Function Contracts: Interior Mutability Tests

[pi314mm]

Test cases for interior mutability using Cell, OnceCell, UnsafeCell, and RefCell.

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[feliperodri]

We also need to update our RFC to describe any limitations around interior mutability.

[feliperodri]

The transmute is safe here because Cell and UnsafeCell are guaranteed to have the same memory layout as their underlying type. https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell.html#memory-layout

This comment should be included in the test cases.

[celinval]

I don't think you need the transmute in any of these cases. You should be able to use the public methods to get the data address or a copy of the data.

We have two problems, and I think it might be helpful to think about them separately, although you might want to add a test that has both. The first problem is interior mutability, and the second one is encapsulation.

Using transmute is necessary if you want to break encapsulation. The interior mutability could be a problem for identifying expressions that are side-effect free.