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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, fo= r 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= of >>>>> 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 avai= lable 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 cou= ld work >> for raw pointers). > > I really would like to see some docs of that, I didn't find anything in > the reference, official docs, or nomicon. The reference does say [1] > that: > > `dyn Trait` metadata must be a pointer to a compiler-generated > vtable for Trait. (For raw pointers, this requirement remains a > subject of some debate.) > > Do you know where it was decided? I did find this [2] UCG issue that > covers it, but that doesn't seem like the decision, just the discussion. The FCP is here: https://github.com/rust-lang/rust/issues/101336 The gist: for a fat raw pointer with `dyn Trait` metadata, the safety invar= iant requires a fully valid vtable. Validity invariant is not yet decided. > > [1]: https://doc.rust-lang.org/reference/behavior-considered-undefined.ht= ml#r-undefined.validity.wide > [2]: https://github.com/rust-lang/unsafe-code-guidelines/issues/516 > >> 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. > > Well size_of_val_raw is `unsafe` and only valid to call in certain > conditions. It asks in the case of slices that the length is an > initialized integer and that the entire value must fit into `isize`. > This to me just further indicates that `*mut T` has safety > requirements to obtaining the size of an arbitrary pointer. > > In the special cases of `T: Sized` and `T =3D=3D [U]`, we have safe ways = of > getting their size. Hmm, the `isize` fitting requirement is problematic indeed. It's broken cod= e if pointer projection is used with an allocation that exceeds the limit, but I= want the API to be safe, so it'll be good if the API is defined to just be wrapp= ing and safe (it may return values that doesn't make sense, but that'll be on t= he user). Anyhow this is moot as we're going the `KnownSize` route. Best, Gary > > Cheers, > Benno