[PATCH v4 05/15] rust: pin-init: rewrite `derive(Zeroable)` and `derive(MaybeZeroable)` using `syn`

[Benno Lossin <lossin@kernel.org>]

Rewrite the two derive macros for `Zeroable` using `syn`. One positive
side effect of this change is that tuple structs are now supported by
them. Additionally, syntax errors and the error emitted when trying to
use one of the derive macros on an `enum` are improved. Otherwise no
functional changes intended.

For example:

    #[derive(Zeroable)]
    enum Num {
        A(u32),
        B(i32),
    }

Produced this error before this commit:

    error: no rules expected keyword `enum`
     --> tests/ui/compile-fail/zeroable/enum.rs:5:1
      |
    5 | enum Num {
      | ^^^^ no rules expected this token in macro call
      |
    note: while trying to match keyword `struct`
     --> src/macros.rs
      |
      |             $vis:vis struct $name:ident
      |                      ^^^^^^

Now the error is:

    error: cannot derive `Zeroable` for an enum
     --> tests/ui/compile-fail/zeroable/enum.rs:5:1
      |
    5 | enum Num {
      | ^^^^

    error: cannot derive `Zeroable` for an enum

Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>
---
Changes in v4: none
Changes in v3:
* use DiagCtxt error handling
Changes in v2:
* improved error handling
---
 rust/pin-init/internal/src/diagnostics.rs |   2 -
 rust/pin-init/internal/src/lib.rs         |   9 +-
 rust/pin-init/internal/src/zeroable.rs    | 155 ++++++++++------------
 rust/pin-init/src/macros.rs               | 124 -----------------
 4 files changed, 74 insertions(+), 216 deletions(-)

diff --git a/rust/pin-init/internal/src/diagnostics.rs b/rust/pin-init/internal/src/diagnostics.rs
index 555876c01bab..3bdb477c2f2b 100644
--- a/rust/pin-init/internal/src/diagnostics.rs
+++ b/rust/pin-init/internal/src/diagnostics.rs
@@ -9,14 +9,12 @@
 pub(crate) struct ErrorGuaranteed(());
 
 impl DiagCtxt {
-    #[expect(dead_code)]
     pub(crate) fn error(&mut self, span: impl Spanned, msg: impl Display) -> ErrorGuaranteed {
         let error = Error::new(span.span(), msg);
         self.0.extend(error.into_compile_error());
         ErrorGuaranteed(())
     }
 
-    #[expect(dead_code)]
     pub(crate) fn with(
         fun: impl FnOnce(&mut DiagCtxt) -> Result<TokenStream, ErrorGuaranteed>,
     ) -> TokenStream {
diff --git a/rust/pin-init/internal/src/lib.rs b/rust/pin-init/internal/src/lib.rs
index 0e1a4724549d..4cc9b7b0cda1 100644
--- a/rust/pin-init/internal/src/lib.rs
+++ b/rust/pin-init/internal/src/lib.rs
@@ -11,6 +11,9 @@
 #![allow(missing_docs)]
 
 use proc_macro::TokenStream;
+use syn::parse_macro_input;
+
+use crate::diagnostics::DiagCtxt;
 
 mod diagnostics;
 mod helpers;
@@ -30,10 +33,12 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 
 #[proc_macro_derive(Zeroable)]
 pub fn derive_zeroable(input: TokenStream) -> TokenStream {
-    zeroable::derive(input.into()).into()
+    let input = parse_macro_input!(input);
+    DiagCtxt::with(|dcx| zeroable::derive(input, dcx)).into()
 }
 
 #[proc_macro_derive(MaybeZeroable)]
 pub fn maybe_derive_zeroable(input: TokenStream) -> TokenStream {
-    zeroable::maybe_derive(input.into()).into()
+    let input = parse_macro_input!(input);
+    DiagCtxt::with(|dcx| zeroable::maybe_derive(input, dcx)).into()
 }
diff --git a/rust/pin-init/internal/src/zeroable.rs b/rust/pin-init/internal/src/zeroable.rs
index d8a5ef3883f4..05683319b0f7 100644
--- a/rust/pin-init/internal/src/zeroable.rs
+++ b/rust/pin-init/internal/src/zeroable.rs
@@ -1,99 +1,78 @@
 // SPDX-License-Identifier: GPL-2.0
 
-use crate::helpers::{parse_generics, Generics};
-use proc_macro2::{TokenStream, TokenTree};
+use proc_macro2::TokenStream;
 use quote::quote;
+use syn::{parse_quote, Data, DeriveInput, Field, Fields};
 
-pub(crate) fn parse_zeroable_derive_input(
-    input: TokenStream,
-) -> (
-    Vec<TokenTree>,
-    Vec<TokenTree>,
-    Vec<TokenTree>,
-    Option<TokenTree>,
-) {
-    let (
-        Generics {
-            impl_generics,
-            decl_generics: _,
-            ty_generics,
-        },
-        mut rest,
-    ) = parse_generics(input);
-    // This should be the body of the struct `{...}`.
-    let last = rest.pop();
-    // Now we insert `Zeroable` as a bound for every generic parameter in `impl_generics`.
-    let mut new_impl_generics = Vec::with_capacity(impl_generics.len());
-    // Are we inside of a generic where we want to add `Zeroable`?
-    let mut in_generic = !impl_generics.is_empty();
-    // Have we already inserted `Zeroable`?
-    let mut inserted = false;
-    // Level of `<>` nestings.
-    let mut nested = 0;
-    for tt in impl_generics {
-        match &tt {
-            // If we find a `,`, then we have finished a generic/constant/lifetime parameter.
-            TokenTree::Punct(p) if nested == 0 && p.as_char() == ',' => {
-                if in_generic && !inserted {
-                    new_impl_generics.extend(quote! { : ::pin_init::Zeroable });
-                }
-                in_generic = true;
-                inserted = false;
-                new_impl_generics.push(tt);
-            }
-            // If we find `'`, then we are entering a lifetime.
-            TokenTree::Punct(p) if nested == 0 && p.as_char() == '\'' => {
-                in_generic = false;
-                new_impl_generics.push(tt);
-            }
-            TokenTree::Punct(p) if nested == 0 && p.as_char() == ':' => {
-                new_impl_generics.push(tt);
-                if in_generic {
-                    new_impl_generics.extend(quote! { ::pin_init::Zeroable + });
-                    inserted = true;
-                }
-            }
-            TokenTree::Punct(p) if p.as_char() == '<' => {
-                nested += 1;
-                new_impl_generics.push(tt);
-            }
-            TokenTree::Punct(p) if p.as_char() == '>' => {
-                assert!(nested > 0);
-                nested -= 1;
-                new_impl_generics.push(tt);
-            }
-            _ => new_impl_generics.push(tt),
+use crate::{diagnostics::ErrorGuaranteed, DiagCtxt};
+
+pub(crate) fn derive(
+    input: DeriveInput,
+    dcx: &mut DiagCtxt,
+) -> Result<TokenStream, ErrorGuaranteed> {
+    let fields = match input.data {
+        Data::Struct(data_struct) => data_struct.fields,
+        Data::Union(data_union) => Fields::Named(data_union.fields),
+        Data::Enum(data_enum) => {
+            return Err(dcx.error(data_enum.enum_token, "cannot derive `Zeroable` for an enum"));
         }
+    };
+    let name = input.ident;
+    let mut generics = input.generics;
+    for param in generics.type_params_mut() {
+        param.bounds.insert(0, parse_quote!(::pin_init::Zeroable));
     }
-    assert_eq!(nested, 0);
-    if in_generic && !inserted {
-        new_impl_generics.extend(quote! { : ::pin_init::Zeroable });
-    }
-    (rest, new_impl_generics, ty_generics, last)
+    let (impl_gen, ty_gen, whr) = generics.split_for_impl();
+    let field_type = fields.iter().map(|field| &field.ty);
+    Ok(quote! {
+        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
+        #[automatically_derived]
+        unsafe impl #impl_gen ::pin_init::Zeroable for #name #ty_gen
+            #whr
+        {}
+        const _: () = {
+            fn assert_zeroable<T: ?::core::marker::Sized + ::pin_init::Zeroable>() {}
+            fn ensure_zeroable #impl_gen ()
+                #whr
+            {
+                #(
+                    assert_zeroable::<#field_type>();
+                )*
+            }
+        };
+    })
 }
 
-pub(crate) fn derive(input: TokenStream) -> TokenStream {
-    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
-    quote! {
-        ::pin_init::__derive_zeroable!(
-            parse_input:
-                @sig(#(#rest)*),
-                @impl_generics(#(#new_impl_generics)*),
-                @ty_generics(#(#ty_generics)*),
-                @body(#last),
-        );
+pub(crate) fn maybe_derive(
+    input: DeriveInput,
+    dcx: &mut DiagCtxt,
+) -> Result<TokenStream, ErrorGuaranteed> {
+    let fields = match input.data {
+        Data::Struct(data_struct) => data_struct.fields,
+        Data::Union(data_union) => Fields::Named(data_union.fields),
+        Data::Enum(data_enum) => {
+            return Err(dcx.error(data_enum.enum_token, "cannot derive `Zeroable` for an enum"));
+        }
+    };
+    let name = input.ident;
+    let mut generics = input.generics;
+    for param in generics.type_params_mut() {
+        param.bounds.insert(0, parse_quote!(::pin_init::Zeroable));
     }
-}
-
-pub(crate) fn maybe_derive(input: TokenStream) -> TokenStream {
-    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
-    quote! {
-        ::pin_init::__maybe_derive_zeroable!(
-            parse_input:
-                @sig(#(#rest)*),
-                @impl_generics(#(#new_impl_generics)*),
-                @ty_generics(#(#ty_generics)*),
-                @body(#last),
-        );
+    for Field { ty, .. } in fields {
+        generics
+            .make_where_clause()
+            .predicates
+            // the `for<'__dummy>` HRTB makes this not error without the `trivial_bounds`
+            // feature <https://github.com/rust-lang/rust/issues/48214#issuecomment-2557829956>.
+            .push(parse_quote!(#ty: for<'__dummy> ::pin_init::Zeroable));
     }
+    let (impl_gen, ty_gen, whr) = generics.split_for_impl();
+    Ok(quote! {
+        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
+        #[automatically_derived]
+        unsafe impl #impl_gen ::pin_init::Zeroable for #name #ty_gen
+            #whr
+        {}
+    })
 }
diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
index 682c61a587a0..53ed5ce860fc 100644
--- a/rust/pin-init/src/macros.rs
+++ b/rust/pin-init/src/macros.rs
@@ -1551,127 +1551,3 @@ fn assert_zeroable<T: $crate::Zeroable>(_: *mut T) {}
         );
     };
 }
-
-#[doc(hidden)]
-#[macro_export]
-macro_rules! __derive_zeroable {
-    (parse_input:
-        @sig(
-            $(#[$($struct_attr:tt)*])*
-            $vis:vis struct $name:ident
-            $(where $($whr:tt)*)?
-        ),
-        @impl_generics($($impl_generics:tt)*),
-        @ty_generics($($ty_generics:tt)*),
-        @body({
-            $(
-                $(#[$($field_attr:tt)*])*
-                $field_vis:vis $field:ident : $field_ty:ty
-            ),* $(,)?
-        }),
-    ) => {
-        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
-        #[automatically_derived]
-        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
-        where
-            $($($whr)*)?
-        {}
-        const _: () = {
-            fn assert_zeroable<T: ?::core::marker::Sized + $crate::Zeroable>() {}
-            fn ensure_zeroable<$($impl_generics)*>()
-                where $($($whr)*)?
-            {
-                $(assert_zeroable::<$field_ty>();)*
-            }
-        };
-    };
-    (parse_input:
-        @sig(
-            $(#[$($struct_attr:tt)*])*
-            $vis:vis union $name:ident
-            $(where $($whr:tt)*)?
-        ),
-        @impl_generics($($impl_generics:tt)*),
-        @ty_generics($($ty_generics:tt)*),
-        @body({
-            $(
-                $(#[$($field_attr:tt)*])*
-                $field_vis:vis $field:ident : $field_ty:ty
-            ),* $(,)?
-        }),
-    ) => {
-        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
-        #[automatically_derived]
-        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
-        where
-            $($($whr)*)?
-        {}
-        const _: () = {
-            fn assert_zeroable<T: ?::core::marker::Sized + $crate::Zeroable>() {}
-            fn ensure_zeroable<$($impl_generics)*>()
-                where $($($whr)*)?
-            {
-                $(assert_zeroable::<$field_ty>();)*
-            }
-        };
-    };
-}
-
-#[doc(hidden)]
-#[macro_export]
-macro_rules! __maybe_derive_zeroable {
-    (parse_input:
-        @sig(
-            $(#[$($struct_attr:tt)*])*
-            $vis:vis struct $name:ident
-            $(where $($whr:tt)*)?
-        ),
-        @impl_generics($($impl_generics:tt)*),
-        @ty_generics($($ty_generics:tt)*),
-        @body({
-            $(
-                $(#[$($field_attr:tt)*])*
-                $field_vis:vis $field:ident : $field_ty:ty
-            ),* $(,)?
-        }),
-    ) => {
-        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
-        #[automatically_derived]
-        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
-        where
-            $(
-                // the `for<'__dummy>` HRTB makes this not error without the `trivial_bounds`
-                // feature <https://github.com/rust-lang/rust/issues/48214#issuecomment-2557829956>.
-                $field_ty: for<'__dummy> $crate::Zeroable,
-            )*
-            $($($whr)*)?
-        {}
-    };
-    (parse_input:
-        @sig(
-            $(#[$($struct_attr:tt)*])*
-            $vis:vis union $name:ident
-            $(where $($whr:tt)*)?
-        ),
-        @impl_generics($($impl_generics:tt)*),
-        @ty_generics($($ty_generics:tt)*),
-        @body({
-            $(
-                $(#[$($field_attr:tt)*])*
-                $field_vis:vis $field:ident : $field_ty:ty
-            ),* $(,)?
-        }),
-    ) => {
-        // SAFETY: Every field type implements `Zeroable` and padding bytes may be zero.
-        #[automatically_derived]
-        unsafe impl<$($impl_generics)*> $crate::Zeroable for $name<$($ty_generics)*>
-        where
-            $(
-                // the `for<'__dummy>` HRTB makes this not error without the `trivial_bounds`
-                // feature <https://github.com/rust-lang/rust/issues/48214#issuecomment-2557829956>.
-                $field_ty: for<'__dummy> $crate::Zeroable,
-            )*
-            $($($whr)*)?
-        {}
-    };
-}
-- 
2.52.0