Fix Option<T> schema generation sometimes producing a schema that is not referenced

schemars/src/json_schema_impls.rs

@@ -272,9 +272,11 @@ impl<T: JsonSchema> JsonSchema for Option<T> {
     }
 
     fn json_schema(gen: &mut SchemaGenerator) -> Result {
-        // FIXME this may produce a subschema that is not referenced in the final schema,
+        let mut schema = if gen.settings().option_nullable {
-        // e.g. SingleOrVec_For_InstanceType in schema-openapi3.json
+            T::json_schema(gen)?
-        let mut schema = gen.subschema_for::<T>()?;
+        } else {
+            gen.subschema_for::<T>()?
+        };
         if gen.settings().option_add_null_type {
             schema = match schema {
                 Schema::Bool(true) => Schema::Bool(true),

schemars/tests/expected/schema-openapi3.json

@@ -24,6 +24,17 @@
         "integer"
       ]
     },
+    "Ref": {
+      "type": "object",
+      "properties": {
+        "$ref": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "$ref"
+      ]
+    },
     "Schema": {
       "anyOf": [
         {
@@ -144,43 +155,6 @@
         }
       },
       "additionalProperties": true
-    },
-    "Ref": {
-      "type": "object",
-      "properties": {
-        "$ref": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "$ref"
-      ]
-    },
-    "SingleOrVec_For_InstanceType": {
-      "anyOf": [
-        {
-          "$ref": "#/components/schemas/InstanceType"
-        },
-        {
-          "type": "array",
-          "items": {
-            "$ref": "#/components/schemas/InstanceType"
-          }
-        }
-      ]
-    },
-    "SingleOrVec_For_Schema": {
-      "anyOf": [
-        {
-          "$ref": "#/components/schemas/Schema"
-        },
-        {
-          "type": "array",
-          "items": {
-            "$ref": "#/components/schemas/Schema"
-          }
-        }
-      ]
     }
   }
 }