rust: pin-init: add std and alloc support from the user-space version

To synchronize the kernel's version of pin-init with the user-space
version, introduce support for `std` and `alloc`. While the kernel uses
neither, the user-space version has to support both. Thus include the
required `#[cfg]`s and additional code.

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/20250308110339.2997091-17-benno.lossin@proton.me
[ Undo the temporary `--extern force:alloc` since now we have contents
  for `alloc` here. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -121,7 +121,7 @@ rustdoc-pin_init_internal: $(src)/pin-init/internal/src/lib.rs FORCE
 
 rustdoc-pin_init: private rustdoc_host = yes
 rustdoc-pin_init: private rustc_target_flags = --extern pin_init_internal \
-    --extern macros --extern force:alloc --cfg kernel --cfg feature=\"alloc\"
+    --extern macros --extern alloc --cfg kernel --cfg feature=\"alloc\"
 rustdoc-pin_init: $(src)/pin-init/src/lib.rs rustdoc-pin_init_internal \
     rustdoc-macros FORCE
 	+$(call if_changed,rustdoc)

rust/pin-init/src/__internal.rs

@@ -189,6 +189,33 @@ pub fn init<E>(self: Pin<&mut Self>, init: impl PinInit<T, E>) -> Result<Pin<&mu
     }
 }
 
+#[test]
+fn stack_init_reuse() {
+    use ::std::{borrow::ToOwned, println, string::String};
+    use core::pin::pin;
+
+    #[derive(Debug)]
+    struct Foo {
+        a: usize,
+        b: String,
+    }
+    let mut slot: Pin<&mut StackInit<Foo>> = pin!(StackInit::uninit());
+    let value: Result<Pin<&mut Foo>, core::convert::Infallible> =
+        slot.as_mut().init(crate::init!(Foo {
+            a: 42,
+            b: "Hello".to_owned(),
+        }));
+    let value = value.unwrap();
+    println!("{value:?}");
+    let value: Result<Pin<&mut Foo>, core::convert::Infallible> =
+        slot.as_mut().init(crate::init!(Foo {
+            a: 24,
+            b: "world!".to_owned(),
+        }));
+    let value = value.unwrap();
+    println!("{value:?}");
+}
+
 /// When a value of this type is dropped, it drops a `T`.
 ///
 /// Can be forgotten to prevent the drop.

rust/pin-init/src/alloc.rs

@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+use alloc::{boxed::Box, sync::Arc};
+#[cfg(feature = "alloc")]
+use core::alloc::AllocError;
+use core::{mem::MaybeUninit, pin::Pin};
+#[cfg(feature = "std")]
+use std::sync::Arc;
+
+#[cfg(not(feature = "alloc"))]
+type AllocError = core::convert::Infallible;
+
+use crate::{
+    init_from_closure, pin_init_from_closure, InPlaceWrite, Init, PinInit, ZeroableOption,
+};
+
+pub extern crate alloc;
+
+// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
+//
+// In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant and there
+// is no problem with a VTABLE pointer being null.
+unsafe impl<T: ?Sized> ZeroableOption for Box<T> {}
+
+/// Smart pointer that can initialize memory in-place.
+pub trait InPlaceInit<T>: Sized {
+    /// 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>) -> Result<Pin<Self>, 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(init: impl PinInit<T>) -> Result<Pin<Self>, AllocError> {
+        // SAFETY: We delegate to `init` and only change the error type.
+        let init = unsafe {
+            pin_init_from_closure(|slot| match init.__pinned_init(slot) {
+                Ok(()) => Ok(()),
+                Err(i) => match i {},
+            })
+        };
+        Self::try_pin_init(init)
+    }
+
+    /// Use the given initializer to in-place initialize a `T`.
+    fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
+    where
+        E: From<AllocError>;
+
+    /// Use the given initializer to in-place initialize a `T`.
+    fn init(init: impl Init<T>) -> Result<Self, AllocError> {
+        // SAFETY: We delegate to `init` and only change the error type.
+        let init = unsafe {
+            init_from_closure(|slot| match init.__init(slot) {
+                Ok(()) => Ok(()),
+                Err(i) => match i {},
+            })
+        };
+        Self::try_init(init)
+    }
+}
+
+#[cfg(feature = "alloc")]
+macro_rules! try_new_uninit {
+    ($type:ident) => {
+        $type::try_new_uninit()?
+    };
+}
+#[cfg(all(feature = "std", not(feature = "alloc")))]
+macro_rules! try_new_uninit {
+    ($type:ident) => {
+        $type::new_uninit()
+    };
+}
+
+impl<T> InPlaceInit<T> for Box<T> {
+    #[inline]
+    fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
+    where
+        E: From<AllocError>,
+    {
+        try_new_uninit!(Box).write_pin_init(init)
+    }
+
+    #[inline]
+    fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
+    where
+        E: From<AllocError>,
+    {
+        try_new_uninit!(Box).write_init(init)
+    }
+}
+
+impl<T> InPlaceInit<T> for Arc<T> {
+    #[inline]
+    fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
+    where
+        E: From<AllocError>,
+    {
+        let mut this = try_new_uninit!(Arc);
+        let Some(slot) = Arc::get_mut(&mut this) else {
+            // SAFETY: the Arc has just been created and has no external references
+            unsafe { core::hint::unreachable_unchecked() }
+        };
+        let slot = slot.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 and this is the only `Arc` to that data.
+        Ok(unsafe { Pin::new_unchecked(this.assume_init()) })
+    }
+
+    #[inline]
+    fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
+    where
+        E: From<AllocError>,
+    {
+        let mut this = try_new_uninit!(Arc);
+        let Some(slot) = Arc::get_mut(&mut this) else {
+            // SAFETY: the Arc has just been created and has no external references
+            unsafe { core::hint::unreachable_unchecked() }
+        };
+        let slot = slot.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() })
+    }
+}
+
+impl<T> InPlaceWrite<T> for Box<MaybeUninit<T>> {
+    type Initialized = Box<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())
+    }
+}

rust/pin-init/src/lib.rs

@@ -204,8 +204,16 @@
 //! [structurally pinned fields]:
 //!     https://doc.rust-lang.org/std/pin/index.html#pinning-is-structural-for-field
 //! [stack]: crate::stack_pin_init
-//! [`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html
-//! [`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html
+#![cfg_attr(
+    kernel,
+    doc = "[`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html"
+)]
+#![cfg_attr(
+    kernel,
+    doc = "[`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html"
+)]
+#![cfg_attr(not(kernel), doc = "[`Arc<T>`]: alloc::alloc::sync::Arc")]
+#![cfg_attr(not(kernel), doc = "[`Box<T>`]: alloc::alloc::boxed::Box")]
 //! [`impl PinInit<Foo>`]: PinInit
 //! [`impl PinInit<T, E>`]: PinInit
 //! [`impl Init<T, E>`]: Init
@@ -239,6 +247,11 @@
 #[doc(hidden)]
 pub mod macros;
 
+#[cfg(any(feature = "std", feature = "alloc"))]
+mod alloc;
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub use alloc::InPlaceInit;
+
 /// Used to specify the pinning information of the fields of a struct.
 ///
 /// This is somewhat similar in purpose as
@@ -914,8 +927,16 @@ macro_rules! assert_pinned {
 ///     - `slot` is not partially initialized.
 /// - while constructing the `T` at `slot` it upholds the pinning invariants of `T`.
 ///
-/// [`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html
-/// [`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html
+#[cfg_attr(
+    kernel,
+    doc = "[`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html"
+)]
+#[cfg_attr(
+    kernel,
+    doc = "[`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html"
+)]
+#[cfg_attr(not(kernel), doc = "[`Arc<T>`]: alloc::alloc::sync::Arc")]
+#[cfg_attr(not(kernel), doc = "[`Box<T>`]: alloc::alloc::boxed::Box")]
 #[must_use = "An initializer must be used in order to create its value."]
 pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// Initializes `slot`.
@@ -1005,8 +1026,16 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
 /// Contrary to its supertype [`PinInit<T, E>`] the caller is allowed to
 /// move the pointee after initialization.
 ///
-/// [`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html
-/// [`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html
+#[cfg_attr(
+    kernel,
+    doc = "[`Arc<T>`]: https://rust.docs.kernel.org/kernel/sync/struct.Arc.html"
+)]
+#[cfg_attr(
+    kernel,
+    doc = "[`Box<T>`]: https://rust.docs.kernel.org/kernel/alloc/kbox/struct.Box.html"
+)]
+#[cfg_attr(not(kernel), doc = "[`Arc<T>`]: alloc::alloc::sync::Arc")]
+#[cfg_attr(not(kernel), doc = "[`Box<T>`]: alloc::alloc::boxed::Box")]
 #[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`.