Re: [PATCH 07/10] rust: alloc: update `VecExt` to take allocation flags

Wedson Almeida Filho
2024-03-26T00:04:10Z

On Mon, 25 Mar 2024 at 17:44, Boqun Feng <boqun.feng@gmail.com> wrote:
>
> On Mon, Mar 25, 2024 at 04:54:15PM -0300, Wedson Almeida Filho wrote:
> [...]
> > + fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
> > + <Self as VecExt<_>>::reserve(self, 1, flags)?;
> > + let (ptr, len, cap) = destructure(self);
> > + // SAFETY: ptr is valid for `cap` elements. And `cap` is greater (by at least 1) than
> > + // `len` because of the call to `reserve` above. So the pointer after offsetting by `len`
> > + // elements is valid for write.
> > + unsafe { ptr.wrapping_add(len).write(v) };
> > +
> > + // SAFETY: The only difference from the values returned by `destructure` is that `length`
> > + // is incremented by 1, which is fine because we have just initialised the element at
> > + // offset `length`.
> > + unsafe { rebuild(self, ptr, len + 1, cap) };
>
> probably use spare_capacity_mut() here to avoid `destructure` and
> `rebuild`?

Ah, yes, this sounds like a better approach. I will use this in v2.

>
> https://doc.rust-lang.org/std/vec/struct.Vec.html#method.spare_capacity_mut
>
> // .. after reserve succeed.
> // there must be room for adding one more.
> self.spare_capacity_mut()[0].write(v);
> // or unsafe { self.spare_capacity_mut().as_mut_ptr().cast().write(v); }
>
> unsafe {
> self.set_len(self.len() + 1);
> }
>
> Thoughts?
>
> Regards,
> Boqun
>
> > Ok(())
> > }
> >
> …