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Date:   Thu, 30 Mar 2023 11:58:08 +0100
From:   Gary Guo <gary@garyguo.net>
To:     y86-dev@protonmail.com
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Alice Ryhl <alice@ryhl.io>, rust-for-linux@vger.kernel.org,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH v3 06/13] rust: init/sync: add `InPlaceInit` trait to
 pin-initialize smart pointers
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On Wed, 29 Mar 2023 22:33:18 +0000
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>
> ---
>  rust/kernel/init.rs     | 97 +++++++++++++++++++++++++++++++++++------
>  rust/kernel/sync/arc.rs | 25 ++++++++++-
>  2 files changed, 108 insertions(+), 14 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 85e8d5f41b60..3d89c7e3bdb5 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -114,10 +114,13 @@
>  //! [`impl Init<T, E>`]: Init
>  //! [`Opaque`]: kernel::types::Opaque
>  //! [`pin_data`]: ::macros::pin_data
> -//! [`UniqueArc<T>`]: kernel::sync::UniqueArc
> 
> +use crate::{
> +    error::{self, Error},
> +    sync::UniqueArc,
> +};
>  use alloc::boxed::Box;
> -use core::{cell::Cell, convert::Infallible, marker::PhantomData, mem::MaybeUninit, ptr};
> +use core::{cell::Cell, convert::Infallible, marker::PhantomData, mem::MaybeUninit, pin::Pin, ptr};
> 
>  #[doc(hidden)]
>  pub mod __internal;
> @@ -309,7 +312,6 @@ pub mod macros;
>  ///
>  /// [`try_pin_init!`]: kernel::try_pin_init
>  /// [`NonNull<Self>`]: core::ptr::NonNull
> -/// [`Error`]: kernel::error::Error
>  // For a detailed example of how this macro works, see the module documentation of the hidden
>  // module `__internal` inside of `init/__internal.rs`.
>  #[macro_export]
> @@ -366,8 +368,6 @@ macro_rules! pin_init {
>  ///     }
>  /// }
>  /// ```
> -///
> -/// [`Error`]: kernel::error::Error
>  // For a detailed example of how this macro works, see the module documentation of the hidden
>  // module `__internal` inside of `init/__internal.rs`.
>  #[macro_export]
> @@ -589,8 +589,6 @@ macro_rules! try_pin_init {
>  ///
>  /// This initializer is for initializing data in-place that might later be moved. If you want to
>  /// pin-initialize, use [`pin_init!`].
> -///
> -/// [`Error`]: kernel::error::Error
>  // For a detailed example of how this macro works, see the module documentation of the hidden
>  // module `__internal` inside of `init/__internal.rs`.
>  #[macro_export]
> @@ -641,8 +639,6 @@ macro_rules! init {
>  ///     }
>  /// }
>  /// ```
> -///
> -/// [`Error`]: kernel::error::Error
>  // For a detailed example of how this macro works, see the module documentation of the hidden
>  // module `__internal` inside of `init/__internal.rs`.
>  #[macro_export]
> @@ -848,7 +844,8 @@ macro_rules! try_init {
>  /// A pinned initializer for `T`.
>  ///
>  /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
> -/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`].
> +/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Use the [`InPlaceInit::pin_init`] function of a
> +/// smart pointer like [`Arc<T>`] on this.
>  ///
>  /// Also see the [module description](self).
>  ///
> @@ -867,7 +864,6 @@ macro_rules! try_init {
>  ///
>  /// [`Arc<T>`]: crate::sync::Arc
>  /// [`Arc::pin_init`]: crate::sync::Arc::pin_init
> -/// [`UniqueArc<T>`]: kernel::sync::UniqueArc
>  #[must_use = "An initializer must be used in order to create its value."]
>  pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
>      /// Initializes `slot`.
> @@ -884,7 +880,8 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
>  /// An initializer for `T`.
>  ///
>  /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
> -/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Because [`PinInit<T, E>`] is a super trait, you can
> +/// be [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Use the [`InPlaceInit::init`] function of a smart
> +/// pointer like [`Arc<T>`] on this. Because [`PinInit<T, E>`] is a super trait, you can
>  /// use every function that takes it as well.
>  ///
>  /// Also see the [module description](self).
> @@ -909,7 +906,6 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
>  /// move the pointee after initialization.
>  ///
>  /// [`Arc<T>`]: crate::sync::Arc
> -/// [`UniqueArc<T>`]: kernel::sync::UniqueArc
>  #[must_use = "An initializer must be used in order to create its value."]
>  pub unsafe trait Init<T: ?Sized, E = Infallible>: PinInit<T, E> {
>      /// Initializes `slot`.
> @@ -1014,3 +1010,78 @@ unsafe impl<T> Init<T> for T {
>          Ok(())
>      }
>  }
> +
> +/// 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>;
> +}
> +
> +impl<T> InPlaceInit<T> for Box<T> {
> +    #[inline]
> +    fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
> +    where
> +        Error: From<E>,
> +    {
> +        let mut this = Box::try_new_uninit()?;
> +        let slot = this.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid and will not be moved because of the `Pin::new_unchecked`
> +        unsafe { init.__pinned_init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { Pin::new_unchecked(this.assume_init()) })

This can be either `Box::into_pin` or just `into()`.

> +    }
> +
> +    #[inline]
> +    fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
> +    where
> +        Error: From<E>,
> +    {
> +        let mut this = Box::try_new_uninit()?;
> +        let slot = this.as_mut_ptr();
> +        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
> +        // slot is valid
> +        unsafe { init.__init(slot)? };
> +        // SAFETY: All fields have been initialized.
> +        Ok(unsafe { this.assume_init() })
> +    }
> +}