Add serde default attribute for fields

When annotated with #[serde(default)], a field gets a default value
using std:default::Default, and will not error when decoding if the
field is missing from the input.

Addresses #9

serde2/serde2_macros/src/lib.rs

@@ -9,6 +9,8 @@ use syntax::ast::{
     Item,
     Expr,
     MutMutable,
+    StructDef,
+    EnumDef,
 };
 use syntax::ast;
 use syntax::codemap::{Span, respan};
@@ -57,8 +59,8 @@ fn expand_derive_serialize(
     sp: Span,
     mitem: &MetaItem,
     item: &Item,
-    mut push: Box<FnMut(P<ast::Item>)>
+    push: &mut FnMut(P<ast::Item>))
-) {
+{
     let inline = cx.meta_word(sp, token::InternedString::new("inline"));
     let attrs = vec!(cx.attribute(sp, inline));
 
@@ -462,8 +464,8 @@ pub fn expand_derive_deserialize(
     sp: Span,
     mitem: &MetaItem,
     item: &Item,
-    mut push: Box<FnMut(P<ast::Item>)>
+    push: &mut FnMut(P<ast::Item>))
-) {
+{
     let inline = cx.meta_word(sp, token::InternedString::new("inline"));
     let attrs = vec!(cx.attribute(sp, inline));
 
@@ -518,7 +520,7 @@ fn deserialize_substructure(cx: &ExtCtxt, span: Span, substr: &Substructure) ->
     let state = substr.nonself_args[0].clone();
 
     match *substr.fields {
-        StaticStruct(_, ref fields) => {
+        StaticStruct(ref struct_def, ref fields) => {
             deserialize_struct(
                 cx,
                 span,
@@ -526,15 +528,17 @@ fn deserialize_substructure(cx: &ExtCtxt, span: Span, substr: &Substructure) ->
                 substr.type_ident,
                 cx.path(span, vec![substr.type_ident]),
                 fields,
-                state)
+                state,
+                struct_def)
         }
-        StaticEnum(_, ref fields) => {
+        StaticEnum(ref enum_def, ref fields) => {
             deserialize_enum(
                 cx,
                 span,
                 substr.type_ident,
                 &fields,
-                state)
+                state,
+                enum_def)
         }
         _ => cx.bug("expected StaticEnum or StaticStruct in derive(Deserialize)")
     }
@@ -548,6 +552,7 @@ fn deserialize_struct(
     struct_path: ast::Path,
     fields: &StaticFields,
     state: P<ast::Expr>,
+    struct_def: &StructDef
 ) -> P<ast::Expr> {
     match *fields {
         Unnamed(ref fields) => {
@@ -578,7 +583,8 @@ fn deserialize_struct(
                 struct_ident,
                 struct_path,
                 &fields[],
-                state)
+                state,
+                struct_def)
         }
     }
 }
@@ -714,6 +720,7 @@ fn deserialize_struct_named_fields(
     struct_path: ast::Path,
     fields: &[(Ident, Span)],
     state: P<ast::Expr>,
+    struct_def: &StructDef,
 ) -> P<ast::Expr> {
     let struct_name = cx.expr_str(span, token::get_ident(struct_ident));
 
@@ -735,6 +742,7 @@ fn deserialize_struct_named_fields(
         struct_path,
         &field_names[],
         fields,
+        struct_def
     );
 
     quote_expr!(cx, {
@@ -842,18 +850,50 @@ fn declare_map_field_deserializer(
     ]
 }
 
+
+fn default_value(field: &ast::StructField) -> bool {
+    field.node.attrs.iter()
+        .any(|sa|
+             match &sa.node.value.node {
+                 &ast::MetaItem_::MetaList(ref n, ref vals) => {
+                     if n == &"serde" {
+                         vals.iter()
+                             .map(|mi|
+                                  match &mi.node {
+                                      &ast::MetaItem_::MetaWord(ref n) => {
+                                          n == &"default"
+                                      },
+                                      _ => false
+                                  })
+                             .any(|x| x)
+                     } else {
+                         false
+                     }
+                 },
+                 _ => false
+             } )
+}
+
 fn declare_visit_map(
     cx: &ExtCtxt,
     span: Span,
     struct_path: ast::Path,
     field_names: &[Ident],
     fields: &[(Ident, Span)],
+    struct_def: &StructDef,
 ) -> P<ast::Expr> {
 
     // Declare each field.
     let let_values: Vec<P<ast::Stmt>> = field_names.iter()
-        .map(|field| {
+        .zip(struct_def.fields.iter())
-            quote_stmt!(cx, let mut $field = None;)
+        .map(|(field, sf)| {
+            if default_value(sf) {
+                quote_stmt!(
+                    cx,
+                    let mut $field = Some(::std::default::Default::default());)
+            } else {
+                quote_stmt!(cx, let mut $field = None;)
+            }
         })
         .collect();
 
@@ -912,12 +952,14 @@ fn deserialize_enum(
     type_ident: Ident,
     fields: &[(Ident, Span, StaticFields)],
     state: P<ast::Expr>,
+    enum_def: &EnumDef,
 ) -> P<ast::Expr> {
     let type_name = cx.expr_str(span, token::get_ident(type_ident));
 
     // Match arms to extract a variant from a string
     let variant_arms: Vec<ast::Arm> = fields.iter()
-        .map(|&(name, span, ref fields)| {
+        .zip(enum_def.variants.iter())
+        .map(|(&(name, span, ref fields), variant_ptr)| {
             let value = deserialize_enum_variant(
                 cx,
                 span,
@@ -925,6 +967,7 @@ fn deserialize_enum(
                 name,
                 fields,
                 cx.expr_ident(span, cx.ident_of("visitor")),
+                variant_ptr,
             );
 
             let s = cx.expr_str(span, token::get_ident(name));
@@ -969,6 +1012,7 @@ fn deserialize_enum_variant(
     variant_ident: Ident,
     fields: &StaticFields,
     state: P<ast::Expr>,
+    variant_ptr: &P<ast::Variant>,
 ) -> P<ast::Expr> {
     let variant_path = cx.path(span, vec![type_ident, variant_ident]);
 
@@ -1030,6 +1074,10 @@ fn deserialize_enum_variant(
                 variant_path,
                 &field_names[],
                 fields,
+                match variant_ptr.node.kind {
+                    ast::VariantKind::StructVariantKind(ref sd) => &*sd,
+                    _ => panic!("Mismatched enum types")
+                },
             );
 
             quote_expr!(cx, {