Add `#[serde(skip_serializing)] to skip serializing some fields

Closes #99

serde_codegen/src/attr.rs

@@ -16,14 +16,20 @@ pub enum FieldNames {
 
 /// Represents field attribute information
 pub struct FieldAttrs {
+    skip_serializing_field: bool,
     names: FieldNames,
     use_default: bool,
 }
 
 impl FieldAttrs {
     /// Create a FieldAttr with a single default field name
-    pub fn new(default_value: bool, name: P<ast::Expr>) -> FieldAttrs {
+    pub fn new(
+        skip_serializing_field: bool,
+        default_value: bool,
+        name: P<ast::Expr>,
+    ) -> FieldAttrs {
         FieldAttrs {
+            skip_serializing_field: skip_serializing_field,
             names: FieldNames::Global(name),
             use_default: default_value,
         }
@@ -31,12 +37,14 @@ impl FieldAttrs {
 
     /// Create a FieldAttr with format specific field names
     pub fn new_with_formats(
+        skip_serializing_field: bool,
         default_value: bool,
         default_name: P<ast::Expr>,
         formats: HashMap<P<ast::Expr>, P<ast::Expr>>,
     ) -> FieldAttrs {
         FieldAttrs {
-            names:  FieldNames::Format {
+            skip_serializing_field: skip_serializing_field,
+            names: FieldNames::Format {
                 formats: formats,
                 default: default_name,
             },
@@ -104,4 +112,9 @@ impl FieldAttrs {
     pub fn use_default(&self) -> bool {
         self.use_default
     }
+
+    /// Predicate for ignoring a field when serializing a value
+    pub fn skip_serializing_field(&self) -> bool {
+        self.skip_serializing_field
+    }
 }

serde_codegen/src/de.rs

@@ -483,6 +483,7 @@ fn deserialize_item_enum(
         enum_def.variants.iter()
             .map(|variant|
                  attr::FieldAttrs::new(
+                     false,
                      true,
                      builder.expr().str(variant.node.name)))
             .collect()

serde_codegen/src/field.rs

@@ -58,10 +58,18 @@ fn default_value(mi: &ast::MetaItem) -> bool {
     }
 }
 
+fn skip_serializing_field(mi: &ast::MetaItem) -> bool {
+    if let ast::MetaItem_::MetaWord(ref n) = mi.node {
+        n == &"skip_serializing"
+    } else {
+        false
+    }
+}
+
 fn field_attrs<'a>(
     builder: &aster::AstBuilder,
     field: &'a ast::StructField,
-) -> (Rename<'a>, bool) {
+) -> (Rename<'a>, bool, bool) {
     field.node.attrs.iter()
         .find(|sa| {
             if let ast::MetaList(ref n, _) = sa.node.value.node {
@@ -73,15 +81,18 @@ fn field_attrs<'a>(
         .and_then(|sa| {
             if let ast::MetaList(_, ref vals) = sa.node.value.node {
                 attr::mark_used(&sa);
-                Some((vals.iter()
-                      .fold(None, |v, mi| v.or(rename(builder, mi)))
-                      .unwrap_or(Rename::None),
-                      vals.iter().any(|mi| default_value(mi))))
+                Some((
+                    vals.iter()
+                        .fold(None, |v, mi| v.or(rename(builder, mi)))
+                        .unwrap_or(Rename::None),
+                    vals.iter().any(|mi| default_value(mi)),
+                    vals.iter().any(|mi| skip_serializing_field(mi)),
+                ))
             } else {
-                Some((Rename::None, false))
+                Some((Rename::None, false, false))
             }
         })
-        .unwrap_or((Rename::None, false))
+        .unwrap_or((Rename::None, false, false))
 }
 
 pub fn struct_field_attrs(
@@ -92,23 +103,26 @@ pub fn struct_field_attrs(
     struct_def.fields.iter()
         .map(|field| {
             match field_attrs(builder, field) {
-                (Rename::Global(rename), default_value) =>
+                (Rename::Global(rename), default_value, skip_serializing_field) =>
                     FieldAttrs::new(
+                        skip_serializing_field,
                         default_value,
                         builder.expr().build_lit(P(rename.clone()))),
-                (Rename::Format(renames), default_value) => {
+                (Rename::Format(renames), default_value, skip_serializing_field) => {
                     let mut res = HashMap::new();
                     res.extend(
                         renames.into_iter()
                             .map(|(k,v)|
                                  (k, builder.expr().build_lit(P(v.clone())))));
                     FieldAttrs::new_with_formats(
+                        skip_serializing_field,
                         default_value,
                         default_field_name(cx, builder, field.node.kind),
                         res)
                 },
-                (Rename::None, default_value) => {
+                (Rename::None, default_value, skip_serializing_field) => {
                     FieldAttrs::new(
+                        skip_serializing_field,
                         default_value,
                         default_field_name(cx, builder, field.node.kind))
                 }

serde_codegen/src/ser.rs

@@ -550,6 +550,7 @@ fn serialize_struct_visitor<I>(
 
     let arms: Vec<ast::Arm> = field_attrs.into_iter()
         .zip(value_exprs)
+        .filter(|&(ref field, _)| !field.skip_serializing_field())
         .enumerate()
         .map(|(i, (field, value_expr))| {
             let key_expr = field.serializer_key_expr(cx);