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([2001:4bc9:1f91:3f49:880a:9a87:b7c1:e3a3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4406d5a9ed9sm180011125e9.3.2025.04.22.07.17.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Apr 2025 07:17:35 -0700 (PDT) Message-ID: Date: Tue, 22 Apr 2025 16:17:33 +0200 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/8] rust: pin-init: Implement `Wrapper` for `UnsafePinned` behind feature flag. To: Benno Lossin , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Fiona Behrens Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250421221728.528089-1-benno.lossin@proton.me> <20250421221728.528089-4-benno.lossin@proton.me> <4df4e3a0-9caa-4253-bf65-66c238bf0291@gmail.com> Content-Language: en-US, de-DE From: Christian Schrefl In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 22.04.25 1:21 PM, Benno Lossin wrote: > On Tue Apr 22, 2025 at 11:42 AM CEST, Christian Schrefl wrote: >> On 22.04.25 12:18 AM, Benno Lossin wrote: >>> @@ -1557,3 +1561,11 @@ fn pin_init(value_init: impl PinInit) -> impl PinInit { >>> unsafe { cast_pin_init(value_init) } >>> } >>> } >>> + >>> +#[cfg(all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED))] >>> +impl Wrapper for core::pin::UnsafePinned { >>> + fn pin_init(init: impl PinInit) -> impl PinInit { >>> + // SAFETY: `UnsafePinned` has a compatible layout to `T`. >>> + unsafe { cast_pin_init(init) } >>> + } >>> +} >> >> I've realized that for us to use `UnsafePinned` in `Opaque` internally >> we need to have a `T: ?Sized` for this implementation. `cast_pin_init` >> won't work for that since we can't cast pointers between different DSTs. >> We could add something similar that uses a closure to convert a >> `*mut T` to `*mut U`. > > I don't follow, which types do you need to wrap in `Opaque` that are not > sized? > > Since `Opaque` uses `MaybeUninit` under the hood, it cannot support > `!Sized` types anyways, so I'm a bit confused. Sorry I got an `?Sized` error and convinced myself that `Opaque` had a `T: ?Sized` bound. After looking at it again there is no problem. I'll work on V2 of my `UnsafePinned` patch with a second commit for using it in `Opaque`. Cheers Chrisitan