Re: [PATCH v2] rust: alloc: Add doctest for `ArrayLayout`

[Danilo Krummrich <dakr@kernel.org>]

Hi Jimmy,

Thanks for the patch!

On Thu, Dec 05, 2024 at 02:56:27AM -0800, Jimmy Ostler wrote:
> 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.

Please split this into a separate patch.

> 
> 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 <boqun.feng@gmail.com>
> Link: https://github.com/Rust-for-Linux/linux/issues/1131
> Signed-off-by: Jimmy Ostler <jtostler1@gmail.com>
> ---
> v1: https://lore.kernel.org/lkml/20241203051843.291729-1-jtostler1@gmail.com/T/#u
> v1 -> v2 changes:
> - Add third assert where length is smaller but still overflows
> - Remove rustdoc markdown codeblock languge signifier
> - Change tests to return results using `?` instead of panic
> - Remove `#[derive(Debug)]` for `LayoutError`
> - Add `From<LayoutError> for Error` implementation
> ---
>  rust/kernel/alloc/layout.rs | 19 +++++++++++++++++++
>  rust/kernel/error.rs        | 13 ++++++++++---
>  2 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/alloc/layout.rs b/rust/kernel/alloc/layout.rs
> index 4b3cd7fdc816..0e053dcc7941 100644
> --- a/rust/kernel/alloc/layout.rs
> +++ b/rust/kernel/alloc/layout.rs
> @@ -43,6 +43,25 @@ pub const fn empty() -> Self {
>      /// # 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 => {
> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> index 52c502432447..ac8526140d7a 100644
> --- a/rust/kernel/error.rs
> +++ b/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,
> +};

I think this part of the change would be enough, since we don't make use of the
`From` implementation of `core::alloc::LayoutError` anywhere.

I think we can add it (again), once it's needed.

>  
>  use core::fmt;
>  use core::num::NonZeroI32;
> @@ -223,6 +224,12 @@ fn from(_: LayoutError) -> 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
> 
> base-commit: 1dc707e647bc919834eff9636c8d00b78c782545
> -- 
> 2.47.1
>