Copy conditional compilation flags onto created constructors

src/expand.rs

@@ -14,6 +14,10 @@ pub(crate) fn expand_struct(mut item: ItemStruct) -> proc_macro::TokenStream {
 
             let default: TokenStream = attr.parse_args().unwrap();
 
+            // copy all the same #[cfg] conditional compilations flags for the field onto our built "constructor"
+            // otherwise, it's possible to create a constructor for a type that may be filtered by the same #[cfg]'s, breaking compilation
+            let cfg_attrs = field.attrs.iter().filter(|a| a.path().is_ident("cfg"));
+
             let fn_name_lit = format!("__serde_inline_default_{}_{}", item.ident, i);
             let fn_name_ident = Ident::new(&fn_name_lit, Span::call_site());
             let mut return_type = field.ty.clone();
@@ -24,6 +28,7 @@ pub(crate) fn expand_struct(mut item: ItemStruct) -> proc_macro::TokenStream {
             inline_fns.push(quote! {
                 #[doc(hidden)]
                 #[allow(non_snake_case)]
+                #( #cfg_attrs )*
                 fn #fn_name_ident () -> #return_type {
                     #default
                 }

tests/test_serde_inline_default.rs

@@ -43,3 +43,26 @@ fn test_lifetime() {
 
     assert_eq!(lifetime_test.test_str, "test");
 }
+
+#[test]
+#[allow(dead_code)]
+fn test_conditional_compilation() {
+    #[cfg(debug_assertions)]
+    #[derive(Deserialize)]
+    struct TypeA(u8);
+
+    #[cfg(not(debug_assertions))]
+    #[derive(Deserialize)]
+    struct TypeB(u8);
+
+    #[serde_inline_default]
+    #[derive(Deserialize)]
+    struct Test {
+        #[cfg(debug_assertions)]
+        #[serde_inline_default(TypeA(1))]
+        val_a: TypeA,
+        #[cfg(not(debug_assertions))]
+        #[serde_inline_default(TypeB(1))]
+        val_b: TypeB,
+    }
+}