https://news.ycombinator.com/item?id=41944121
The TLDR is that there’s all sorts of invariants that the Rust compiler internals assume about Rust code (eg pointers don’t alias). In C such assumptions can’t be made. Unsafe rust requires you to uphold all the same invariants that safe rust does without any guardrails and it has more such assumptions than C. C however is uniformly dangerous - there isn’t a safe subset to write your code in.
One example would be that you could have something like:
let x = 5;
do_something(&x as *const _ as *mut _);
There are fairly straightforward ways to disclaim these invariants to a greater or lesser extent as appropriate, such as accepting &Cell<T> arguments (or even, e.g. Option<&Cell<T>>) in lieu of &mut T. But the Rust standard library is not yet comprehensively built with this in mind, so this might be an area where Zig's library facilities have a real advantage unless an effort is made to address this gap.