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/src/_private.rs

@@ -2,6 +2,8 @@ use crate::flatten::Merge;
 use crate::gen::SchemaGenerator;
 use crate::schema::{Metadata, Schema, SchemaObject};
 use crate::JsonSchema;
+use serde::Serialize;
+use serde_json::Value;
 
 // Helper for generating schemas for flattened `Option` fields.
 pub fn json_schema_for_flatten<T: ?Sized + JsonSchema>(
@@ -32,3 +34,34 @@ pub fn apply_metadata(schema: Schema, metadata: Metadata) -> Schema {
         Schema::Object(schema_obj)
     }
 }
+
+/// Hack to simulate specialization:
+/// `MaybeSerializeWrapper(x).maybe_to_value()` will resolve to either
+/// - The inherent method `MaybeSerializeWrapper::maybe_to_value(...)` if x is `Serialize`
+/// - The trait method `NoSerialize::maybe_to_value(...)` from the blanket impl otherwise
+#[doc(hidden)]
+#[macro_export]
+macro_rules! _schemars_maybe_to_value {
+    ($expression:expr) => {{
+        #[allow(unused_imports)]
+        use $crate::_private::{MaybeSerializeWrapper, NoSerialize as _};
+
+        MaybeSerializeWrapper($expression).maybe_to_value()
+    }};
+}
+
+pub struct MaybeSerializeWrapper<T>(pub T);
+
+pub trait NoSerialize: Sized {
+    fn maybe_to_value(self) -> Option<Value> {
+        None
+    }
+}
+
+impl<T> NoSerialize for T {}
+
+impl<T: Serialize> MaybeSerializeWrapper<T> {
+    pub fn maybe_to_value(self) -> Option<Value> {
+        serde_json::value::to_value(self.0).ok()
+    }
+}