Summary

Allow equality, but not order, comparisons between fat raw pointers of the same type.

Motivation

Currently, fat raw pointers can’t be compared via either PartialEq or PartialOrd (currently this causes an ICE). It seems to me that a primitive type like a fat raw pointer should implement equality in some way.

However, there doesn’t seem to be a sensible way to order raw fat pointers unless we take vtable addresses into account, which is relatively weird.

Detailed design

Implement PartialEq/Eq for fat raw pointers, defined as comparing both the unsize-info and the address. This means that these are true:

    &s as &fmt::Debug as *const _ == &s as &fmt::Debug as *const _ // of course
    &s.first_field as &fmt::Debug as *const _
        != &s as &fmt::Debug as *const _ // these are *different* (one
	                                 // prints only the first field,
					 // the other prints all fields).

But

    &s.first_field as &fmt::Debug as *const _ as *const () ==
        &s as &fmt::Debug as *const _ as *const () // addresses are equal

Drawbacks

Order comparisons may be useful for putting fat raw pointers into ordering-based data structures (e.g. BinaryTree).

Alternatives

@nrc suggested to implement heterogeneous comparisons between all thin raw pointers and all fat raw pointers. I don’t like this because equality between fat raw pointers of different traits is false most of the time (unless one of the traits is a supertrait of the other and/or the only difference is in free lifetimes), and anyway you can always compare by casting both pointers to a common type.

It is also possible to implement ordering too, either in unsize -> addr lexicographic order or addr -> unsize lexicographic order.

Unresolved questions

What form of ordering should be adopted, if any?