[PATCH v2 1/2] rust: macros: Add derive Display for enums

[Maurice Hieronymus <mhi@mailbox.org>]

Add a derive macro that implements kernel::fmt::Display for enums.
The macro outputs the exact variant name as written, preserving case.

This supports all enum variant types: unit, tuple, and struct variants.
For variants with data, only the variant name is displayed.

Signed-off-by: Maurice Hieronymus <mhi@mailbox.org>
---
 rust/macros/display.rs | 52 ++++++++++++++++++++++++++++++++++++++++++
 rust/macros/lib.rs     | 42 ++++++++++++++++++++++++++++++++++
 2 files changed, 94 insertions(+)
 create mode 100644 rust/macros/display.rs

diff --git a/rust/macros/display.rs b/rust/macros/display.rs
new file mode 100644
index 000000000000..5cd396d3900e
--- /dev/null
+++ b/rust/macros/display.rs
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Derive macro for `Display` on enums.
+//!
+//! This module provides a derive macro that implements `kernel::fmt::Display`
+//! for enums, outputting the exact variant name as written.
+
+use proc_macro::TokenStream;
+
+pub(crate) fn derive_display(input: TokenStream) -> TokenStream {
+    let input: syn::DeriveInput = syn::parse(input).expect("failed to parse input");
+
+    let data = match &input.data {
+        syn::Data::Enum(data) => data,
+        syn::Data::Struct(_) => {
+            panic!("derive(Display) only supports enums, not structs");
+        }
+        syn::Data::Union(_) => {
+            panic!("derive(Display) only supports enums, not unions");
+        }
+    };
+
+    // Generate match arms for each variant.
+    let match_arms = data.variants.iter().map(|variant| {
+        let variant_ident = &variant.ident;
+        let variant_name = variant_ident.to_string();
+
+        // Handle different variant types: unit, tuple, and struct.
+        let pattern = match &variant.fields {
+            syn::Fields::Unit => quote::quote! { Self::#variant_ident },
+            syn::Fields::Unnamed(_) => quote::quote! { Self::#variant_ident(..) },
+            syn::Fields::Named(_) => quote::quote! { Self::#variant_ident { .. } },
+        };
+
+        quote::quote! {
+            #pattern => f.write_str(#variant_name)
+        }
+    });
+
+    let name = &input.ident;
+    let expanded = quote::quote! {
+        impl ::kernel::fmt::Display for #name {
+            fn fmt(&self, f: &mut ::kernel::fmt::Formatter<'_>) -> ::kernel::fmt::Result {
+                match self {
+                    #(#match_arms),*
+                }
+            }
+        }
+    };
+
+    expanded.into()
+}
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index b38002151871..4c95a132fefe 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -14,6 +14,7 @@
 #[macro_use]
 mod quote;
 mod concat_idents;
+mod display;
 mod export;
 mod fmt;
 mod helpers;
@@ -475,3 +476,44 @@ pub fn paste(input: TokenStream) -> TokenStream {
 pub fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
     kunit::kunit_tests(attr, ts)
 }
+
+/// Derives the [`Display`] trait for enums.
+///
+/// This macro generates an implementation of [`kernel::fmt::Display`] for enums
+/// that outputs the exact variant name as written (case-preserved).
+///
+/// # Requirements
+///
+/// - Can only be applied to enums (not structs or unions).
+/// - Supports unit variants, tuple variants, and struct variants.
+/// - For variants with data, only the variant name is displayed.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::fmt::Adapter;
+/// use kernel::macros::Display;
+///
+/// #[allow(non_camel_case_types)]
+/// #[derive(Display)]
+/// enum TestEnum {
+///     Foo,
+///     bAr(u8),
+///     baZ { value: u8 },
+/// }
+///
+/// let foo = TestEnum::Foo;
+/// let bar = TestEnum::bAr(42);
+/// let baz = TestEnum::baZ { value: 0 };
+///
+/// assert!(format!("{}", Adapter(&foo)) == "Foo");
+/// assert!(format!("{}", Adapter(&bar)) == "bAr");
+/// assert!(format!("{}", Adapter(&baz)) == "baZ");
+/// ```
+///
+/// [`Display`]: ../kernel/fmt/trait.Display.html
+/// [`kernel::fmt::Display`]: ../kernel/fmt/trait.Display.html
+#[proc_macro_derive(Display)]
+pub fn derive_display(input: TokenStream) -> TokenStream {
+    display::derive_display(input)
+}
-- 
2.51.2