Unsound usages of unsafe implementation from Object to T

[llooFlashooll]

Hi, I am scanning the objc in the latest version with my own static analyzer tool.

Unsafe conversion found at: src/runtime.rs#L506

let ptr = {
    let self_ptr: *mut Object = self;
    (self_ptr as *mut u8).offset(offset) as *mut T
};
&mut *ptr

This unsound implementation would create a misalignment issues if the type size of Object is smaller than the type size of T.

This would potentially cause undefined behaviors in Rust. If we further manipulate the problematic converted types, it would potentially lead to different consequences such as access out-of-bound. I am reporting this issue for your attention.

[madsmtm]

It is true that Object should ideally be an extern type so that accesses such as this one would be correctly calculated, but extern types are not yet stable in Rust, so instead we use a ZST, which allow some of the same things (for now).

So nothing for objc to do here, Rust needs to progress further with extern types first.