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

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Date: Mon, 25 Mar 2024 21:03:58 -0300
From: Wedson Almeida Filho <wedsonaf@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Alice Ryhl <aliceryhl@...gle.com>, linux-kernel@...r.kernel.org, 
	Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH 07/10] rust: alloc: update `VecExt` to take allocation flags

On Mon, 25 Mar 2024 at 17:44, Boqun Feng <boqun.feng@...il.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(())
> >      }
> >
> [...]