rust: init: add `stack_pin_init!` macro

The `stack_pin_init!` macro allows pin-initializing a value on the
stack. It accepts a `impl PinInit<T, E>` to initialize a `T`. It allows
propagating any errors via `?` or handling it normally via `match`.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20230408122429.1103522-11-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

2 files changed, 191 insertions, 6 deletions

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 37e8159df24d..99751375e7c8 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -12,7 +12,8 @@
 //!
 //! 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`.
+//! - a memory location that can hold your `struct` (this can be the [stack], an [`Arc<T>`],
+//!   [`UniqueArc<T>`], [`Box<T>`] or any other smart pointer that implements [`InPlaceInit`]).
 //!
 //! To get an in-place constructor there are generally three options:
 //! - directly creating an in-place constructor using the [`pin_init!`] macro,
@@ -180,6 +181,7 @@
 //! [pinning]: https://doc.rust-lang.org/std/pin/index.html
 //! [structurally pinned fields]:
 //!     https://doc.rust-lang.org/std/pin/index.html#pinning-is-structural-for-field
+//! [stack]: crate::stack_pin_init
 //! [`Arc<T>`]: crate::sync::Arc
 //! [`impl PinInit<Foo>`]: PinInit
 //! [`impl PinInit<T, E>`]: PinInit
@@ -202,6 +204,132 @@ pub mod __internal;
 #[doc(hidden)]
 pub mod macros;
 
+/// Initialize and pin a type directly on the stack.
+///
+/// # Examples
+///
+/// ```rust
+/// # #![allow(clippy::disallowed_names, clippy::new_ret_no_self)]
+/// # use kernel::{init, pin_init, stack_pin_init, init::*, sync::Mutex, new_mutex};
+/// # use macros::pin_data;
+/// # use core::pin::Pin;
+/// #[pin_data]
+/// struct Foo {
+///     #[pin]
+///     a: Mutex<usize>,
+///     b: Bar,
+/// }
+///
+/// #[pin_data]
+/// struct Bar {
+///     x: u32,
+/// }
+///
+/// stack_pin_init!(let foo = pin_init!(Foo {
+///     a <- new_mutex!(42),
+///     b: Bar {
+///         x: 64,
+///     },
+/// }));
+/// let foo: Pin<&mut Foo> = foo;
+/// pr_info!("a: {}", &*foo.a.lock());
+/// ```
+///
+/// # Syntax
+///
+/// A normal `let` binding with optional type annotation. The expression is expected to implement
+/// [`PinInit`]/[`Init`] with the error type [`Infallible`]. If you want to use a different error
+/// type, then use [`stack_try_pin_init!`].
+#[macro_export]
+macro_rules! stack_pin_init {
+    (let $var:ident $(: $t:ty)? = $val:expr) => {
+        let val = $val;
+        let mut $var = ::core::pin::pin!($crate::init::__internal::StackInit$(::<$t>)?::uninit());
+        let mut $var = match $crate::init::__internal::StackInit::init($var, val) {
+            Ok(res) => res,
+            Err(x) => {
+                let x: ::core::convert::Infallible = x;
+                match x {}
+            }
+        };
+    };
+}
+
+/// Initialize and pin a type directly on the stack.
+///
+/// # Examples
+///
+/// ```rust
+/// # #![allow(clippy::disallowed_names, clippy::new_ret_no_self)]
+/// # use kernel::{init, pin_init, stack_try_pin_init, init::*, sync::Mutex, new_mutex};
+/// # use macros::pin_data;
+/// # use core::{alloc::AllocError, pin::Pin};
+/// #[pin_data]
+/// struct Foo {
+///     #[pin]
+///     a: Mutex<usize>,
+///     b: Box<Bar>,
+/// }
+///
+/// struct Bar {
+///     x: u32,
+/// }
+///
+/// stack_try_pin_init!(let foo: Result<Pin<&mut Foo>, AllocError> = pin_init!(Foo {
+///     a <- new_mutex!(42),
+///     b: Box::try_new(Bar {
+///         x: 64,
+///     })?,
+/// }));
+/// let foo = foo.unwrap();
+/// pr_info!("a: {}", &*foo.a.lock());
+/// ```
+///
+/// ```rust
+/// # #![allow(clippy::disallowed_names, clippy::new_ret_no_self)]
+/// # use kernel::{init, pin_init, stack_try_pin_init, init::*, sync::Mutex, new_mutex};
+/// # use macros::pin_data;
+/// # use core::{alloc::AllocError, pin::Pin};
+/// #[pin_data]
+/// struct Foo {
+///     #[pin]
+///     a: Mutex<usize>,
+///     b: Box<Bar>,
+/// }
+///
+/// struct Bar {
+///     x: u32,
+/// }
+///
+/// stack_try_pin_init!(let foo: Pin<&mut Foo> =? pin_init!(Foo {
+///     a <- new_mutex!(42),
+///     b: Box::try_new(Bar {
+///         x: 64,
+///     })?,
+/// }));
+/// pr_info!("a: {}", &*foo.a.lock());
+/// # Ok::<_, AllocError>(())
+/// ```
+///
+/// # Syntax
+///
+/// A normal `let` binding with optional type annotation. The expression is expected to implement
+/// [`PinInit`]/[`Init`]. This macro assigns a result to the given variable, adding a `?` after the
+/// `=` will propagate this error.
+#[macro_export]
+macro_rules! stack_try_pin_init {
+    (let $var:ident $(: $t:ty)? = $val:expr) => {
+        let val = $val;
+        let mut $var = ::core::pin::pin!($crate::init::__internal::StackInit$(::<$t>)?::uninit());
+        let mut $var = $crate::init::__internal::StackInit::init($var, val);
+    };
+    (let $var:ident $(: $t:ty)? =? $val:expr) => {
+        let val = $val;
+        let mut $var = ::core::pin::pin!($crate::init::__internal::StackInit$(::<$t>)?::uninit());
+        let mut $var = $crate::init::__internal::StackInit::init($var, val)?;
+    };
+}
+
 /// Construct an in-place, pinned initializer for `struct`s.
 ///
 /// This macro defaults the error to [`Infallible`]. If you need [`Error`], then use
@@ -913,8 +1041,8 @@ macro_rules! try_init {
 /// 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 [`Box<T>`], [`Arc<T>`], [`UniqueArc<T>`]. Use the [`InPlaceInit::pin_init`] function of a
-/// smart pointer like [`Arc<T>`] on this.
+/// be [`Box<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.
 ///
 /// Also see the [module description](self).
 ///
@@ -949,9 +1077,9 @@ 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>`]. 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.
+/// be [`Box<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
+/// [`PinInit<T, E>`] is a super trait, you can use every function that takes it as well.
 ///
 /// Also see the [module description](self).
 ///
diff --git a/rust/kernel/init/__internal.rs b/rust/kernel/init/__internal.rs
index 774cb620afa7..44751fb62b51 100644
--- a/rust/kernel/init/__internal.rs
+++ b/rust/kernel/init/__internal.rs
@@ -112,6 +112,63 @@ unsafe impl<T: ?Sized> HasInitData for T {
     }
 }
 
+/// Stack initializer helper type. Use [`stack_pin_init`] instead of this primitive.
+///
+/// # Invariants
+///
+/// If `self.is_init` is true, then `self.value` is initialized.
+///
+/// [`stack_pin_init`]: kernel::stack_pin_init
+pub struct StackInit<T> {
+    value: MaybeUninit<T>,
+    is_init: bool,
+}
+
+impl<T> Drop for StackInit<T> {
+    #[inline]
+    fn drop(&mut self) {
+        if self.is_init {
+            // SAFETY: As we are being dropped, we only call this once. And since `self.is_init` is
+            // true, `self.value` is initialized.
+            unsafe { self.value.assume_init_drop() };
+        }
+    }
+}
+
+impl<T> StackInit<T> {
+    /// Creates a new [`StackInit<T>`] that is uninitialized. Use [`stack_pin_init`] instead of this
+    /// primitive.
+    ///
+    /// [`stack_pin_init`]: kernel::stack_pin_init
+    #[inline]
+    pub fn uninit() -> Self {
+        Self {
+            value: MaybeUninit::uninit(),
+            is_init: false,
+        }
+    }
+
+    /// Initializes the contents and returns the result.
+    #[inline]
+    pub fn init<E>(self: Pin<&mut Self>, init: impl PinInit<T, E>) -> Result<Pin<&mut T>, E> {
+        // SAFETY: We never move out of `this`.
+        let this = unsafe { Pin::into_inner_unchecked(self) };
+        // The value is currently initialized, so it needs to be dropped before we can reuse
+        // the memory (this is a safety guarantee of `Pin`).
+        if this.is_init {
+            this.is_init = false;
+            // SAFETY: `this.is_init` was true and therefore `this.value` is initialized.
+            unsafe { this.value.assume_init_drop() };
+        }
+        // SAFETY: The memory slot is valid and this type ensures that it will stay pinned.
+        unsafe { init.__pinned_init(this.value.as_mut_ptr())? };
+        // INVARIANT: `this.value` is initialized above.
+        this.is_init = true;
+        // SAFETY: The slot is now pinned, since we will never give access to `&mut T`.
+        Ok(unsafe { Pin::new_unchecked(this.value.assume_init_mut()) })
+    }
+}
+
 /// When a value of this type is dropped, it drops a `T`.
 ///
 /// Can be forgotten to prevent the drop.