Add vec![ptr::null{,_mut}(); n] optimization, like vec![0; n]
vec![0; n], via implementations of SpecFromElem, has an optimization that uses with_capacity_zeroed instead of with_capacity, which will use calloc instead of malloc, and avoid an extra memset. This adds the same optimization for vec![ptr::null(); n] and vec![ptr::null_mut(); n], assuming their bit value is 0 (which is true on all currently supported platforms). This does so by adding an intermediate trait IsZero, which looks very much like nonzero::Zeroable, but that one is on the way out, and doesn't apply to pointers anyways. Adding such a trait allows to avoid repeating the logic using with_capacity_zeroed or with_capacity, or making the macro more complex to support generics.
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