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

[Ezekiel Newren <ezekielnewren@xxxxxxxxx>]

On Sun, Aug 24, 2025 at 7:31 AM Ben Knoble <ben.knoble@xxxxxxxxx> 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.

=== Rust version 1.61.0 ===
from: https://doc.rust-lang.org/1.61.0/src/alloc/vec/mod.rs.html#400
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Vec")]
#[rustc_insignificant_dtor]
pub struct Vec<T, #[unstable(feature = "allocator_api", issue =
"32838")] A: Allocator = Global> {
    buf: RawVec<T, A>,
    len: usize,
}

from: https://doc.rust-lang.org/1.61.0/src/alloc/raw_vec.rs.html#52
#[allow(missing_debug_implementations)]
pub(crate) struct RawVec<T, A: Allocator = Global> {
    ptr: Unique<T>,
    cap: usize,
    alloc: A,
}

=== Rust version 1.89.0 ===
from: https://doc.rust-lang.org/1.89.0/src/alloc/vec/mod.rs.html#414
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "Vec"]
#[rustc_insignificant_dtor]
pub struct Vec<T, #[unstable(feature = "allocator_api", issue =
"32838")] A: Allocator = Global> {
    buf: RawVec<T, A>,
    len: usize,
}

from: https://doc.rust-lang.org/1.89.0/src/alloc/raw_vec/mod.rs.html#74
#[allow(missing_debug_implementations)]
pub(crate) struct RawVec<T, A: Allocator = Global> {
    inner: RawVecInner<A>,
    _marker: PhantomData<T>,
}

from: https://doc.rust-lang.org/1.89.0/src/alloc/raw_vec/mod.rs.html#86
#[allow(missing_debug_implementations)]
struct RawVecInner<A: Allocator = Global> {
    ptr: Unique<u8>,
    /// Never used for ZSTs; it's `capacity()`'s responsibility to
return usize::MAX in that case.
    ///
    /// # Safety
    ///
    /// `cap` must be in the `0..=isize::MAX` range.
    cap: Cap,
    alloc: A,
}

> 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.