[PATCH 1/6] rust: alloc: add Vec::push_init

Add `Vec::push_init` which initializes a new element in place. We can't
modify the existing `Vec::push` signature to take an `impl Init<T, E>`
without changing its Error type.

Signed-off-by: Eliot Courtney <[email protected]>
---
 rust/kernel/alloc/kvec.rs | 42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index c7546b9da4fa..9f6f25d7e218 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -52,7 +52,10 @@
     }, //
 };
 
-use pin_init::Zeroable;
+use pin_init::{
+    Init,
+    Zeroable, //
+};
 
 mod errors;
 pub use self::errors::{InsertError, PushError, RemoveError};
@@ -359,6 +362,43 @@ pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
         Ok(())
     }
 
+    /// Appends an element to the back of the [`Vec`] instance by initializing it in place.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// struct Element {
+    ///     buf: KVec<u8>,
+    /// }
+    ///
+    /// impl Element {
+    ///     fn new() -> impl Init<Self, Error> {
+    ///         try_init!(Element {
+    ///             buf: KVec::with_capacity(16, GFP_KERNEL)?,
+    ///         }? Error)
+    ///     }
+    /// }
+    ///
+    /// let mut v: KVec<Element> = KVec::new();
+    /// v.push_init(Element::new(), GFP_KERNEL)?;
+    /// assert!(v[0].buf.is_empty());
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn push_init<E>(&mut self, init: impl Init<T, E>, flags: Flags) -> Result<(), E>
+    where
+        E: From<AllocError>,
+    {
+        self.reserve(1, flags)?;
+        // SAFETY: The call to `reserve` was successful, so there is at least one spare slot; the
+        // pointer therefore refers to allocated, aligned memory valid for a write of one `T`.
+        unsafe { init.__init(self.spare_capacity_mut().as_mut_ptr().cast::<T>())? };
+        // SAFETY: The call to `__init` returned `Ok`, so the first spare slot now holds an
+        // initialized `T`. The new length does not exceed the capacity because `reserve` ensured
+        // the capacity is greater than the length by at least one.
+        unsafe { self.inc_len(1) };
+        Ok(())
+    }
+
     /// Appends an element to the back of the [`Vec`] instance without reallocating.
     ///
     /// Fails if the vector does not have capacity for the new element.

-- 
2.55.0