Re: [PATCH v3 1/2] rust: add projection infrastructure

[Alice Ryhl]

On Tue, Mar 03, 2026 at 10:17:01AM +0000, Gary Guo wrote:
> On Tue Mar 3, 2026 at 9:14 AM GMT, Benno Lossin wrote:
> > On Mon Mar 2, 2026 at 11:19 PM CET, Gary Guo wrote:
> >> 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 == [U]`, we have safe ways of
> > getting their size.
> 
> Hmm, the `isize` fitting requirement is problematic indeed. It's broken code 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 wrapping
> and safe (it may return values that doesn't make sense, but that'll be on the
> user).
> 
> Anyhow this is moot as we're going the `KnownSize` route.

It sounds like that's no different from dyn trait vtable case. Fat
pointer must have valid metadata, even if raw pointer.

Alice