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Date: Sat, 03 Aug 2024 14:23:42 +0000
To: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, Andreas Hindborg <a.hindborg@samsung.com>, Alice Ryhl <aliceryhl@google.com>
From: Benno Lossin <benno.lossin@proton.me>
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/2] rust: kernel: add `drop_contents` to `BoxExt`
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On 03.08.24 16:16, Benno Lossin wrote:
> @@ -53,4 +69,12 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<=
T>>, AllocError> {
>          // zero-sized types, we use `NonNull::dangling`.
>          Ok(unsafe { Box::from_raw(ptr) })
>      }
> +
> +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
> +        let ptr =3D Box::into_raw(this);
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { ptr::drop_in_place(ptr) };
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.

I just noticed that I missed another comment from Boqun here. Got
confused with the two mails. I would replace the comment above with

    // CAST: `T` and `MaybeUninit<T>` have the same layout.
    let ptr =3D ptr.cast::<MaybeUninit<T>>();
    // SAFETY: `ptr` is valid for writes, because it came from `Box::into_r=
aw` and it is valid for
    // reads, since the pointer came from `Box::into_raw` and the type is `=
MaybeUninit<T>`.

Let me know if you want another version.

---
Cheers,
Benno

> +        unsafe { Box::from_raw(ptr.cast()) }
> +    }
>  }
> --
> 2.45.2
>=20
>=20
>=20