feat(codegen): Add more attributes to skip serializing

These attributes are `#[serde(skip_serializing_if_none)]` and
`#[serde(skip_serializing_if_empty)]`.

serde_codegen/src/attr.rs

@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 
 use syntax::ast;
+use syntax::attr;
 use syntax::ext::base::ExtCtxt;
 use syntax::ptr::P;
 
@@ -21,6 +22,8 @@ pub enum FieldNames {
 #[derive(Debug)]
 pub struct FieldAttrs {
     skip_serializing_field: bool,
+    skip_serializing_field_if_empty: bool,
+    skip_serializing_field_if_none: bool,
     names: FieldNames,
     use_default: bool,
 }
@@ -44,10 +47,10 @@ impl FieldAttrs {
     ///
     /// The resulting expression assumes that `S` refers to a type
     /// that implements `Serializer`.
-    pub fn serializer_key_expr(self, cx: &ExtCtxt) -> P<ast::Expr> {
+    pub fn serializer_key_expr(&self, cx: &ExtCtxt) -> P<ast::Expr> {
         match self.names {
-            FieldNames::Global(name) => name,
+            FieldNames::Global(ref name) => name.clone(),
-            FieldNames::Format { formats, default } => {
+            FieldNames::Format { ref formats, ref default } => {
                 let arms = formats.iter()
                     .map(|(fmt, lit)| {
                         quote_arm!(cx, $fmt => { $lit })
@@ -90,11 +93,21 @@ impl FieldAttrs {
     pub fn skip_serializing_field(&self) -> bool {
         self.skip_serializing_field
     }
+
+    pub fn skip_serializing_field_if_empty(&self) -> bool {
+        self.skip_serializing_field_if_empty
+    }
+
+    pub fn skip_serializing_field_if_none(&self) -> bool {
+        self.skip_serializing_field_if_none
+    }
 }
 
 pub struct FieldAttrsBuilder<'a> {
     builder: &'a aster::AstBuilder,
     skip_serializing_field: bool,
+    skip_serializing_field_if_empty: bool,
+    skip_serializing_field_if_none: bool,
     name: Option<P<ast::Expr>>,
     format_rename: HashMap<P<ast::Expr>, P<ast::Expr>>,
     use_default: bool,
@@ -105,6 +118,8 @@ impl<'a> FieldAttrsBuilder<'a> {
         FieldAttrsBuilder {
             builder: builder,
             skip_serializing_field: false,
+            skip_serializing_field_if_empty: false,
+            skip_serializing_field_if_none: false,
             name: None,
             format_rename: HashMap::new(),
             use_default: false,
@@ -129,6 +144,7 @@ impl<'a> FieldAttrsBuilder<'a> {
     pub fn attr(self, attr: &ast::Attribute) -> FieldAttrsBuilder<'a> {
         match attr.node.value.node {
             ast::MetaList(ref name, ref items) if name == &"serde" => {
+                attr::mark_used(&attr);
                 items.iter().fold(self, FieldAttrsBuilder::meta_item)
             }
             _ => {
@@ -164,6 +180,12 @@ impl<'a> FieldAttrsBuilder<'a> {
             ast::MetaWord(ref name) if name == &"skip_serializing" => {
                 self.skip_serializing_field()
             }
+            ast::MetaWord(ref name) if name == &"skip_serializing_if_empty" => {
+                self.skip_serializing_field_if_empty()
+            }
+            ast::MetaWord(ref name) if name == &"skip_serializing_if_none" => {
+                self.skip_serializing_field_if_none()
+            }
             _ => {
                 // Ignore unknown meta variables for now.
                 self
@@ -176,6 +198,16 @@ impl<'a> FieldAttrsBuilder<'a> {
         self
     }
 
+    pub fn skip_serializing_field_if_empty(mut self) -> FieldAttrsBuilder<'a> {
+        self.skip_serializing_field_if_empty = true;
+        self
+    }
+
+    pub fn skip_serializing_field_if_none(mut self) -> FieldAttrsBuilder<'a> {
+        self.skip_serializing_field_if_none = true;
+        self
+    }
+
     pub fn name(mut self, name: P<ast::Expr>) -> FieldAttrsBuilder<'a> {
         self.name = Some(name);
         self
@@ -204,6 +236,8 @@ impl<'a> FieldAttrsBuilder<'a> {
 
         FieldAttrs {
             skip_serializing_field: self.skip_serializing_field,
+            skip_serializing_field_if_empty: self.skip_serializing_field_if_empty,
+            skip_serializing_field_if_none: self.skip_serializing_field_if_none,
             names: names,
             use_default: self.use_default,
         }

serde_codegen/src/ser.rs

@@ -580,23 +580,31 @@ fn serialize_struct_visitor<I>(
 ) -> (P<ast::Item>, P<ast::Item>)
     where I: Iterator<Item=P<ast::Expr>>,
 {
+    let value_exprs = value_exprs.collect::<Vec<_>>();
+
     let field_attrs = struct_field_attrs(cx, builder, struct_def);
 
-    let len = struct_def.fields.len() - field_attrs.iter()
+    let arms: Vec<ast::Arm> = field_attrs.iter()
-        .fold(0, |sum, field| {
+        .zip(value_exprs.iter())
-            sum + if field.skip_serializing_field() { 1 } else { 0 }
-        });
-
-    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))| {
+        .map(|(i, (ref field, value_expr))| {
             let key_expr = field.serializer_key_expr(cx);
+
+            let stmt = if field.skip_serializing_field_if_empty() {
+                quote_stmt!(cx, if $value_expr.is_empty() { continue; })
+            } else if field.skip_serializing_field_if_none() {
+                quote_stmt!(cx, if $value_expr.is_none() { continue; })
+            } else {
+                quote_stmt!(cx, {})
+            };
+
             quote_arm!(cx,
                 $i => {
                     self.state += 1;
-                    Ok(
+                    $stmt
+
+                    return Ok(
                         Some(
                             try!(
                                 serializer.visit_struct_elt(
@@ -605,7 +613,7 @@ fn serialize_struct_visitor<I>(
                                 )
                             )
                         )
-                    )
+                    );
                 }
             )
         })
@@ -622,6 +630,21 @@ fn serialize_struct_visitor<I>(
         .strip_bounds()
         .build();
 
+    let len = field_attrs.iter()
+        .zip(value_exprs.iter())
+        .map(|(field, value_expr)| {
+            if field.skip_serializing_field() {
+                quote_expr!(cx, 0)
+            } else if field.skip_serializing_field_if_empty() {
+                quote_expr!(cx, if $value_expr.is_empty() { 0 } else { 1 })
+            } else if field.skip_serializing_field_if_none() {
+                quote_expr!(cx, if $value_expr.is_none() { 0 } else { 1 })
+            } else {
+                quote_expr!(cx, 1)
+            }
+        })
+        .fold(quote_expr!(cx, 0), |sum, expr| quote_expr!(cx, $sum + $expr));
+
     (
         quote_item!(cx,
             struct Visitor $visitor_impl_generics $where_clause {
@@ -640,9 +663,11 @@ fn serialize_struct_visitor<I>(
                 fn visit<S>(&mut self, serializer: &mut S) -> ::std::result::Result<Option<()>, S::Error>
                     where S: ::serde::ser::Serializer,
                 {
-                    match self.state {
+                    loop {
-                        $arms
+                        match self.state {
-                        _ => Ok(None)
+                            $arms
+                            _ => { return Ok(None); }
+                        }
                     }
                 }
 

serde_tests/tests/test_annotations.rs

@@ -39,6 +39,20 @@ struct SkipSerializingFields<A: default::Default> {
     b: A,
 }
 
+#[derive(Debug, PartialEq, Deserialize, Serialize)]
+struct SkipSerializingIfEmptyFields<A: default::Default> {
+    a: i8,
+    #[serde(skip_serializing_if_empty, default)]
+    b: Vec<A>,
+}
+
+#[derive(Debug, PartialEq, Deserialize, Serialize)]
+struct SkipSerializingIfNoneFields<A: default::Default> {
+    a: i8,
+    #[serde(skip_serializing_if_none, default)]
+    b: Option<A>,
+}
+
 #[test]
 fn test_default() {
     assert_de_tokens(
@@ -169,3 +183,161 @@ fn test_skip_serializing_fields() {
         ]
     );
 }
+
+#[test]
+fn test_skip_serializing_fields_if_empty() {
+    assert_ser_tokens(
+        &SkipSerializingIfEmptyFields::<i32> {
+            a: 1,
+            b: vec![],
+        },
+        &[
+            Token::StructStart("SkipSerializingIfEmptyFields", Some(1)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_de_tokens(
+        &SkipSerializingIfEmptyFields::<i32> {
+            a: 1,
+            b: vec![],
+        },
+        vec![
+            Token::StructStart("SkipSerializingIfEmptyFields", Some(1)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_ser_tokens(
+        &SkipSerializingIfEmptyFields {
+            a: 1,
+            b: vec![2],
+        },
+        &[
+            Token::StructStart("SkipSerializingIfEmptyFields", Some(2)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapSep,
+            Token::Str("b"),
+            Token::SeqStart(Some(1)),
+            Token::SeqSep,
+            Token::I32(2),
+            Token::SeqEnd,
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_de_tokens(
+        &SkipSerializingIfEmptyFields {
+            a: 1,
+            b: vec![2],
+        },
+        vec![
+            Token::StructStart("SkipSerializingIfEmptyFields", Some(2)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapSep,
+            Token::Str("b"),
+            Token::SeqStart(Some(1)),
+            Token::SeqSep,
+            Token::I32(2),
+            Token::SeqEnd,
+
+            Token::MapEnd,
+        ]
+    );
+}
+
+#[test]
+fn test_skip_serializing_fields_if_none() {
+    assert_ser_tokens(
+        &SkipSerializingIfNoneFields::<i32> {
+            a: 1,
+            b: None,
+        },
+        &[
+            Token::StructStart("SkipSerializingIfNoneFields", Some(1)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_de_tokens(
+        &SkipSerializingIfNoneFields::<i32> {
+            a: 1,
+            b: None,
+        },
+        vec![
+            Token::StructStart("SkipSerializingIfNoneFields", Some(1)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_ser_tokens(
+        &SkipSerializingIfNoneFields {
+            a: 1,
+            b: Some(2),
+        },
+        &[
+            Token::StructStart("SkipSerializingIfNoneFields", Some(2)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapSep,
+            Token::Str("b"),
+            Token::Option(true),
+            Token::I32(2),
+
+            Token::MapEnd,
+        ]
+    );
+
+    assert_de_tokens(
+        &SkipSerializingIfNoneFields {
+            a: 1,
+            b: Some(2),
+        },
+        vec![
+            Token::StructStart("SkipSerializingIfNoneFields", Some(2)),
+
+            Token::MapSep,
+            Token::Str("a"),
+            Token::I8(1),
+
+            Token::MapSep,
+            Token::Str("b"),
+            Token::Option(true),
+            Token::I32(2),
+
+            Token::MapEnd,
+        ]
+    );
+}