From 8a863f3b320fcdc77e9ea94d825319770c77f68c Mon Sep 17 00:00:00 2001
From: nullstalgia <nullstalgia@gmail.com>
Date: Tue, 2 Sep 2025 04:43:29 -0700
Subject: [PATCH] Copy conditional compilation flags onto created constructors
 (#13)

(Closes #12)
---
 src/expand.rs                      |  5 +++++
 tests/test_serde_inline_default.rs | 23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/src/expand.rs b/src/expand.rs
index 6aec480..496dc8b 100644
--- a/src/expand.rs
+++ b/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
                 }
diff --git a/tests/test_serde_inline_default.rs b/tests/test_serde_inline_default.rs
index cc07fc2..ed5208f 100644
--- a/tests/test_serde_inline_default.rs
+++ b/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,
+    }
+}