rust: alloc: Add doctest for ArrayLayout

Add a rustdoc example and Kunit test to the `ArrayLayout` struct's
`ArrayLayout::new()` function.

Add an implementation of `From<LayoutError> for Error` for the
`kernel::alloc::LayoutError`. This is necessary for the new test to
compile.

Change the `From` implementation on `core::alloc::LayoutError` to avoid
collisions with `kernel::alloc::LayoutError`, and modify imports to
explicitly import `kernel::alloc::LayoutError` instead.

Suggested-by: Boqun Feng <[email protected]>
Link: Rust-for-Linux#1131
Signed-off-by: Jimmy Ostler <[email protected]>

rust/kernel/alloc/layout.rs

@@ -43,6 +43,25 @@ impl<T> ArrayLayout<T> {
     /// # Errors
     ///
     /// When `len * size_of::<T>()` overflows or when `len * size_of::<T>() > isize::MAX`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::alloc::layout::{ArrayLayout, LayoutError};
+    /// let layout = ArrayLayout::<i32>::new(15)?;
+    /// assert_eq!(layout.len(), 15);
+    ///
+    /// // Errors because `len * size_of::<T>()` overflows
+    /// let layout = ArrayLayout::<i32>::new(isize::MAX as usize);
+    /// assert!(layout.is_err());
+    ///
+    /// // Errors because `len * size_of::<i32>() > isize::MAX`,
+    /// // even though `len < isize::MAX`
+    /// let layout = ArrayLayout::<i32>::new(isize::MAX as usize / 2);
+    /// assert!(layout.is_err());
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
     pub const fn new(len: usize) -> Result<Self, LayoutError> {
         match len.checked_mul(core::mem::size_of::<T>()) {
             Some(size) if size <= ISIZE_MAX => {

rust/kernel/error.rs

@@ -4,9 +4,10 @@
 //!
 //! C header: [`include/uapi/asm-generic/errno-base.h`](srctree/include/uapi/asm-generic/errno-base.h)
 
-use crate::{alloc::AllocError, str::CStr};
-
-use core::alloc::LayoutError;
+use crate::{
+    alloc::{layout::LayoutError, AllocError},
+    str::CStr,
+};
 
 use core::fmt;
 use core::num::NonZeroI32;
@@ -223,6 +224,12 @@ impl From<LayoutError> for Error {
     }
 }
 
+impl From<core::alloc::LayoutError> for Error {
+    fn from(_: core::alloc::LayoutError) -> Error {
+        code::ENOMEM
+    }
+}
+
 impl From<core::fmt::Error> for Error {
     fn from(_: core::fmt::Error) -> Error {
         code::EINVAL