[PATCH v3] rust: macros: improve `#[vtable]` documentation

[Benno Lossin <benno.lossin@proton.me>]

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>
---
v2 -> v3:
- don't hide the import of the constant in the example
- fixed "function" typo
- improve paragraph about optional functions
- do not remove the `Send + Sync + Sized` bounds on the example

v1 -> v2:
- removed imperative mode in the paragraph describing optional
  functions.

 rust/kernel/error.rs |  4 ++++
 rust/macros/lib.rs   | 37 ++++++++++++++++++++++++++++++-------
 2 files changed, 34 insertions(+), 7 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..917a51183c23 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -87,27 +87,48 @@ 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 trait method to be optional, it must have a default implementation.
+/// This is also the case for traits annotated with `#[vtable]`, but in this
+/// case the default implementation will never be executed. The reason for this
+/// is that the functions will be called through function pointers installed in
+/// C side vtables. When an optional method is not implemented on a `#[vtable]`
+/// trait, a NULL entry is installed in the vtable. Thus the default
+/// implementation is never called. Since these traits are not designed to be
+/// used on the Rust side, it should not be possible to call the default
+/// implementation. This is done to ensure that we call the vtable methods
+/// through the C vtable, and not through the Rust vtable. Therefore, the
+/// default implementation should call `kernel::build_error`, which prevents
+/// calls to this function at compile time:
+///
+/// ```compile_fail
+/// # use kernel::error::VTABLE_DEFAULT_ERROR;
+/// kernel::build_error(VTABLE_DEFAULT_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 functions are required.
 ///
 /// # Examples
 ///
 /// ```ignore
+/// use kernel::error::VTABLE_DEFAULT_ERROR;
 /// use kernel::prelude::*;
 ///
 /// // Declares a `#[vtable]` trait
 /// #[vtable]
 /// pub trait Operations: Send + Sync + Sized {
 ///     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 +146,8 @@ 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: 3857af38e57a80b15b994e19d1f4301cac796481
-- 
2.41.0