zerocopy: Deny structs which have sneaky fields

crates/musli-macros/Cargo.toml

@@ -21,6 +21,7 @@ path = "src/lib.rs"
 
 [features]
 test = []
+sneaky-fields = []
 
 [dependencies]
 proc-macro2 = "1.0.69"

crates/musli-macros/src/lib.rs

@@ -17,6 +17,8 @@ mod de;
 mod en;
 mod expander;
 mod internals;
+#[cfg(feature = "sneaky-fields")]
+mod sneaky_fields;
 #[cfg(feature = "test")]
 mod test;
 mod types;
@@ -137,8 +139,8 @@ pub fn visitor(attr: TokenStream, input: TokenStream) -> TokenStream {
 
 #[proc_macro_derive(ZeroCopy, attributes(zero_copy))]
 pub fn zero_copy(input: TokenStream) -> TokenStream {
-    let input = syn::parse_macro_input!(input as syn::DeriveInput);
-    let expander = zero_copy::Expander::new(&input);
+    let input = syn::parse_macro_input!(input as syn::Item);
+    let expander = zero_copy::Expander::new(input);
 
     match expander.expand() {
         Ok(stream) => stream.into(),
@@ -157,6 +159,14 @@ pub fn visit(input: TokenStream) -> TokenStream {
     }
 }
 
+// NB: Only used in UI tests.
+#[proc_macro_attribute]
+#[doc(hidden)]
+#[cfg(feature = "sneaky-fields")]
+pub fn sneaky_fields(attr: TokenStream, item: TokenStream) -> TokenStream {
+    sneaky_fields::expand(attr, item)
+}
+
 fn to_compile_errors(errors: Vec<syn::Error>) -> proc_macro2::TokenStream {
     let mut output = proc_macro2::TokenStream::new();
 

crates/musli-macros/src/sneaky_fields.rs

@@ -0,0 +1,44 @@
+use proc_macro::TokenStream;
+use quote::ToTokens;
+
+/// Replace the fields of a struct.
+pub(super) fn expand(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let mut input = syn::parse_macro_input!(item as syn::Item);
+    let ty = syn::parse_macro_input!(attr as syn::Type);
+
+    match &mut input {
+        syn::Item::Enum(e) => {
+            for v in &mut e.variants {
+                match &mut v.fields {
+                    syn::Fields::Named(named) => {
+                        let extra: syn::FieldsNamed = syn::parse_quote!({ sneaky_field: #ty });
+                        named.named.extend(extra.named);
+                    }
+                    syn::Fields::Unnamed(unnamed) => {
+                        let extra: syn::FieldsUnnamed = syn::parse_quote!(( #ty ));
+                        unnamed.unnamed.extend(extra.unnamed);
+                    }
+                    syn::Fields::Unit => {
+                        v.fields = syn::Fields::Named(syn::parse_quote!({ sneaky_field: #ty }));
+                    }
+                }
+            }
+        }
+        syn::Item::Struct(st) => match &mut st.fields {
+            syn::Fields::Named(named) => {
+                let extra: syn::FieldsNamed = syn::parse_quote!({ sneaky_field: #ty });
+                named.named.extend(extra.named);
+            }
+            syn::Fields::Unnamed(unnamed) => {
+                let extra: syn::FieldsUnnamed = syn::parse_quote!(( #ty ));
+                unnamed.unnamed.extend(extra.unnamed);
+            }
+            syn::Fields::Unit => {
+                st.fields = syn::Fields::Named(syn::parse_quote!({ sneaky_field: #ty }));
+            }
+        },
+        _ => (),
+    }
+
+    input.to_token_stream().into()
+}