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Date:   Thu, 30 Mar 2023 16:37:01 +0200
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Subject: Re: [PATCH v3 06/13] rust: init/sync: add `InPlaceInit` trait to
 pin-initialize smart pointers
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To:     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>
References: <20230329223239.138757-1-y86-dev@protonmail.com>
 <20230329223239.138757-7-y86-dev@protonmail.com>
From:   Alice Ryhl <alice@ryhl.io>
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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>;
}