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Date:   Thu, 30 Mar 2023 15:28:17 +0000
To:     Alice Ryhl <alice@ryhl.io>
From:   Benno Lossin <y86-dev@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, 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>
Subject: Re: [PATCH v3 06/13] rust: init/sync: add `InPlaceInit` trait to pin-initialize smart pointers
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On 30.03.23 16:37, Alice Ryhl wrote:
> On 3/30/23 00:33, y86-dev@protonmail.com wrote:
>> From: Benno Lossin <y86-dev@protonmail.com>
>>
>> The `InPlaceInit` trait that provides two functions, for initializing
>> using `PinInit<T, E>` and `Init<T>`. It is implemented by `Arc<T>`,
>> `UniqueArc<T>` and `Box<T>`.
>>
>> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
>   > ---
>   >
>> +/// Smart pointer that can initialize memory in-place.
>> +pub trait InPlaceInit<T>: Sized {
>> +    /// Use the given initializer to in-place initialize a `T`.
>> +    ///
>> +    /// If `T: !Unpin` it will not be able to move afterwards.
>> +    fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>=
>
>> +    where
>> +        Error: From<E>;
>> +
>> +    /// Use the given initializer to in-place initialize a `T`.
>> +    fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
>> +    where
>> +        Error: From<E>;
>> +}
>
> This definition is potentially rather limiting, because it can only be
> used with error types that can be converted into a `kernel::Error`. What
> do you think of this alternative?
>
> pub trait InPlaceInit<T>: Sized {
>       fn pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
>       where
>           E: From<AllocError>;
>
>       fn init<E>(init: impl Init<T, E>) -> Result<Self, E>
>       where
>           E: From<AllocError>;
> }

I initially implemented it like this, but it required almost always that
`E` is specified, I will try and see if the situation is any different now,
but I do not think so. In the user-space version of this API (see [1]) I
have four functions, normal variants that return an `AllocError` and `try`
variants that look exactly like what you suggested. In the kernel, we could
make the normal variants as they are now and add the `try` variants as you
described.

[1]: https://docs.rs/pinned-init/0.0.5/pinned_init/trait.InPlaceInit.html

--
Cheers,
Benno