Fix autorust handling of optional datetime fields

sdk/core/src/date/mod.rs

@@ -141,8 +141,20 @@ pub fn diff(first: OffsetDateTime, second: OffsetDateTime) -> Duration {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serde::{Deserialize, Serialize};
+    use serde_json;
     use time::macros::datetime;
 
+    #[derive(Serialize, Deserialize)]
+    struct ExampleState {
+        #[serde(with = "crate::date::rfc3339")]
+        created_time: time::OffsetDateTime,
+
+        // Note: Must specify "default" in serde options when using "with"
+        #[serde(default, with = "crate::date::rfc3339::option")]
+        deleted_time: Option<time::OffsetDateTime>,
+    }
+
     #[test]
     fn test_roundtrip_rfc3339() -> crate::Result<()> {
         let s = "2019-10-12T07:20:50.52Z";
@@ -192,4 +204,36 @@ mod tests {
         );
         Ok(())
     }
+
+    #[test]
+    fn test_serde_rfc3339_none_optional() -> crate::Result<()> {
+        let json_state = r#"{
+            "created_time": "2021-07-01T10:45:02Z"
+        }"#;
+
+        let state: ExampleState = serde_json::from_str(json_state).unwrap();
+
+        assert_eq!(parse_rfc3339("2021-07-01T10:45:02Z")?, state.created_time);
+        assert_eq!(state.deleted_time, None);
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_serde_rfc3339_some_optional() -> crate::Result<()> {
+        let json_state = r#"{
+            "created_time": "2021-07-01T10:45:02Z",
+            "deleted_time": "2022-03-28T11:05:31Z"
+        }"#;
+
+        let state: ExampleState = serde_json::from_str(json_state).unwrap();
+
+        assert_eq!(parse_rfc3339("2021-07-01T10:45:02Z")?, state.created_time);
+        assert_eq!(
+            state.deleted_time,
+            Some(parse_rfc3339("2022-03-28T11:05:31Z").unwrap())
+        );
+
+        Ok(())
+    }
 }

services/autorust/codegen/src/codegen_models.rs

@@ -641,9 +641,11 @@ fn create_struct(cg: &CodeGen, schema: &SchemaGen, struct_name: &str, pageable:
             }
         } else {
             if type_name.is_date_time() {
-                serde_attrs.push(quote! { with = "azure_core::date::rfc3339::option"});
+                // Must specify `default` when using `with` for `Option`
+                serde_attrs.push(quote! { default, with = "azure_core::date::rfc3339::option"});
             } else if type_name.is_date_time_rfc1123() {
-                serde_attrs.push(quote! { with = "azure_core::date::rfc1123::option"});
+                // Must specify `default` when using `with` for `Option`
+                serde_attrs.push(quote! { default, with = "azure_core::date::rfc1123::option"});
             } else if type_name.is_vec() {
                 serde_attrs.push(quote! { default, skip_serializing_if = "Vec::is_empty"});
             } else {