
        input * input
    });

fails with a compiler error:

error[E0596]: cannot borrow data in an `Arc` as mutable
  --> src/main.rs:50:5
   |
50 |     mock.expect_get_current()
   |     ^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `std::sync::Arc<MockMyService>`

I'm wondering if it makes sense to support DerefMut to make testing with smart pointers easier?

[asomers]

No. The problem here is that setting expectations requires mutable access to the mock. But Arc does not provide interior mutability. What you want is either to use an Arc<Mutex<MockMyService>>, or else set the expectations before you Arc it, like this:

let mut mock = MockMyService::new();
mock.expect_get_current()
        .times(1)
        .returning(|input| {
        input * input
    });
let arcmock = Arc::new(mock);

[bradforj287]

Yup, setting expectations before mocking is the workaround i went with. There is another workaround involving unsafe code. Example:

#![feature(get_mut_unchecked)]
use std::sync::Arc;

fn main() {

    let mut v: Vec<i32> = Vec::new();
    v.push(1);

    println!("data: {:?}", v);
    let a = Arc::new(v);

    unsafe {
        let mut aa = a.clone();
        let mut d = Arc::get_mut_unchecked(&mut aa);
        d.push(55);
    }
    println!("data: {:?}", a);
}

[YarekTyshchenko]

After some searching I found a solution to this: #327 (comment)