What is the real difference between `*const T` and `*mut T` raw pointers?

[RalfJung]

@matklad

Is this really unsound though? In my current view of the world, this is not unsound (by itself). What matters is how did you get that pointer. If you originally had an &mut __m256i then using *const or *mut should be OK. If you originally had an &__m256i , then both are unsound.

I agree. *const T and *mut T are equivalent in terms of UB.

However, as @ExpHP wrote

@ExpHP

The fact that &T coerces to *const T does make the signature surprising and dangerous.

Using *const T could lead to people thinking they can pass in an &T, and that would be UB.