MendSlice *might* be UB.

[eddyb]

IIUC, it's fine within a memory allocation (what C calls "an object"), but across them, you're usually not allowed to do a lot. cc @RalfJung @solson

[bluss]

Good point, makes sense

[eddyb]

Note that I'm not sure, hoping @RalfJung can answer, or bring up in "unsafe code guidelines" discussions.

[bluss]

Context link to the docs https://docs.rs/odds/0.3.1/odds/slice/struct.MendSlices.html

[RalfJung]

What exactly is mend_slices doing? The docs link to https://docs.rs/odds/0.3.1/odds/trait.Itertools.html#method.mend_slices but that doesn't exist.

EDIT: Found https://docs.rs/odds/0.3.1/odds/slice/trait.SliceIterExt.html#method.mend_slices but I still didn't find the actual code that @eddyb asks whether it is UB.

[RalfJung]

Oh I think I found it at https://docs.rs/odds/0.3.1/src/odds/slice/mod.rs.html#309

Yeah I think this doesn't work. So you are comparing the end address of one slice with the beginning of another. That's an extremely gray area of memory models -- an area where C and C++ differ and we have no idea what the heck LLVM implements. But LLVM is pretty clear that you can do getelementptr only within an allocation, so if it happens to be the case that two allocations exist right next to each other, and you "mend" them together, I am pretty sure you are on UB territory here.

The funny thing is that this will actually work reliably on miri because that assumes allocations never sit right next to each other. ;)

[bluss]

It's very old code, don't think it is used. It can be removed if this library is updated, because I think mend_slices was more of a cool trick and not a utility.

Is the whole stack an allocation? I assume we are not allowed to "mend" together slices of two adjacent arrays on the stack.

[RalfJung]

Is the whole stack an allocation?

No. Every local variable is its own allocation.

[HeroicKatora]

The code here unfortunately does not only exhibit UB for the cases of mending separate allocations. It also does not ensure that the new slice with length a.len() + b.len() does not lead to a slice which has more than isize::MAX bytes.

[RalfJung]

Also see oberien/str-concat#8