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But since the functions of this trait aren't `unsafe`, it >>>>> must be sound to pass `ptr::null` to them. >>>> >>>> The "in bounds" here is the conceptual bounds of the pointer. So, for = a pointer >>>> with size `x`, the address of the returned pointer lies between `ptr .= . ptr + >>>> x`. >>> >>> Okay, I haven't really seen that as a concept. Also, what is the size o= f >>> an invalid pointer? >> >> It's `size_of::()` for sized types, and `size_of::() * slice.len()= ` for a >> raw slice pointer. > > And for `dyn Trait`? > > Do you have a link to somewhere? For `dyn Trait` it would be the size in the vtable, which is always availab= le as vtable metadata on a raw pointer is required to be valid anyway (this is something that lang team has already decided so that trait upcasting could = work for raw pointers). I am basically just having `size_of_val_raw` in mind when writing this. So = the current `KnownSize` comment in v4 is something that I am happy about. > >> The projection semantics is same regardless whether it's valid or not. T= he I/O >> projection work will rely on this, as many I/O impls will act on pointer= s that >> are not "valid" in Rust sense because they refer to a different address = space. >> But they're still legit pointers with proper meaning. > > I like the solution with `KnownSize`. The previous safety requirement > was nebulous IMO. Best, Gary