Re: [PATCH v1 28/28] rust: types: add `Opaque` type

[Gary Guo <gary@garyguo.net>]

On Thu, 10 Nov 2022 17:41:40 +0100
Miguel Ojeda <ojeda@kernel.org> wrote:

> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add the `Opaque` type, which is meant to be used with FFI objects
> that are never interpreted by Rust code, e.g.:
> 
>     struct Waiter {
>         completion: Opaque<bindings::completion>,
>         next: *mut Waiter,
>     }
> 
> It has the advantage that the objects don't have to be
> zero-initialised before calling their init functions, making
> the code performance closer to C.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
>  rust/kernel/types.rs | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 3b0c44769708..e84e51ec9716 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -2,6 +2,31 @@
>  
>  //! Kernel types.
>  
> +use core::{cell::UnsafeCell, mem::MaybeUninit};
> +
> +/// Stores an opaque value.
> +///
> +/// This is meant to be used with FFI objects that are never interpreted by Rust code.
> +#[repr(transparent)]
> +pub struct Opaque<T>(MaybeUninit<UnsafeCell<T>>);

Shouldn't this be `UnsafeCell<MaybeUninit<T>>`?

> +
> +impl<T> Opaque<T> {
> +    /// Creates a new opaque value.
> +    pub const fn new(value: T) -> Self {
> +        Self(MaybeUninit::new(UnsafeCell::new(value)))
> +    }
> +
> +    /// Creates an uninitialised value.
> +    pub const fn uninit() -> Self {
> +        Self(MaybeUninit::uninit())
> +    }
> +
> +    /// Returns a raw pointer to the opaque data.
> +    pub fn get(&self) -> *mut T {
> +        UnsafeCell::raw_get(self.0.as_ptr())
> +    }
> +}
> +
>  /// A sum type that always holds either a value of type `L` or `R`.
>  pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `L`.