[pas]

I meant that you are not forced to use generics, you can use composition, boxing, dyn, etc. (And unsafe too, but PhantomData seems simpler.)

> For example, u64 and u128 are not From<usize> because...well I have no idea.

Because usize is not known statically, it's target arch dependent. Yes, it's silly, but that's how technical safety works. (Also, just as a silly technical counter example the AS/400 virtual instruction set has 128 bit sized pointers.)

There's no question about the need for more ergonomics. That's what this whole post is about after all. For example in some cases where currently PhantomData is needed the intent of the programmer can be easily and unambiguously figured out, but for this folks need to be at least sufficiently certain that this makes things easier, makes code readable, doesn't constraint later language evolution, etc. (If I understand correctly some associated trait bound enhancements will lead to more readable code, less PhantomData.)