[clangd] Return empty results on spurious completion triggers

[ilya-biryukov]

Summary

We currently return an error, this causes coc.nvim and VSCode to
show an error message in the logs.

Returning empty list of completions allows to avod the error message
without altering other user-visible behavior.

[sammccall]

clang-tools-extra/clangd/ClangdLSPServer.cpp

733

this error code seems bogus, we should have a comment explaining that returning and empty list breaks vscode and any other error code breaks coc.nvim.

(Might want to file a vscode or LSP bug - trigger char is a heuristic, if [] isn't the preferred way for a server to say "no" then we should spell it out, if it is then vscode is wrong)

[ilya-biryukov]

• Return empty results instead of an error

[ilya-biryukov]

clang-tools-extra/clangd/ClangdLSPServer.cpp

733

Returning empty completion results seems to work just fine.
Added a comment anyway, but I don't think we need an LSP bug for this, returning empty results seems less cheesy than a 'request cancelled' error.

clang-tools-extra/clangd/ClangdLSPServer.cpp

 #include "llvm/Support/Error.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/ScopedPrinter.h"
 
 namespace clang {
 namespace clangd {
 namespace {
-class IgnoreCompletionError : public llvm::ErrorInfo<CancelledError> {
-public:
-  void log(llvm::raw_ostream &OS) const override {
-    OS << "ignored auto-triggered completion, preceding char did not match";
-  }
-  std::error_code convertToErrorCode() const override {
-    return std::make_error_code(std::errc::operation_canceled);
-  }
-};
-
 /// Transforms a tweak into a code action that would apply it if executed.
 /// EXPECTS: T.prepare() was called and returned true.
 CodeAction toCodeAction(const ClangdServer::TweakRef &T, const URIForFile &File,
                         Range Selection) {
   CodeAction CA;
   CA.title = T.Title;
   CA.kind = CodeAction::REFACTOR_KIND;
   // This tweak may have an expensive second stage, we only run it if the user
void ClangdLSPServer::onCodeAction(const CodeActionParams &Params,
   Server->enumerateTweaks(File.file(), Params.range,
                           Bind(ConsumeActions, std::move(Reply), File,
                                std::move(*Code), Params.range,
                                std::move(FixIts)));
 }
 
 void ClangdLSPServer::onCompletion(const CompletionParams &Params,
                                    Callback<CompletionList> Reply) {
-  if (!shouldRunCompletion(Params))
-    return Reply(llvm::make_error<IgnoreCompletionError>());
+  if (!shouldRunCompletion(Params)) {
+    // Clients sometimes auto-trigger completions in undesired places (e.g.
+    // 'a >^ '), we return empty results in those cases.
+    vlog("ignored auto-triggered completion, preceding char did not match");
+    return Reply(CompletionList());
+  }
   Server->codeComplete(Params.textDocument.uri.file(), Params.position, CCOpts,
                        Bind(
                            [this](decltype(Reply) Reply,
                                   llvm::Expected<CodeCompleteResult> List) {
                              if (!List)
                                return Reply(List.takeError());
                              CompletionList LSPList;
                              LSPList.isIncomplete = List->HasMore;

clang-tools-extra/clangd/test/completion-auto-trigger.test

 # RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
 ---
 {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"namespace ns { int ns_member; } struct vector { int size; static int default_capacity; };\nvoid test(vector *a, vector *b) {\n  if (a > b) {} \n  a->size = 10;\n\n  a ? a : b;\n  ns::ns_member = 10;\n}"}}}
 ---
 {"jsonrpc":"2.0","id":1,"method":"textDocument/completion","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":2,"character":9},"context":{"triggerKind":2,"triggerCharacter":">"}}}
-#      CHECK:  "error": {
-# CHECK-NEXT:    "code": -32001,
-# CHECK-NEXT:    "message": "ignored auto-triggered completion, preceding char did not match"
-# CHECK-NEXT:  },
-# CHECK-NEXT:  "id": 1,
+#      CHECK:  "id": 1,
+# CHECK-NEXT:  "jsonrpc": "2.0"
+# CHECK-NEXT:  "result": {
+# CHECK-NEXT:    "isIncomplete": false,
+# CHECK-NEXT:    "items": []
+# CHECK-NEXT:  }
 ---
 {"jsonrpc":"2.0","id":2,"method":"textDocument/completion","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":3,"character":5},"context":{"triggerKind":2,"triggerCharacter":">"}}}
 #      CHECK:  "id": 2,
 # CHECK-NEXT:  "jsonrpc": "2.0"
 # CHECK-NEXT:  "result": {
 # CHECK-NEXT:    "isIncomplete": false,
 # CHECK-NEXT:    "items": [
 # CHECK-NEXT:       {

 # CHECK-NEXT:             }
 # CHECK-NEXT:           }
 # CHECK-NEXT:         }
 # CHECK-NEXT:       }
 # CHECK-NEXT:     ]
 # CHECK-NEXT:   }
 ---
 {"jsonrpc":"2.0","id":3,"method":"textDocument/completion","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":5,"character":9},"context":{"triggerKind":2,"triggerCharacter":":"}}}
-#      CHECK:  "error": {
-# CHECK-NEXT:    "code": -32001,
-# CHECK-NEXT:    "message": "ignored auto-triggered completion, preceding char did not match"
-# CHECK-NEXT:  },
-# CHECK-NEXT:  "id": 3,
+#      CHECK:  "id": 3,
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": {
+# CHECK-NEXT:    "isIncomplete": false,
+# CHECK-NEXT:    "items": []
+# CHECK-NEXT:  }
 ---
 {"jsonrpc":"2.0","id":4,"method":"textDocument/completion","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":6,"character":6},"context":{"triggerKind":2,"triggerCharacter":":"}}}
 ---
 #      CHECK:  "id": 4,
 # CHECK-NEXT:  "jsonrpc": "2.0"
 # CHECK-NEXT:  "result": {
 # CHECK-NEXT:    "isIncomplete": false,
 # CHECK-NEXT:    "items": [