rust: pin-init: move InPlaceInit and impls of InPlaceWrite into the kernel crate

In order to make pin-init a standalone crate, move kernel-specific code
directly into the kernel crate. This includes the `InPlaceInit<T>`
trait, its implementations and the implementations of `InPlaceWrite` for
`Arc` and `UniqueArc`. All of these use the kernel's error type which
will become unavailable in pin-init.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-9-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: y86-dev

rust/kernel/alloc/kbox.rs

@@ -15,7 +15,8 @@ use core::pin::Pin;
 use core::ptr::NonNull;
 use core::result::Result;
 
-use crate::init::{InPlaceInit, InPlaceWrite, Init, PinInit};
+use crate::init::{InPlaceWrite, Init, PinInit};
+use crate::init_ext::InPlaceInit;
 use crate::types::ForeignOwnable;
 
 /// The kernel's [`Box`] type -- a heap allocation for a single value of type `T`.

rust/kernel/init.rs

@@ -134,6 +134,61 @@
 //! }
 //! ```
 
+use crate::{
+    alloc::{AllocError, Flags},
+    error::{self, Error},
+    init::{init_from_closure, pin_init_from_closure, Init, PinInit},
+};
+
+/// Smart pointer that can initialize memory in-place.
+pub trait InPlaceInit<T>: Sized {
+    /// Pinned version of `Self`.
+    ///
+    /// If a type already implicitly pins its pointee, `Pin<Self>` is unnecessary. In this case use
+    /// `Self`, otherwise just use `Pin<Self>`.
+    type PinnedSelf;
+
+    /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
+    /// type.
+    ///
+    /// If `T: !Unpin` it will not be able to move afterwards.
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
+    where
+        E: From<AllocError>;
+
+    /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
+    /// type.
+    ///
+    /// If `T: !Unpin` it will not be able to move afterwards.
+    fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self::PinnedSelf>
+    where
+        Error: From<E>,
+    {
+        // SAFETY: We delegate to `init` and only change the error type.
+        let init = unsafe {
+            pin_init_from_closure(|slot| init.__pinned_init(slot).map_err(|e| Error::from(e)))
+        };
+        Self::try_pin_init(init, flags)
+    }
+
+    /// Use the given initializer to in-place initialize a `T`.
+    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
+    where
+        E: From<AllocError>;
+
+    /// Use the given initializer to in-place initialize a `T`.
+    fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
+    where
+        Error: From<E>,
+    {
+        // SAFETY: We delegate to `init` and only change the error type.
+        let init = unsafe {
+            init_from_closure(|slot| init.__pinned_init(slot).map_err(|e| Error::from(e)))
+        };
+        Self::try_init(init, flags)
+    }
+}
+
 /// Construct an in-place fallible initializer for `struct`s.
 ///
 /// This macro defaults the error to [`Error`]. If you need [`Infallible`], then use

rust/kernel/prelude.rs

@@ -36,6 +36,7 @@ pub use super::error::{code::*, Error, Result};
 
 pub use super::{str::CStr, ThisModule};
 
-pub use super::init::{InPlaceInit, InPlaceWrite, Init, PinInit};
+pub use super::init::{InPlaceWrite, Init, PinInit};
+pub use super::init_ext::InPlaceInit;
 
 pub use super::current;

rust/kernel/sync/arc.rs

@@ -19,7 +19,8 @@
 use crate::{
     alloc::{AllocError, Flags, KBox},
     bindings,
-    init::{self, InPlaceInit, Init, PinInit},
+    init::{self, InPlaceWrite, Init, PinInit},
+    init_ext::InPlaceInit,
     try_init,
     types::{ForeignOwnable, Opaque},
 };
@@ -202,6 +203,26 @@ unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
 // the reference count reaches zero and `T` is dropped.
 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 
+impl<T> InPlaceInit<T> for Arc<T> {
+    type PinnedSelf = Self;
+
+    #[inline]
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::try_pin_init(init, flags).map(|u| u.into())
+    }
+
+    #[inline]
+    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::try_init(init, flags).map(|u| u.into())
+    }
+}
+
 impl<T> Arc<T> {
     /// Constructs a new reference counted instance of `T`.
     pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
@@ -659,6 +680,48 @@ pub struct UniqueArc<T: ?Sized> {
     inner: Arc<T>,
 }
 
+impl<T> InPlaceInit<T> for UniqueArc<T> {
+    type PinnedSelf = Pin<Self>;
+
+    #[inline]
+    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::new_uninit(flags)?.write_pin_init(init)
+    }
+
+    #[inline]
+    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
+    where
+        E: From<AllocError>,
+    {
+        UniqueArc::new_uninit(flags)?.write_init(init)
+    }
+}
+
+impl<T> InPlaceWrite<T> for UniqueArc<MaybeUninit<T>> {
+    type Initialized = UniqueArc<T>;
+
+    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
+        let slot = self.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 { self.assume_init() })
+    }
+
+    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
+        let slot = self.as_mut_ptr();
+        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+        // slot is valid and will not be moved, because we pin it later.
+        unsafe { init.__pinned_init(slot)? };
+        // SAFETY: All fields have been initialized.
+        Ok(unsafe { self.assume_init() }.into())
+    }
+}
+
 impl<T> UniqueArc<T> {
     /// Tries to allocate a new [`UniqueArc`] instance.
     pub fn new(value: T, flags: Flags) -> Result<Self, AllocError> {

rust/pin-init/src/lib.rs

@@ -10,7 +10,7 @@
 //! To initialize a `struct` with an in-place constructor you will need two things:
 //! - an in-place constructor,
 //! - a memory location that can hold your `struct` (this can be the [stack], an [`Arc<T>`],
-//!   [`UniqueArc<T>`], [`KBox<T>`] or any other smart pointer that implements [`InPlaceInit`]).
+//!   [`KBox<T>`] or any other smart pointer that supports this library).
 //!
 //! To get an in-place constructor there are generally three options:
 //! - directly creating an in-place constructor using the [`pin_init!`] macro,
@@ -212,10 +212,7 @@
 //! [`pin_init!`]: crate::pin_init!
 
 use crate::{
-    alloc::{AllocError, Flags, KBox},
-    error::{self, Error},
-    sync::Arc,
-    sync::UniqueArc,
+    alloc::KBox,
     types::{Opaque, ScopeGuard},
 };
 use core::{
@@ -891,8 +888,7 @@ macro_rules! assert_pinned {
 /// A pin-initializer for the type `T`.
 ///
 /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
-/// be [`KBox<T>`], [`Arc<T>`], [`UniqueArc<T>`] or even the stack (see [`stack_pin_init!`]). Use
-/// the [`InPlaceInit::pin_init`] function of a smart pointer like [`Arc<T>`] on this.
+/// be [`KBox<T>`], [`Arc<T>`] or even the stack (see [`stack_pin_init!`]).
 ///
 /// Also see the [module description](self).
 ///
@@ -910,7 +906,6 @@ macro_rules! assert_pinned {
 /// - while constructing the `T` at `slot` it upholds the pinning invariants of `T`.
 ///
 /// [`Arc<T>`]: crate::sync::Arc
-/// [`Arc::pin_init`]: crate::sync::Arc::pin_init
 #[must_use = "An initializer must be used in order to create its value."]
 pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// Initializes `slot`.
@@ -976,8 +971,7 @@ where
 /// An initializer for `T`.
 ///
 /// To use this initializer, you will need a suitable memory location that can hold a `T`. This can
-/// be [`KBox<T>`], [`Arc<T>`], [`UniqueArc<T>`] or even the stack (see [`stack_pin_init!`]). Use
-/// the [`InPlaceInit::init`] function of a smart pointer like [`Arc<T>`] on this. Because
+/// be [`KBox<T>`], [`Arc<T>`] or even the stack (see [`stack_pin_init!`]). Because
 /// [`PinInit<T, E>`] is a super trait, you can use every function that takes it as well.
 ///
 /// Also see the [module description](self).
@@ -1238,95 +1232,6 @@ unsafe impl<T, E> PinInit<T, E> for T {
     }
 }
 
-/// Smart pointer that can initialize memory in-place.
-pub trait InPlaceInit<T>: Sized {
-    /// Pinned version of `Self`.
-    ///
-    /// If a type already implicitly pins its pointee, `Pin<Self>` is unnecessary. In this case use
-    /// `Self`, otherwise just use `Pin<Self>`.
-    type PinnedSelf;
-
-    /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
-    /// type.
-    ///
-    /// If `T: !Unpin` it will not be able to move afterwards.
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
-    where
-        E: From<AllocError>;
-
-    /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this
-    /// type.
-    ///
-    /// If `T: !Unpin` it will not be able to move afterwards.
-    fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> error::Result<Self::PinnedSelf>
-    where
-        Error: From<E>,
-    {
-        // SAFETY: We delegate to `init` and only change the error type.
-        let init = unsafe {
-            pin_init_from_closure(|slot| init.__pinned_init(slot).map_err(|e| Error::from(e)))
-        };
-        Self::try_pin_init(init, flags)
-    }
-
-    /// Use the given initializer to in-place initialize a `T`.
-    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
-    where
-        E: From<AllocError>;
-
-    /// Use the given initializer to in-place initialize a `T`.
-    fn init<E>(init: impl Init<T, E>, flags: Flags) -> error::Result<Self>
-    where
-        Error: From<E>,
-    {
-        // SAFETY: We delegate to `init` and only change the error type.
-        let init = unsafe {
-            init_from_closure(|slot| init.__pinned_init(slot).map_err(|e| Error::from(e)))
-        };
-        Self::try_init(init, flags)
-    }
-}
-
-impl<T> InPlaceInit<T> for Arc<T> {
-    type PinnedSelf = Self;
-
-    #[inline]
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
-    where
-        E: From<AllocError>,
-    {
-        UniqueArc::try_pin_init(init, flags).map(|u| u.into())
-    }
-
-    #[inline]
-    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
-    where
-        E: From<AllocError>,
-    {
-        UniqueArc::try_init(init, flags).map(|u| u.into())
-    }
-}
-
-impl<T> InPlaceInit<T> for UniqueArc<T> {
-    type PinnedSelf = Pin<Self>;
-
-    #[inline]
-    fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self::PinnedSelf, E>
-    where
-        E: From<AllocError>,
-    {
-        UniqueArc::new_uninit(flags)?.write_pin_init(init)
-    }
-
-    #[inline]
-    fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
-    where
-        E: From<AllocError>,
-    {
-        UniqueArc::new_uninit(flags)?.write_init(init)
-    }
-}
-
 /// Smart pointer containing uninitialized memory and that can write a value.
 pub trait InPlaceWrite<T> {
     /// The type `Self` turns into when the contents are initialized.
@@ -1343,28 +1248,6 @@ pub trait InPlaceWrite<T> {
     fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E>;
 }
 
-impl<T> InPlaceWrite<T> for UniqueArc<MaybeUninit<T>> {
-    type Initialized = UniqueArc<T>;
-
-    fn write_init<E>(mut self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
-        let slot = self.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 { self.assume_init() })
-    }
-
-    fn write_pin_init<E>(mut self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
-        let slot = self.as_mut_ptr();
-        // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
-        // slot is valid and will not be moved, because we pin it later.
-        unsafe { init.__pinned_init(slot)? };
-        // SAFETY: All fields have been initialized.
-        Ok(unsafe { self.assume_init() }.into())
-    }
-}
-
 /// Trait facilitating pinned destruction.
 ///
 /// Use [`pinned_drop`] to implement this trait safely: