nixd: suppress completion if the node is not leaf (#522)

Fixes: #521

nixd/lib/Controller/Completion.cpp

@@ -6,6 +6,8 @@
 #include "AST.h"
 #include "Convert.h"
 
+#include "lspserver/Protocol.h"
+
 #include "nixd/Controller/Controller.h"
 
 #include <boost/asio/post.hpp>
@@ -277,6 +279,10 @@ void Controller::onCompletion(const CompletionParams &Params,
           Reply(error("cannot find corresponding node on given position"));
           return;
         }
+        if (!Desc->children().empty()) {
+          Reply(CompletionList{});
+          return;
+        }
         CompletionList List;
         try {
           const ParentMapAnalysis &PM = *TU->parentMap();

nixd/tools/nixd/test/completion-comment.md

@@ -0,0 +1,73 @@
+# RUN: nixd --lit-test \
+# RUN: --nixos-options-expr="{ foo.bar = { _type = \"option\"; }; }" \
+# RUN: < %s | FileCheck %s
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities": {
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "method":"textDocument/didOpen",
+   "params":{
+      "textDocument":{
+         "uri":"file:///completion.nix",
+         "languageId":"nix",
+         "version":1,
+         "text":"{ x = 1; y = /*   */2;}"
+      }
+   }
+}
+```
+
+```json
+{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "textDocument/completion",
+    "params": {
+        "textDocument": {
+            "uri": "file:///completion.nix"
+        },
+        "position": {
+            "line": 0,
+            "character": 16
+        },
+        "context": {
+            "triggerKind": 1
+        }
+    }
+}
+```
+
+```
+     CHECK: "id": 1,
+CHECK-NEXT: "jsonrpc": "2.0",
+CHECK-NEXT: "result": {
+CHECK-NEXT:   "isIncomplete": false,
+CHECK-NEXT:   "items": []
+CHECK-NEXT: }
+```
+
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```