feat(rulesets): show warning if callbacks defined in a callback

docs/reference/openapi-rules.md

@@ -923,6 +923,25 @@ servers:
 
 In this example, both **`{region}`** and **`{version}`** variables are properly defined and used in the server URL. Also, the default value for **`region`** is within the allowed values.
 
+### oas3_callbacks_in_callbacks
+
+A callback should not be defined within another callback.
+
+**Recommended:** Yes
+
+**Bad Example**
+
+```yaml
+paths:
+  /path:
+    get:
+      callbacks:
+        onData:
+          /data:
+            post:
+              callbacks: ...
+```
+
 ### oas3_1-servers-in-webhook
 
 Servers should not be defined in a webhook.

packages/rulesets/src/oas/__tests__/oas3-callbacks-in-callbacks.test.ts

@@ -0,0 +1,33 @@
+import { DiagnosticSeverity } from '@stoplight/types';
+import testRule from '../../__tests__/__helpers__/tester';
+
+testRule('oas3-callbacks-in-callbacks', [
+  {
+    name: 'callbacks defined within a callback',
+    document: {
+      openapi: '3.0.0',
+      paths: {
+        '/path': {
+          get: {
+            callbacks: {
+              onData: {
+                '/data': {
+                  post: {
+                    callbacks: {},
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    },
+    errors: [
+      {
+        message: 'Callbacks should not be defined within a callback',
+        path: ['paths', '/path', 'get', 'callbacks', 'onData', '/data', 'post', 'callbacks'],
+        severity: DiagnosticSeverity.Warning,
+      },
+    ],
+  },
+]);

packages/rulesets/src/oas/index.ts

@@ -707,6 +707,15 @@ const ruleset = {
         },
       },
     },
+    'oas3-callbacks-in-callbacks': {
+      message: 'Callbacks should not be defined within a callback',
+      formats: [oas3],
+      recommended: true,
+      given: ['#OperationObject.callbacks[*][*][*].callbacks'],
+      then: {
+        function: undefined,
+      },
+    },
     'oas3_1-servers-in-webhook': {
       message: 'Servers should not be defined in a webhook.',
       formats: [oas3_1],