lsp: Skip textDocument/completion for ignored files (#953)

When files are ignored, they will have no file contents in the cache,
and so completions are impossible. This leads to an error message which
is now avoided by just returning an empty list.

Fixes #879

Signed-off-by: Charlie Egan <charlie@styra.com>

internal/lsp/server.go

@@ -863,6 +863,15 @@ func (l *LanguageServer) handleTextDocumentCompletion(
 		return nil, fmt.Errorf("failed to unmarshal params: %w", err)
 	}
 
+	// when config ignores a file, then we return an empty completion list
+	// as a no-op.
+	if l.ignoreURI(params.TextDocument.URI) {
+		return types.CompletionList{
+			IsIncomplete: false,
+			Items:        []types.CompletionItem{},
+		}, nil
+	}
+
 	// items is allocated here so that the return value is always a non-nil CompletionList
 	items, err := l.completionsManager.Run(params, &providers.Options{
 		ClientIdentifier: l.clientIdentifier,