Allow non-Serialize default values.

Default values that don't implement Serialize are now ignored, rather than causing a compile error.
This is done by simulating specialization using a technique copied from Rocket:
5ebefa97c9/core/lib/src/sentinel.rs (L391-L445)

Fixes #115

schemars/tests/default.rs

@@ -1,6 +1,5 @@
 mod util;
 use schemars::JsonSchema;
-use serde::{Deserialize, Serialize};
 use util::*;
 
 fn is_default<T: Default + PartialEq>(value: &T) -> bool {
@@ -25,7 +24,7 @@ where
     ser.collect_str(&format_args!("i:{} b:{}", value.my_int, value.my_bool))
 }
 
-#[derive(Default, Deserialize, Serialize, JsonSchema, Debug)]
+#[derive(Default, JsonSchema, Debug)]
 #[serde(default)]
 pub struct MyStruct {
     pub my_int: i32,
@@ -37,9 +36,10 @@ pub struct MyStruct {
         skip_serializing_if = "is_default"
     )]
     pub my_struct2_default_skipped: MyStruct2,
+    pub not_serialize: NotSerialize,
 }
 
-#[derive(Default, Deserialize, Serialize, JsonSchema, Debug, PartialEq)]
+#[derive(Default, JsonSchema, Debug, PartialEq)]
 #[serde(default = "ten_and_true")]
 pub struct MyStruct2 {
     #[serde(default = "six")]
@@ -47,6 +47,9 @@ pub struct MyStruct2 {
     pub my_bool: bool,
 }
 
+#[derive(Default, JsonSchema, Debug)]
+pub struct NotSerialize;
+
 #[test]
 fn schema_default_values() -> TestResult {
     test_default_generated_schema::<MyStruct>("default")

schemars/tests/expected/default.json

@@ -22,6 +22,9 @@
     },
     "my_struct2_default_skipped": {
       "$ref": "#/definitions/MyStruct2"
+    },
+    "not_serialize": {
+      "$ref": "#/definitions/NotSerialize"
     }
   },
   "definitions": {
@@ -38,6 +41,9 @@
           "type": "boolean"
         }
       }
+    },
+    "NotSerialize": {
+      "type": "null"
     }
   }
 }