nixd: redirect doc highlight error from UI into log

nixd/lib/Controller/DocumentHighlight.cpp

@@ -58,7 +58,14 @@ void Controller::onDocumentHighlight(
         std::vector<DocumentHighlight> Highlights;
         if (auto Err = highlight(*Desc, *TU->parentMap(), *TU->variableLookup(),
                                  URI, Highlights)) {
-          Reply(std::move(Err));
+          // FIXME: Empty response if there are no def-use chain found.
+          // For document highlights, the specification explicitly specified LSP
+          // should do "fuzzy" things.
+
+          // Empty response on error, don't reply all errors because this method
+          // is very frequently called.
+          Reply(std::vector<DocumentHighlight>{});
+          lspserver::elog("textDocument/documentHighlight failed: {0}", Err);
           return;
         }
         Reply(std::move(Highlights));

nixd/tools/nixd/test/document-highlight-fail.md

@@ -0,0 +1,66 @@
+# RUN: nixd --lit-test < %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:///basic.nix",
+         "languageId":"nix",
+         "version":1,
+         "text":"let x = 1; y = 2; in x + y"
+      }
+   }
+}
+```
+
+<-- textDocument/documentHighlight(2)
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":2,
+   "method":"textDocument/documentHighlight",
+   "params":{
+      "textDocument":{
+         "uri":"file:///basic.nix"
+      },
+      "position":{
+        "line": 0,
+        "character":1
+      }
+   }
+}
+```
+
+```
+     CHECK: "id": 2,
+CHECK-NEXT: "jsonrpc": "2.0",
+CHECK-NEXT: "result": []
+```
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```