[PATCH 07/10] rust: Add arrayvec

[Remo Senekowitsch <remo@buenzli.dev>]

This patch is basically a proof of concept intendend to gather feedback
about how to do this properly. Normally I would want to use the crate
from crates.io[1], but that's not an option in the kernel. We could also
vendor the entire source code of arrayvec. I'm not sure if people will
be happy with that.

[1] https://crates.io/crates/arrayvec

Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
---
 rust/kernel/arrayvec.rs | 81 +++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs      |  1 +
 2 files changed, 82 insertions(+)
 create mode 100644 rust/kernel/arrayvec.rs

diff --git a/rust/kernel/arrayvec.rs b/rust/kernel/arrayvec.rs
new file mode 100644
index 000000000..041e7dcce
--- /dev/null
+++ b/rust/kernel/arrayvec.rs
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Provides [ArrayVec], a stack-allocated vector with static capacity.
+
+use core::mem::MaybeUninit;
+
+/// A stack-allocated vector with statically fixed capacity.
+///
+/// This can be useful to avoid heap allocation and still ensure safety where a
+/// small but dynamic number of elements is needed.
+///
+/// For example, consider a function that returns a variable number of values,
+/// but no more than 8. In C, one might achieve this by passing a pointer to
+/// a stack-allocated array as an out-parameter and making the function return
+/// the actual number of elements. This is not safe, because nothing prevents
+/// the caller from reading elements from the array that weren't actually
+/// initialized by the function. `ArrayVec` solves this problem, users are
+/// prevented from accessing uninitialized elements.
+///
+/// This basically exists already (in a much more mature form) on crates.io:
+/// <https://crates.io/crates/arrayvec>
+#[derive(Debug)]
+pub struct ArrayVec<const N: usize, T> {
+    array: [core::mem::MaybeUninit<T>; N],
+    len: usize,
+}
+
+impl<const N: usize, T> ArrayVec<N, T> {
+    /// Adds a new element to the end of the vector.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the vector is already full.
+    pub fn push(&mut self, elem: T) {
+        if self.len == N {
+            panic!("OOM")
+        }
+        self.array[self.len] = MaybeUninit::new(elem);
+        self.len += 1;
+    }
+
+    /// Returns the length of the vector.
+    pub fn len(&self) -> usize {
+        self.len
+    }
+}
+
+impl<const N: usize, T> Default for ArrayVec<N, T> {
+    fn default() -> Self {
+        Self {
+            array: [const { MaybeUninit::uninit() }; N],
+            len: 0,
+        }
+    }
+}
+
+impl<const N: usize, T> AsRef<[T]> for ArrayVec<N, T> {
+    fn as_ref(&self) -> &[T] {
+        // SAFETY: As per the type invariant, all elements at index < self.len
+        // are initialized.
+        unsafe { core::mem::transmute(&self.array[..self.len]) }
+    }
+}
+
+impl<const N: usize, T> AsMut<[T]> for ArrayVec<N, T> {
+    fn as_mut(&mut self) -> &mut [T] {
+        // SAFETY: As per the type invariant, all elements at index < self.len
+        // are initialized.
+        unsafe { core::mem::transmute(&mut self.array[..self.len]) }
+    }
+}
+
+impl<const N: usize, T> Drop for ArrayVec<N, T> {
+    fn drop(&mut self) {
+        unsafe {
+            let slice: &mut [T] =
+                core::slice::from_raw_parts_mut(self.array.as_mut_ptr().cast(), self.len);
+            core::ptr::drop_in_place(slice);
+        }
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ca233fd20..0777f7a42 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -36,6 +36,7 @@
 pub use ffi;
 
 pub mod alloc;
+pub mod arrayvec;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
 #[doc(hidden)]
-- 
2.49.0