Re: [PATCH v2 2/7] rust: macros: auto-insert ThisModule in #[vtable]

On Thu May 21, 2026 at 8:52 AM BST, Alvin Sun wrote:
> Auto-add `type ThisModule: ::kernel::ModuleMetadata;` as a required
> associated type on the trait side if not already defined, and
> auto-insert `type ThisModule = crate::LocalModule;` on the impl side
> if not explicitly provided, eliminating the need to manually declare
> and implement `ThisModule` in every vtable trait and impl.
>
> Signed-off-by: Alvin Sun <[email protected]>

Suggested-by: Gary Guo <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

> ---
>  rust/macros/lib.rs    |  6 ++++++
>  rust/macros/vtable.rs | 38 +++++++++++++++++++++++++++++++++++++-
>  2 files changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index 2cfd59e0f9e7c..d35e45ea745c0 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -176,6 +176,12 @@ pub fn module(input: TokenStream) -> TokenStream {
>  ///
>  /// This macro should not be used when all functions are required.
>  ///
> +/// Additionally, this macro automatically handles the `ThisModule`
> +/// associated type: on the trait side, `type ThisModule: ModuleMetadata;`
> +/// is added as a required associated type if not already defined; on the
> +/// impl side, `type ThisModule = LocalModule;` is automatically inserted
> +/// if not explicitly defined.
> +///
>  /// # Examples
>  ///
>  /// ```
> diff --git a/rust/macros/vtable.rs b/rust/macros/vtable.rs
> index c6510b0c4ea1d..d3d0e9cbd7172 100644
> --- a/rust/macros/vtable.rs
> +++ b/rust/macros/vtable.rs
> @@ -23,6 +23,7 @@
>  
>  fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
>      let mut gen_items = Vec::new();
> +    let mut has_this_module = false;
>  
>      gen_items.push(parse_quote! {
>           /// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable)
> @@ -30,6 +31,28 @@ fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
>           const USE_VTABLE_ATTR: ();
>      });
>  
> +    // Detect existing type ThisModule so we don't add a duplicate.
> +    for i in &item.items {
> +        if let TraitItem::Type(type_item) = i {
> +            if type_item.ident == "ThisModule" {
> +                has_this_module = true;
> +            }
> +        }
> +    }
> +
> +    // Add `type ThisModule: ModuleMetadata` as a required associated type if
> +    // the trait does not already define it. No default is used because
> +    // `associated_type_defaults` is unstable (issue #29661).

I don't think this is relevant. What's the sensible default anyway?

> +    if !has_this_module {

Perhaps just make this an one liner :

    if !item.items.iter().any(|i| matches!(item, TraitItem::Type(t) if t.ident == "ThisModule")) {

> +        gen_items.push(parse_quote! {
> +            /// The module implementing this vtable trait.
> +            ///
> +            /// Automatically set to `crate::LocalModule` by the `#[vtable]`
> +            /// impl macro.
> +            type ThisModule: ::kernel::ModuleMetadata;
> +        });
> +    }
> +
>      for item in &item.items {
>          if let TraitItem::Fn(fn_item) = item {
>              let name = &fn_item.sig.ident;
> @@ -58,18 +81,31 @@ fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
>  fn handle_impl(mut item: ItemImpl) -> Result<ItemImpl> {
>      let mut gen_items = Vec::new();
>      let mut defined_consts = HashSet::new();
> +    let mut defined_types = HashSet::new();

I'd just rename `defined_consts` to `defined_items` to reuse the same set as
there cannot be assoc items with same name anyway.

Best,
Gary

>  
> -    // Iterate over all user-defined constants to gather any possible explicit overrides.
> +    // Iterate over all user-defined constants and types to gather any possible explicit overrides.
>      for item in &item.items {
>          if let ImplItem::Const(const_item) = item {
>              defined_consts.insert(const_item.ident.clone());
>          }
> +        if let ImplItem::Type(type_item) = item {
> +            defined_types.insert(type_item.ident.clone());
> +        }
>      }
>  
>      gen_items.push(parse_quote! {
>          const USE_VTABLE_ATTR: () = ();
>      });
>  
> +    // Auto-insert `type ThisModule = crate::LocalModule` if not explicitly defined.
> +    // `crate::LocalModule` resolves to the real module type (via `module!`) or a
> +    // dummy fallback in non-module contexts (e.g., doctests).
> +    if !defined_types.contains(&parse_quote!(ThisModule)) {
> +        gen_items.push(parse_quote! {
> +            type ThisModule = crate::LocalModule;
> +        });
> +    }
> +
>      for item in &item.items {
>          if let ImplItem::Fn(fn_item) = item {
>              let name = &fn_item.sig.ident;