Re: [PATCH v2] rust: macros: improve `#[vtable]` documentation

[Gary Guo <gary@garyguo.net>]

On Thu, 19 Oct 2023 17:15:53 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> Traits marked with `#[vtable]` need to provide default implementations
> for optional functions. The C side represents these with `NULL` in the
> vtable, so the default functions are never actually called. We do not
> want to replicate the default behavior from C in Rust, because that is
> not maintainable. Therefore we should use `build_error` in those default
> implementations. The error message for that is provided at
> `kernel::error::VTABLE_DEFAULT_ERROR`.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
> v1 -> v2:
> - removed imperative mode in the paragraph describing optional
>   functions.
> 
>  rust/kernel/error.rs |  4 ++++
>  rust/macros/lib.rs   | 32 ++++++++++++++++++++++++--------
>  2 files changed, 28 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> index 05fcab6abfe6..1373cde025ef 100644
> --- a/rust/kernel/error.rs
> +++ b/rust/kernel/error.rs
> @@ -335,3 +335,7 @@ pub(crate) fn from_result<T, F>(f: F) -> T
>          Err(e) => T::from(e.to_errno() as i16),
>      }
>  }
> +
> +/// Error message for calling a default function of a [`#[vtable]`](macros::vtable) trait.
> +pub const VTABLE_DEFAULT_ERROR: &str =
> +    "This function must not be called, see the #[vtable] documentation.";
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index c42105c2ff96..daf1ef8baa62 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -87,27 +87,41 @@ pub fn module(ts: TokenStream) -> TokenStream {
>  /// implementation could just return `Error::EINVAL`); Linux typically use C
>  /// `NULL` pointers to represent these functions.
>  ///
> -/// This attribute is intended to close the gap. Traits can be declared and
> -/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
> -/// will be generated for each method in the trait, indicating if the implementor
> -/// has overridden a method.
> +/// This attribute closes that gap. A trait can be annotated with the `#[vtable]` attribute.
> +/// Implementers of the trait will then also have to annotate the trait with `#[vtable]`. This
> +/// attribute generates a `HAS_*` associated constant bool for each method in the trait that is set
> +/// to true if the implementer has overridden the associated method.
> +///
> +/// For a function to be optional, it must have a default implementation. But this default
> +/// implementation will never be executed, since these functions are exclusively called from
> +/// callbacks from the C side. This is because the vtable will have a `NULL` entry and the C side
> +/// will execute the default behavior. Since it is not maintainable to replicate the default
> +/// behavior in Rust, the default implementation should be:
> +///
> +/// ```compile_fail
> +/// # use kernel::error::VTABLE_DEFAULT_ERROR;
> +/// kernel::build_error(VTABLE_DEFAULT_ERROR)

Note that `build_error` function is considered impl detail and is
hidden. This should use the macro version instead:

kernel::build_error!(VTABLE_DEFAULT_ERROR)

Actually, the string here provides little use other than documentation,
since the string provided to build_error is only visible in const eval,
so this you might just omit that and write

kernel::build_error!()

> +/// ```
> +///
> +/// note that you might need to import [`kernel::error::VTABLE_DEFAULT_ERROR`].
>  ///
> -/// This attribute is not needed if all methods are required.
> +/// This macro should not be used when all function are required.
>  ///
>  /// # Examples
>  ///
>  /// ```ignore
> +/// # use kernel::error::VTABLE_DEFAULT_ERROR;
>  /// use kernel::prelude::*;
>  ///
>  /// // Declares a `#[vtable]` trait
>  /// #[vtable]
> -/// pub trait Operations: Send + Sync + Sized {
> +/// pub trait Operations {
>  ///     fn foo(&self) -> Result<()> {
> -///         Err(EINVAL)
> +///         kernel::build_error(VTABLE_DEFAULT_ERROR)
>  ///     }
>  ///
>  ///     fn bar(&self) -> Result<()> {
> -///         Err(EINVAL)
> +///         kernel::build_error(VTABLE_DEFAULT_ERROR)
>  ///     }
>  /// }
>  ///
> @@ -125,6 +139,8 @@ pub fn module(ts: TokenStream) -> TokenStream {
>  /// assert_eq!(<Foo as Operations>::HAS_FOO, true);
>  /// assert_eq!(<Foo as Operations>::HAS_BAR, false);
>  /// ```
> +///
> +/// [`kernel::error::VTABLE_DEFAULT_ERROR`]: ../kernel/error/constant.VTABLE_DEFAULT_ERROR.html
>  #[proc_macro_attribute]
>  pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream {
>      vtable::vtable(attr, ts)
> 
> base-commit: a7135d10754760f0c038497b44c2c2f2b0fb5651