fix: nested structure

filecaster-derive/src/from_file.rs

@@ -2,7 +2,7 @@ use proc_macro2::TokenStream;
 use quote::{format_ident, quote};
 use syn::{
     Attribute, Data, DeriveInput, Error, Expr, Field, Fields, FieldsNamed, GenericParam, Generics,
-    Lit, Meta, MetaList, Result, Type, TypePath, WhereClause, WherePredicate, parse_quote, parse2,
+    Ident, Lit, Meta, MetaList, Result, Type, parse_quote,
 };
 
 const WITH_MERGE: bool = cfg!(feature = "merge");
@@ -17,9 +17,8 @@ pub fn impl_from_file(input: &DeriveInput) -> Result<TokenStream> {
     let file_ident = format_ident!("{name}File");
 
     let fields = extract_named_fields(input)?;
-    let (field_assignments, file_fields, default_bounds) = process_fields(fields)?;
+    let (field_assignments, file_fields) = process_fields(fields)?;
 
-    let where_clause = build_where_clause(where_clause.cloned(), default_bounds)?;
     let derive_clause = build_derive_clause();
 
     Ok(quote! {
@@ -71,13 +70,15 @@ fn extract_named_fields(input: &DeriveInput) -> Result<&FieldsNamed> {
 }
 
 /// Build the shadow field + assignment for one original field
-fn build_file_field(field: &Field) -> Result<(TokenStream, TokenStream, Option<TokenStream>)> {
+fn build_file_field(field: &Field) -> Result<(TokenStream, TokenStream)> {
     let ident = field
         .ident
         .as_ref()
         .ok_or_else(|| Error::new_spanned(field, "Expected named fields"))?;
     let ty = &field.ty;
 
+    let default_override = parse_from_file_default_attr(&field.attrs)?;
+
     let field_attrs = if WITH_MERGE {
         quote! { #[merge(strategy = merge::option::overwrite_none)] }
     } else {
@@ -85,59 +86,41 @@ fn build_file_field(field: &Field) -> Result<(TokenStream, TokenStream, Option<T
     };
 
     // Nested struct -> delegate to its own `FromFile` impl
+    let shadow_ty = quote! { <#ty as filecaster::FromFile>::Shadow };
     let field_decl = quote! {
         #field_attrs
-        pub #ident: Option<#ty>
+        pub #ident: Option<#shadow_ty>
-    };
-    let assign = quote! {
-            #ident: <#ty as filecaster::FromFile>::from_file(file.#ident)
     };
 
-    let default_bound = Some(quote! { #ty: Default });
+    let assign = build_file_assing(ident, ty, default_override);
 
-    Ok((field_decl, assign, default_bound))
+    Ok((field_decl, assign))
+}
+
+fn build_file_assing(ident: &Ident, ty: &Type, default_override: Option<Expr>) -> TokenStream {
+    if let Some(expr) = default_override {
+        return quote! {
+            #ident: file.#ident.map(|inner| <#ty as filecaster::FromFile>::from_file(Some(inner))).unwrap_or(#expr)
+        };
+    }
+    quote! {
+        #ident: <#ty as filecaster::FromFile>::from_file(file.#ident)
+    }
 }
 
 /// Process all fields
-fn process_fields(
+fn process_fields(fields: &FieldsNamed) -> Result<(Vec<TokenStream>, Vec<TokenStream>)> {
-    fields: &FieldsNamed,
-) -> Result<(Vec<TokenStream>, Vec<TokenStream>, Vec<TokenStream>)> {
     fields.named.iter().try_fold(
-        (Vec::new(), Vec::new(), Vec::new()),
+        (Vec::new(), Vec::new()),
-        |(mut assignments, mut file_fields, mut defaults), field| {
+        |(mut assignments, mut file_fields), field| {
-            let (file_field, assignment, default_value) = build_file_field(field)?;
+            let (file_field, assignment) = build_file_field(field)?;
             file_fields.push(file_field);
             assignments.push(assignment);
-            if let Some(value) = default_value {
+            Ok((assignments, file_fields))
-                defaults.push(value);
-            }
-            Ok((assignments, file_fields, defaults))
         },
     )
 }
 
-/// Where-clause helpers
-fn build_where_clause(
-    where_clause: Option<WhereClause>,
-    default_bounds: Vec<TokenStream>,
-) -> Result<Option<WhereClause>> {
-    if default_bounds.is_empty() {
-        return Ok(where_clause);
-    }
-
-    let mut where_clause = where_clause;
-    if let Some(wc) = &mut where_clause {
-        for bound in default_bounds {
-            let predicate = parse2::<WherePredicate>(bound.clone())
-                .map_err(|_| Error::new_spanned(&bound, "Failed to parse where predicate"))?;
-            wc.predicates.push(predicate);
-        }
-    } else {
-        where_clause = Some(parse_quote!(where #(#default_bounds),*));
-    }
-    Ok(where_clause)
-}
-
 /// Derive clause for the shadow struct
 fn build_derive_clause() -> TokenStream {
     quote! {
@@ -150,8 +133,8 @@ fn build_derive_clause() -> TokenStream {
 /// Add Default bound to every generic parameter
 fn add_trait_bounds(mut generics: Generics) -> Generics {
     for param in &mut generics.params {
-        if let GenericParam::Type(type_param) = param {
+        if let GenericParam::Type(ty) = param {
-            type_param.bounds.push(parse_quote!(Default));
+            ty.bounds.push(parse_quote!(Default));
         }
     }
     generics
@@ -250,63 +233,10 @@ mod tests {
                 b: String,
             }
         };
-        let (assign, file_fields, bounds) = process_fields(&fields).unwrap();
+        let (assign, file_fields) = process_fields(&fields).unwrap();
         // two fields
         assert_eq!(assign.len(), 2);
         assert_eq!(file_fields.len(), 2);
-        // a uses unwrap_or_else
-        assert!(
-            assign[0]
-                .to_string()
-                .contains("a : file . a . unwrap_or_else")
-        );
-        // b uses unwrap_or_default
-        assert!(
-            assign[1]
-                .to_string()
-                .contains("b : file . b . unwrap_or_default")
-        );
-        // default-bound should only mention String
-        assert_eq!(bounds.len(), 1);
-        assert!(bounds[0].to_string().contains("String : Default"));
-    }
-
-    #[test]
-    fn build_where_clause_to_new() {
-        let bounds = vec![quote! { A: Default }, quote! { B: Default }];
-        let wc = build_where_clause(None, bounds).unwrap().unwrap();
-        let s = wc.to_token_stream().to_string();
-        assert!(s.contains("where A : Default , B : Default"));
-    }
-
-    #[test]
-    fn build_where_clause_append_existing() {
-        let orig: WhereClause = parse_quote!(where X: Clone);
-        let bounds = vec![quote! { Y: Default }];
-        let wc = build_where_clause(Some(orig.clone()), bounds)
-            .unwrap()
-            .unwrap();
-        let preds: Vec<_> = wc
-            .predicates
-            .iter()
-            .map(|p| p.to_token_stream().to_string())
-            .collect();
-        assert!(preds.contains(&"X : Clone".to_string()));
-        assert!(preds.contains(&"Y : Default".to_string()));
-    }
-
-    #[test]
-    fn build_where_clause_no_bounds_keeps_original() {
-        let orig: WhereClause = parse_quote!(where Z: Eq);
-        let wc = build_where_clause(Some(orig.clone()), vec![])
-            .unwrap()
-            .unwrap();
-        let preds: Vec<_> = wc
-            .predicates
-            .iter()
-            .map(|p| p.to_token_stream().to_string())
-            .collect();
-        assert_eq!(preds, vec!["Z : Eq".to_string()]);
     }
 
     #[test]

filecaster-derive/src/lib.rs

@@ -83,7 +83,7 @@ use proc_macro::TokenStream;
 use proc_macro_error2::proc_macro_error;
 use syn::{DeriveInput, parse_macro_input};
 
-/// Implements the `FromFile` derive macro.
+/// Implements the [`FromFile`] trait.
 ///
 /// This macro processes the `#[from_file]` attribute on structs to generate
 /// code for loading data from files.

filecaster/src/lib.rs

@@ -2,26 +2,10 @@ pub use filecaster_derive::FromFile;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
-/// Marker for types that can be built from an `Option<Shadow>` produced by the macro.
+/// Marker for types that can be built from an [`Option<Shadow>`] produced by the macro.
 pub trait FromFile: Sized {
-    fn from_file(file: Option<Self::Shadow>) -> Self;
-
-    /// Associated shadow type generated by the macro.
-    #[cfg(feature = "serde")]
-    type Shadow: Default + Serialize + for<'de> Deserialize<'de>;
-    #[cfg(not(feature = "serde"))]
     type Shadow: Default;
-}
+    fn from_file(file: Option<Self::Shadow>) -> Self;
-
-#[cfg(feature = "serde")]
-impl<T> FromFile for T
-where
-    T: Default + Serialize + for<'de> Deserialize<'de>,
-{
-    type Shadow = T;
-    fn from_file(file: Option<Self::Shadow>) -> Self {
-        file.unwrap_or_default()
-    }
 }
 
 #[cfg(not(feature = "serde"))]
@@ -30,7 +14,18 @@ where
     T: Default,
 {
     type Shadow = T;
-    fn from_file(file: Option<Self::Shadow>) -> Self {
+    fn from_file(file: Option<Self>) -> Self {
+        file.unwrap_or_default()
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<T> FromFile for T
+where
+    T: Default + Serialize + for<'de> Deserialize<'de>,
+{
+    type Shadow = T;
+    fn from_file(file: Option<Self>) -> Self {
         file.unwrap_or_default()
     }
 }

filecaster/tests/nested_structure.rs

@@ -1,14 +1,141 @@
 use filecaster::FromFile;
 
-#[derive(Debug, Default, Clone, PartialEq, FromFile)]
+#[derive(Debug, Clone, PartialEq, FromFile)]
 pub struct Coordinates {
     x: i32,
     y: i32,
 }
 
+impl Coordinates {
+    fn new(x: i32, y: i32) -> Self {
+        Self { x, y }
+    }
+}
+
+impl CoordinatesFile {
+    fn new(x: i32, y: i32) -> Self {
+        Self {
+            x: Some(x),
+            y: Some(y),
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, FromFile)]
+struct Wrapper {
+    parent: Parent,
+}
+
+// And one more level
+#[derive(Debug, Clone, PartialEq, FromFile)]
+struct DoubleWrapper {
+    wrapper: Wrapper,
+}
+
 #[derive(Debug, Clone, PartialEq, FromFile)]
 pub struct Parent {
     #[from_file(default = "Foo")]
     name: String,
     coordinates: Coordinates,
 }
+
+#[test]
+fn parent_all_defaults() {
+    let p = Parent::from_file(None);
+    assert_eq!(p.name, "Foo".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn parent_partial_shadow_merges_defaults() {
+    let shadow = ParentFile {
+        name: None,
+        coordinates: Some(CoordinatesFile::new(1, 2)),
+    };
+    let p = Parent::from_file(Some(shadow));
+    assert_eq!(p.name, "Foo".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(1, 2));
+}
+
+#[test]
+fn parent_full_shadow_overrides_everything() {
+    let shadow = ParentFile {
+        name: Some("Bar".into()),
+        coordinates: Some(CoordinatesFile::new(42, 24)),
+    };
+    let p = Parent::from_file(Some(shadow));
+    assert_eq!(p.name, "Bar".to_string());
+    assert_eq!(p.coordinates, Coordinates::new(42, 24));
+}
+
+#[test]
+fn wrapper_all_defaults() {
+    // None → WrapperFile::default() → parent = Parent::from_file(None)
+    let w = Wrapper::from_file(None);
+    assert_eq!(w.parent.name, "Foo".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn wrapper_partial_parent() {
+    // We supply only coordinates
+    let shadow = WrapperFile {
+        parent: Some(ParentFile {
+            name: None,
+            coordinates: Some(CoordinatesFile::new(5, -2)),
+        }),
+    };
+    let w = Wrapper::from_file(Some(shadow));
+    assert_eq!(w.parent.name, "Foo".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(5, -2));
+}
+
+#[test]
+fn wrapper_full_parent_override() {
+    let shadow = WrapperFile {
+        parent: Some(ParentFile {
+            name: Some("Baz".into()),
+            coordinates: Some(CoordinatesFile::new(1, 1)),
+        }),
+    };
+    let w = Wrapper::from_file(Some(shadow));
+    assert_eq!(w.parent.name, "Baz".to_string());
+    assert_eq!(w.parent.coordinates, Coordinates::new(1, 1));
+}
+
+#[test]
+fn double_wrapper_all_defaults() {
+    let dw = DoubleWrapper::from_file(None);
+    assert_eq!(dw.wrapper.parent.name, "Foo".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(0, 0));
+}
+
+#[test]
+fn double_wrapper_partial_deep() {
+    let shadow = DoubleWrapperFile {
+        wrapper: Some(WrapperFile {
+            parent: Some(ParentFile {
+                name: None,
+                coordinates: Some(CoordinatesFile::new(10, 20)),
+            }),
+        }),
+    };
+    let dw = DoubleWrapper::from_file(Some(shadow));
+    assert_eq!(dw.wrapper.parent.name, "Foo".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(10, 20));
+}
+
+#[test]
+fn double_wrapper_full_override_deep() {
+    let shadow = DoubleWrapperFile {
+        wrapper: Some(WrapperFile {
+            parent: Some(ParentFile {
+                name: Some("Deep".into()),
+                coordinates: Some(CoordinatesFile::new(3, 4)),
+            }),
+        }),
+    };
+    let dw = DoubleWrapper::from_file(Some(shadow));
+    assert_eq!(dw.wrapper.parent.name, "Deep".to_string());
+    assert_eq!(dw.wrapper.parent.coordinates, Coordinates::new(3, 4));
+}