fix: Incorrect codegen for const generic params with default

fieldx/tests/builder_const_generic.rs

@@ -0,0 +1,19 @@
+use fieldx::fxstruct;
+
+#[fxstruct(builder)]
+struct Foo<const N: usize = 1> {
+    #[fieldx(get, default(N))]
+    v: usize,
+}
+
+#[test]
+fn generic_default() {
+    let foo: Foo = Foo::builder().build().unwrap();
+    assert_eq!(foo.v, 1, "default generic value is 1");
+}
+
+#[test]
+fn generic_2() {
+    let foo = Foo::<2>::builder().build().unwrap();
+    assert_eq!(foo.v, 2, "generic value is 2");
+}

fieldx_derive/src/codegen.rs

@@ -320,8 +320,7 @@ impl FXRewriter {
         let builder_ident = ctx.builder_ident();
         let builders = ctx.builders_combined();
         let input_ident = ctx.input_ident();
-        let generics = ctx.input().generics();
-        let where_clause = &generics.where_clause;
+        let (impl_generics, _, where_clause) = ctx.input().generics().split_for_impl();
         let generic_params = ctx.struct_generic_params();
         let attributes = ctx.args().builder_impl_attributes();
         let post_build_ident = ctx.builder_post_build_ident();
@@ -398,7 +397,7 @@ impl FXRewriter {
 
         quote![
             #attributes
-            impl #generics #builder_ident #generic_params
+            impl #impl_generics #builder_ident #generic_params
             #where_clause
             {
                 #fn_new