rewrite derive(Zeroable) and derive(MaybeZeroable) using syn

BennoLossin · BennoLossin · commit 47d64a7d0ed0 · 2025-10-22T22:09:17.000+02:00
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`
     --&gt; 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`
     --&gt; src/macros.rs
      |
      |             $vis:vis struct $name:ident
      |                      ^^^^^^

Now the error is:

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

    error: cannot derive `Zeroable` for an enum

Signed-off-by: Benno Lossin &lt;lossin@kernel.org&gt;
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   code at that point.
 - Make the `[try_][pin_]init!` macros expose initialized fields via a `let`
   binding as `&mut T` or `Pin<&mut T>` for later fields.
+- `derive([Maybe]Zeroable)` now support tuple structs
 
 ## [0.0.10] - 2025-08-19
 
diff --git a/internal/src/lib.rs b/internal/src/lib.rs
@@ -11,6 +11,7 @@
 #![allow(missing_docs)]
 
 use proc_macro::TokenStream;
+use syn::parse_macro_input;
 
 mod helpers;
 mod pin_data;
@@ -29,10 +30,10 @@ 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()
+    zeroable::derive(parse_macro_input!(input as _)).into()
 }
 
 #[proc_macro_derive(MaybeZeroable)]
 pub fn maybe_derive_zeroable(input: TokenStream) -> TokenStream {
-    zeroable::maybe_derive(input.into()).into()
+    zeroable::maybe_derive(parse_macro_input!(input as _)).into()
 }
diff --git a/internal/src/zeroable.rs b/internal/src/zeroable.rs
@@ -1,99 +1,71 @@
 // SPDX-License-Identifier: GPL-2.0
 
-use crate::helpers::{parse_generics, Generics};
-use proc_macro2::{TokenStream, TokenTree};
-use quote::quote;
+use proc_macro2::TokenStream;
+use quote::{quote, quote_spanned};
+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),
+pub(crate) fn derive(input: DeriveInput) -> TokenStream {
+    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 quote_spanned! {data_enum.enum_token.span=>
+                ::core::compile_error!("cannot derive `Zeroable` for an enum");
+            };
         }
+    };
+    let name = input.ident;
+    let mut generics = input.generics;
+    for param in generics.type_params_mut() {
+        param.bounds.push(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)
-}
-
-pub(crate) fn derive(input: TokenStream) -> TokenStream {
-    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
+    let (impl_gen, ty_gen, whr) = generics.split_for_impl();
+    let field_type = fields.iter().map(|field| &field.ty);
     quote! {
-        ::pin_init::__derive_zeroable!(
-            parse_input:
-                @sig(#(#rest)*),
-                @impl_generics(#(#new_impl_generics)*),
-                @ty_generics(#(#ty_generics)*),
-                @body(#last),
-        );
+        // 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 maybe_derive(input: TokenStream) -> TokenStream {
-    let (rest, new_impl_generics, ty_generics, last) = parse_zeroable_derive_input(input);
+pub(crate) fn maybe_derive(input: DeriveInput) -> TokenStream {
+    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 quote_spanned! {data_enum.enum_token.span=>
+                compile_error!("cannot derive `Zeroable` for an enum");
+            };
+        }
+    };
+    let name = input.ident;
+    let mut generics = input.generics;
+    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();
     quote! {
-        ::pin_init::__maybe_derive_zeroable!(
-            parse_input:
-                @sig(#(#rest)*),
-                @impl_generics(#(#new_impl_generics)*),
-                @ty_generics(#(#ty_generics)*),
-                @body(#last),
-        );
+        // 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/src/macros.rs b/src/macros.rs
@@ -1551,127 +1551,3 @@ macro_rules! __init_internal {
         );
     };
 }
-
-#[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)*)?
-        {}
-    };
-}
diff --git a/tests/ui/compile-fail/zeroable/enum.stderr b/tests/ui/compile-fail/zeroable/enum.stderr
@@ -1,23 +1,11 @@
-error: no rules expected keyword `enum`
+error: cannot derive `Zeroable` for an 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
-  |                      ^^^^^^
+  | ^^^^
 
-error: no rules expected keyword `enum`
+error: cannot derive `Zeroable` for an enum
   --> tests/ui/compile-fail/zeroable/enum.rs:11:1
    |
 11 | enum Num2 {
-   | ^^^^ no rules expected this token in macro call
-   |
-note: while trying to match keyword `struct`
-  --> src/macros.rs
-   |
-   |             $vis:vis struct $name:ident
-   |                      ^^^^^^
+   | ^^^^
diff --git a/tests/ui/compile-fail/zeroable/not_all_fields_zeroable.stderr b/tests/ui/compile-fail/zeroable/not_all_fields_zeroable.stderr
@@ -9,7 +9,7 @@ note: required by a bound in `assert_zeroable`
   |
 4 | #[derive(Zeroable)]
   |          ^^^^^^^^ required by this bound in `assert_zeroable`
-  = note: this error originates in the macro `::pin_init::__derive_zeroable` which comes from the expansion of the derive macro `Zeroable` (in Nightly builds, run with -Z macro-backtrace for more info)
+  = note: this error originates in the derive macro `Zeroable` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider removing the leading `&`-reference
   |
 7 -     b: &'static Foo,
diff --git a/tests/ui/expand/zeroable.expanded.rs b/tests/ui/expand/zeroable.expanded.rs
@@ -31,11 +31,11 @@ struct WithGenerics<'a, T, U: Trait> {
 unsafe impl<
     'a,
     T: ::pin_init::Zeroable,
-    U: ::pin_init::Zeroable + Trait,
+    U: Trait + ::pin_init::Zeroable,
 > ::pin_init::Zeroable for WithGenerics<'a, T, U> {}
 const _: () = {
     fn assert_zeroable<T: ?::core::marker::Sized + ::pin_init::Zeroable>() {}
-    fn ensure_zeroable<'a, T: ::pin_init::Zeroable, U: ::pin_init::Zeroable + Trait>() {
+    fn ensure_zeroable<'a, T: ::pin_init::Zeroable, U: Trait + ::pin_init::Zeroable>() {
         assert_zeroable::<T>();
         assert_zeroable::<&'a U>();
     }

Original file line number	Diff line number	Diff line change
@@ -9,7 +9,7 @@ note: required by a bound in `assert_zeroable`
`9`	`9`	`\|`
`10`	`10`	`4 \| #[derive(Zeroable)]`
`11`	`11`	\| ^^^^^^^^ required by this bound in `assert_zeroable`
`12`		- = note: this error originates in the macro `::pin_init::__derive_zeroable` which comes from the expansion of the derive macro `Zeroable` (in Nightly builds, run with -Z macro-backtrace for more info)
	`12`	+ = note: this error originates in the derive macro `Zeroable` (in Nightly builds, run with -Z macro-backtrace for more info)
`13`	`13`	help: consider removing the leading `&`-reference
`14`	`14`	`\|`
`15`	`15`	`7 - b: &'static Foo,`