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

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Date: Mon, 25 Mar 2024 13:44:23 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Wedson Almeida Filho <wedsonaf@...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, 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`?
	
	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(())
>      }
>  
[...]