[wip] Practice good Self hygiene

Fixes #2116
google · Dec 3, 2024 · 1a1dc4a · 1a1dc4a
1 parent 7431cbd
commit 1a1dc4a
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Showing 2 changed files with 37 additions and 4 deletions.
diff --git a/src/util/macro_util.rs b/src/util/macro_util.rs
@@ -32,6 +32,11 @@ use crate::{
     Immutable, IntoBytes, Ptr, TryFromBytes, Unalign, ValidityError,
 };
 
+/// TODO
+pub trait Field<const IDX: usize> {
+    type Type: ?Sized;
+}
+
 #[cfg_attr(
     zerocopy_diagnostic_on_unimplemented_1_78_0,
     diagnostic::on_unimplemented(

diff --git a/zerocopy-derive/src/lib.rs b/zerocopy-derive/src/lib.rs
@@ -266,11 +266,32 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                 Default::default()
             };
 
+            let trailing_field_index = leading_fields.len();
+            let leading_field_indices = 0..trailing_field_index;
+
+            let trailing_field_ty = quote! {
+                <#ident #ty_generics as
+                    ::zerocopy::util::macro_util::Field<{#trailing_field_index}>
+                >::Type
+            };
+
             let methods = make_methods(&parse_quote! {
                 <#trailing_field_ty as ::zerocopy::KnownLayout>::MaybeUninit
             });
 
+            let field_impls = fields.iter().enumerate().map(|(idx, (_, ty))| quote!{
+                impl #impl_generics ::zerocopy::util::macro_util::Field<{#idx}> for #ident #ty_generics
+                where
+                    #trailing_field_ty: ::zerocopy::KnownLayout,
+                    #predicates
+                {
+                    type Type = #ty;
+                }
+            });
+
             quote! {
+                #(#field_impls)*
+
                 // SAFETY: This has the same layout as the derive target type,
                 // except that it admits uninit bytes. This is ensured by using the
                 // same repr as the target type, and by using field types which have
@@ -279,11 +300,17 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                 #repr
                 #[doc(hidden)]
                 #vis struct __ZerocopyKnownLayoutMaybeUninit<#params> (
-                    #(::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<#leading_fields_tys>,)*
+                    #(::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
+                        <#ident #ty_generics as
+                            ::zerocopy::util::macro_util::Field<{#leading_field_indices}>
+                        >::Type
+                    >,)*
                     <#trailing_field_ty as ::zerocopy::KnownLayout>::MaybeUninit
                 )
                 where
-                    #trailing_field_ty: ::zerocopy::KnownLayout,
+                    #trailing_field_ty: ::zerocopy::KnownLayout<
+                        PointerMetadata = <#ident #ty_generics as KnownLayout>::PointerMetadata
+                    >,
                     #predicates;
 
                 // SAFETY: We largely defer to the `KnownLayout` implementation on
@@ -294,10 +321,11 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                 // `__ZerocopyKnownLayoutMaybeUninit` admits uninit bytes.
                 unsafe impl #impl_generics ::zerocopy::KnownLayout for __ZerocopyKnownLayoutMaybeUninit #ty_generics
                 where
-                    #trailing_field_ty: ::zerocopy::KnownLayout,
                     // This bound may appear to be superfluous, but is required
                     // on our MSRV (1.55) to avoid an ICE.
-                    <#trailing_field_ty as ::zerocopy::KnownLayout>::MaybeUninit: ::zerocopy::KnownLayout,
+                    #trailing_field_ty: ::zerocopy::KnownLayout<
+                        PointerMetadata = <#ident #ty_generics as KnownLayout>::PointerMetadata
+                    >,
                     #predicates
                 {
                     #[allow(clippy::missing_inline_in_public_items)]