Vec::as_ptr for empty vectors is undocumented

[nbaksalyar]

Vec::as_ptr returns a pointer to address 0x1 for empty vectors. As it seems, this is an internal implementation detail for RawVec and allocation.

This behavior is unexpected and not documented. In certain scenarios it might cause a segmentation fault (e.g. due to failed checks for a null pointer when dealing with FFI).

[steveklabnik]

Yes, we should document this for sure.

[Stebalien]

due to failed checks for a null pointer when dealing with FFI

FYI, this is precisely why rust uses uses 0x1 for zero-sized things instead of 0x0 (null); it does this to distinguish between "nothing" and "something empty/zero sized".

[frewsxcv]

Would it be accurate to say that it returning 0x1 is an implementation detail?

[steveklabnik]

Yes, I would say something like "the pointer value of an empty vector is meaningless", but uh, worded better. Not after midnight 😉

[Stebalien]

Not exactly. It *can't* be 0 (null pointer optimization) and setting it to 1 ensures that most accidental dereferences will result in a segfault (most operating systems avoid mapping memory into the first few pages of address space).

[Cobrand]

This not only happen for Vec, but for other things of 0 size allocated on the heap as well, (namely Box<()>). While you might say it's pointless to do Box::new(()), it can still happen and it should be documented.

I suspect it works the same way for Rc<()> and other things like that, so a chapter in the nomicon would be great I think. At the very least, say something in Box::into_raw_ptr() about 0 size structs, and add a link from Vec::as_ptr() to this part.

[frewsxcv]

Opened a pull request here: #39757.

In the pull request, I used the term 'meaningless' @steveklabnik mentioned above. I don't think it's a great term, but I can't think of anything better right now.

Please please share better ideas!

[nagisa]

There are multiple aspects to this issue:

1. You are not supposed to dereference pointer of 0-capacity vector anyway. It makes no difference whether the pointer is 0, 1 or 0xFRESHVEGGIES.
2. 0-sized pointer dereference (including pointer returned by Vec<ZeroSized>::as_ptr) is no-op and dereferencing 0, 1 or 0xFRESHVEGGIES are all equally valid (i.e. no-op) in current implementation, although not specified.
3. These pointers are not meaningless. You need to pass the same pointers back into from_raw and friends to insure the correct behaviour of data structures and smart pointers afterwards.

In that sense the only real change that’s possible in documentation is

This function may return non-null non-dereferenceable pointers.