rust: macros: add `#[vtable]` proc macro

This procedural macro attribute provides a simple way to declare
a trait with a set of operations that later users can partially
implement, providing compile-time `HAS_*` boolean associated
constants that indicate whether a particular operation was overridden.

This is useful as the Rust counterpart to structs like
`file_operations` where some pointers may be `NULL`, indicating
an operation is not provided.

For instance:

    #[vtable]
    trait Operations {
        fn read(...) -> Result<usize> {
            Err(EINVAL)
        }

        fn write(...) -> Result<usize> {
            Err(EINVAL)
        }
    }

    #[vtable]
    impl Operations for S {
        fn read(...) -> Result<usize> {
            ...
        }
    }

    assert_eq!(<S as Operations>::HAS_READ, true);
    assert_eq!(<S as Operations>::HAS_WRITE, false);

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Sergio González Collado <sergio.collado@gmail.com>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 52 insertions, 0 deletions

diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index 15555e7ff487..e40caaf0a656 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -5,6 +5,7 @@
 mod concat_idents;
 mod helpers;
 mod module;
+mod vtable;
 
 use proc_macro::TokenStream;
 
@@ -72,6 +73,57 @@ pub fn module(ts: TokenStream) -> TokenStream {
     module::module(ts)
 }
 
+/// Declares or implements a vtable trait.
+///
+/// Linux's use of pure vtables is very close to Rust traits, but they differ
+/// in how unimplemented functions are represented. In Rust, traits can provide
+/// default implementation for all non-required methods (and the default
+/// 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 is not needed if all methods are required.
+///
+/// # Examples
+///
+/// ```ignore
+/// use kernel::prelude::*;
+///
+/// // Declares a `#[vtable]` trait
+/// #[vtable]
+/// pub trait Operations: Send + Sync + Sized {
+///     fn foo(&self) -> Result<()> {
+///         Err(EINVAL)
+///     }
+///
+///     fn bar(&self) -> Result<()> {
+///         Err(EINVAL)
+///     }
+/// }
+///
+/// struct Foo;
+///
+/// // Implements the `#[vtable]` trait
+/// #[vtable]
+/// impl Operations for Foo {
+///     fn foo(&self) -> Result<()> {
+/// #        Err(EINVAL)
+///         // ...
+///     }
+/// }
+///
+/// assert_eq!(<Foo as Operations>::HAS_FOO, true);
+/// assert_eq!(<Foo as Operations>::HAS_BAR, false);
+/// ```
+#[proc_macro_attribute]
+pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream {
+    vtable::vtable(attr, ts)
+}
+
 /// Concatenate two identifiers.
 ///
 /// This is useful in macros that need to declare or reference items with names