Fix skip_serializing_if/serialize_with handling

Previously whenever a field with a default value has both `skip_serializing_if` and `with`/`serialize_with` attributes, the value would be converted to a type that performs the custom serialization before checking if it should be serialized. This would cause the wrong type to be given to the skip_serializing_if function, causing a compile error.

Issue #26

schemars/src/gen.rs

@@ -343,12 +343,22 @@ impl SchemaGenerator {
         }
     }
 
-    pub(crate) fn apply_metadata(&self, schema: Schema, metadata: Metadata) -> Schema {
-        let mut schema_obj = schema.into();
+    /// This function is only public for use by schemars_derive.
+    ///
+    /// It should not be considered part of the public API.
+    #[doc(hidden)]
+    pub fn apply_metadata(&self, schema: Schema, metadata: Option<Metadata>) -> Schema {
+        match metadata {
+            None => return schema,
+            Some(metadata) if metadata == Metadata::default() => return schema,
+            Some(metadata) => {
+                let mut schema_obj = schema.into();
 
-        self.make_extensible(&mut schema_obj);
-        schema_obj.metadata = Some(Box::new(metadata)).merge(schema_obj.metadata);
+                self.make_extensible(&mut schema_obj);
+                schema_obj.metadata = Some(Box::new(metadata)).merge(schema_obj.metadata);
 
-        Schema::Object(schema_obj)
+                Schema::Object(schema_obj)
+            }
+        }
     }
 }

schemars/src/json_schema_impls/core.rs

@@ -65,10 +65,7 @@ impl<T: JsonSchema> JsonSchema for Option<T> {
         _required: bool,
     ) {
         let mut schema = gen.subschema_for::<Self>();
-
-        if let Some(metadata) = metadata {
-            schema = gen.apply_metadata(schema, metadata);
-        }
+        schema = gen.apply_metadata(schema, metadata);
 
         let object = parent.object();
         object.properties.insert(name, schema);

schemars/src/lib.rs

@@ -300,10 +300,7 @@ pub trait JsonSchema {
         required: bool,
     ) {
         let mut schema = gen.subschema_for::<Self>();
-
-        if let Some(metadata) = metadata {
-            schema = gen.apply_metadata(schema, metadata);
-        }
+        schema = gen.apply_metadata(schema, metadata);
 
         let object = parent.object();
         if required {

schemars/tests/default.rs

@@ -3,6 +3,10 @@ use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use util::*;
 
+fn is_default<T: Default + PartialEq>(value: &T) -> bool {
+    value == &T::default()
+}
+
 fn ten_and_true() -> MyStruct2 {
     MyStruct2 {
         my_int: 10,
@@ -28,9 +32,14 @@ pub struct MyStruct {
     pub my_bool: bool,
     #[serde(serialize_with = "custom_serialize")]
     pub my_struct2: MyStruct2,
+    #[serde(
+        serialize_with = "custom_serialize",
+        skip_serializing_if = "is_default"
+    )]
+    pub my_struct2_default_skipped: MyStruct2,
 }
 
-#[derive(Default, Deserialize, Serialize, JsonSchema, Debug)]
+#[derive(Default, Deserialize, Serialize, JsonSchema, Debug, PartialEq)]
 #[serde(default = "ten_and_true")]
 pub struct MyStruct2 {
     #[serde(default = "six")]

schemars/tests/expected/default.json

@@ -19,6 +19,9 @@
           "$ref": "#/definitions/MyStruct2"
         }
       ]
+    },
+    "my_struct2_default_skipped": {
+      "$ref": "#/definitions/MyStruct2"
     }
   },
   "definitions": {