Expand slices to separate pointer and size

[kvark]

It would be really nice to support slices by splitting those into separate (pointer, size) values, e.g.

   objects: &[Object],

would be translated into:

   objects: *const Object,
   objects_length: size_t,

where the "_length" suffix is configured like all the other prefixes/suffixes

[emilio]

Can you really rely on that layout and ABI?

[RReverser]

Nope.

[eqrion]

We had a discussion about this in the gfx daily. It sounds like this doesn't have a defined layout currently, but might in the future. This is another thing, like strings, where it'd be nice to be able to autogenerate conversion thunks. But there's not a great way to add support for this in cbindgen today.

So I'll close this for now.

[Manishearth]

Would it be possible for cbindgen to come with a companion library containing wrappers like Slice, etc which are compatible? Otherwise folks have to do this per-project.

[emilio]

Ideally we'd get rust to specify these and then just teach cbindgen about that.

But lacking them, I'd be fine with something like that. We can put it in this repo if it makes sense.

Gecko had a bunch of types for stuff like Box<str>, Box<[T]>, etc which should be copiable pretty much straight-away. For non-owned slices I think we've been mostly passing ptr+length by hand, but a new type so that it has the right ABI would be cool...

[Manishearth]

Yeah, to be clear, cbindgen doesn't need any changes itself, it just needs to maintain a second library that works well with cbindgen that does this.

[Manishearth]

Actually, I didn't realize that cbindgen can't generate types from dependencies at all. That does make things a little bit worse here :/

[emilio]

It does generate types from dependencies, see parse_deps = true and extra_bindings if you need functions and constants from a dependent crate.

[petrochenkov]

I use the cfg(feature = "cbindgen") trick for this

#[repr(C)]
struct S<'a> {
    #[cfg(not(feature = "cbindgen"))]
    slice: &'a [u8],

    #[cfg(feature = "cbindgen")]
    ptr: *const u8,
    #[cfg(feature = "cbindgen")]
    len: usize,
}

(which is pretty useful for workarounds in general, and helps with other things like bitfields).

[Manishearth]

ahhh thanks

[emilio]

Right, but that's still relying on technically unspecified behavior, isn't it? I just want https://github.com/rust-lang/unsafe-code-guidelines/blob/master/reference/src/layout/pointers.md set on stone ;)