Re: [PATCH] rust: alloc: Fix `ArrayLayout` allocations

[Boqun Feng <boqun.feng@gmail.com>]

On Sat, Nov 23, 2024 at 06:39:23PM +0100, Miguel Ojeda wrote:
> On Sat, Nov 23, 2024 at 11:30 AM Asahi Lina <lina@asahilina.net> wrote:
> >
> > We were accidentally allocating a layout for the *square* of the object
> > size due to a variable shadowing mishap.
> 
> Good catch, thanks! (Square?)
> 

While we are at it, I think it'll be good to add some example/tests for
those functions of ArrayLayout, for example, the below will catch this:

I will open a good-first-issue.

Regards,
Boqun

------------------------>8
diff --git a/rust/kernel/alloc/layout.rs b/rust/kernel/alloc/layout.rs
index 7e0c2f46157b..bb3ce3b2218b 100644
--- a/rust/kernel/alloc/layout.rs
+++ b/rust/kernel/alloc/layout.rs
@@ -7,6 +7,7 @@
 use core::{alloc::Layout, marker::PhantomData};
 
 /// Error when constructing an [`ArrayLayout`].
+#[derive(Debug)]
 pub struct LayoutError;
 
 /// A layout for an array `[T; n]`.
@@ -43,6 +44,20 @@ pub const fn empty() -> Self {
     /// # Errors
     ///
     /// When `len * size_of::<T>()` overflows or when `len * size_of::<T>() > isize::MAX`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use kernel::alloc::layout::ArrayLayout;
+    ///
+    /// // No overflow.
+    /// let layout = ArrayLayout::<i32>::new(12);
+    /// assert_eq!(layout.expect("sizeof(i32) * 12 is 48, not overflow").len(), 12);
+    ///
+    /// // Overflow, should return `Err`.
+    /// let layout = ArrayLayout::<i32>::new(isize::MAX as usize);
+    /// assert!(layout.is_err());
+    /// ```
     pub const fn new(len: usize) -> Result<Self, LayoutError> {
         match len.checked_mul(core::mem::size_of::<T>()) {
             Some(len) if len <= ISIZE_MAX => {