[pas]

You don't have to use PhantomData.

> "upstream crates may add a new impl" I have satisfied the compiler but no bugs were prevented.

Trial-and-error debugging which version causes a bug is busywork, preventing it right when someone introduces the potential problem is at best annoying, but very fast compared to the alternative.

[millstone]

You do have to use PhantomData, otherwise it won't compile:

https://play.rust-lang.org/?gist=84883cb7cdd09acdcd919888cef...

It's especially bad if your type is an enum, since there's no obvious place to put the PhantomData:

https://github.com/rust-lang/rust/issues/32739

> Trial-and-error debugging which version causes a bug is busywork, preventing it right when someone introduces the potential problem is at best annoying

It's a silly limitation. For example, u64 and u128 are not From<usize> because...well I have no idea. But you can't make them From, because Rust wants to reserve the right to make them From in the future. And you can't write a function that assumes they are NOT From, for the same reason.

So it's pointlessly hard to write generics over integers. I encounter lots of weird holes like this.

https://play.rust-lang.org/?gist=600a1ca784ee7df02351e58df43...

[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.)