Re: [PATCH v3 06/15] ivec: create a vector type that is interoperable between C and Rust

From: D. Ben Knoble <ben.knoble@gmail.com>
Date: Aug 26, 2025, 13:30 UTC
Message-ID: <CALnO6CASgMQ=cbQ_ijWXV0RMMSZgvS47r8ucTro7Wc4pgZ9_jQ@mail.gmail.com>
In-Reply-To: <CAH=ZcbA=-iEFnJ-TecAZL_EX-f3pAShDhdq=S2XWkQHYgRZV7Q@mail.gmail.com>

On Mon, Aug 25, 2025 at 4:40 PM Ezekiel Newren <ezekielnewren@gmail.com> wrote:

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>
> On Sun, Aug 24, 2025 at 7:31 AM Ben Knoble <ben.knoble@gmail.com> wrote:
> > I’m mildly surprised Vec isn’t a good fit: isn’t it a pointer, length, capacity triple? But it sounds like the main issue is allocator interop… which I would also have thought was supported? At least the current version is documented as being generic against an Allocator, too.
>
> Conceptually yes, semantically and syntactically no. On top of Vec<T>
> not being defined with #[repr(C)] (which ensures field order, C ABI
> layout, padding, etc...) the struct definition for Vec isn't constant
> between Rust versions. I'd be open to suggestions for an alternative
> to my ivec type.

Ah, thanks—I had forgotten about the #[repr(C)] needs and changes. Makes sense.

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> > Am I reading the patch correctly that the ivec implementation is primarily C? I’m not familiar with too many FFI projects in Rust, but I might have hoped we could write parts in Rust to gain any benefits from that, too. Is that a fool’s errand I’m thinking of?
>
> The ivec type is defined and implemented in C (interop/ivec.[ch]) and
> Rust (rust/interop/src/ivec.rs). When I started writing the ivec type
> I didn't know if the Git community would accept a hard dependency on
> Rust, so I made ivec usable in C without needing Rust.

Right—I saw both implementations, but it looked like C did most of the work, which was my main question. Re-reading, it looks like Rust does more work than I thought (with implementations of insert/push/etc.)

That said, I think it's sensible to leave the type useable from just C unless/until Rust becomes required (and then we can move things over).

Thanks!

-- 
D. Ben Knoble