rustdoc: add testcase for traitParent deduplication

tests/rustdoc-js/trait-methods.js

@@ -9,4 +9,24 @@ const EXPECTED = [
             { 'path': 'trait_methods::MyTrait', 'name': 'next' },
         ],
     },
+    // the traitParent deduplication pass should remove
+    // Empty::next, as it would be redundant
+    {
+        'query': 'next',
+        'correction': null,
+        'in_args': [],
+        'others': [
+            { 'path': 'trait_methods::MyTrait', 'name': 'next' },
+        ],
+    },
+    // if the trait does not match, no deduplication happens
+    {
+        'query': '-> option<()>',
+        'correction': null,
+        'in_args': [],
+        'others': [
+            { 'path': 'trait_methods::Empty', 'name': 'next' },
+                    { 'path': 'trait_methods::Void', 'name': 'next' },
+        ],
+    },
 ];

tests/rustdoc-js/trait-methods.rs

@@ -2,3 +2,21 @@ pub trait MyTrait {
     type Item;
     fn next(&mut self) -> Option<Self::Item>;
 }
+
+pub struct Empty;
+
+impl MyTrait for Empty {
+    type Item = ();
+    fn next(&mut self) -> Option<()> {
+        None
+    }
+}
+
+pub struct Void;
+
+impl MyTrait for Void {
+    type Item = ();
+    fn next(&mut self) -> Option<()> {
+        Some(())
+    }
+}