libnixt: stop at concrete options, rather than select into it. (#474)

Fixes:
#469 (comment)

nixd/lib/Eval/AttrSetProvider.cpp

@@ -264,6 +264,11 @@ void AttrSetProvider::onOptionComplete(
       return;
     }
 
+    if (nixt::isOption(state(), Scope)) {
+      Reply(error("scope is already an option"));
+      return;
+    }
+
     std::vector<OptionField> Response;
 
     // FIXME: we may want to use "Trie" to speedup the string searching.

nixd/tools/nixd/test/completion-option-stop.md

@@ -0,0 +1,72 @@
+# 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":"{ bar = 1; foo.bar. }"
+      }
+   }
+}
+```
+
+```json
+{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "textDocument/completion",
+    "params": {
+        "textDocument": {
+            "uri": "file:///completion.nix"
+        },
+        "position": {
+            "line": 0,
+            "character": 19
+        },
+        "context": {
+            "triggerKind": 1
+        }
+    }
+}
+```
+
+```
+     CHECK: "id": 1,
+CHECK-NEXT: "jsonrpc": "2.0",
+CHECK-NEXT: "result": {
+CHECK-NEXT: "isIncomplete": false,
+CHECK-NEXT: "items": []
+```
+
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```