If an uninitialized bson.D is passed to Find, the following error results:

cannot transform type primitive.D to a BSON Document: WriteNull can only write while positioned on a Element or Value but is positioned on a TopLevel

This is a very confusing error for users. As mentioned in GODRIVER-917, we differ from mgo in how we serialize a nil document, but regardless of how we resolve that ticket, I think it would be a better user experience to detect this case earlier in the call chain so we can return a meaningful error message that would help users quickly fix the problem.

I suspect that any API method (or option) that takes a document could have the same problem as Find.

Here is a repro (against master):

package main

import (
	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

func main() {
	opts := options.Client().ApplyURI("mongodb://localhost:27017")
	client, err := mongo.Connect(nil, opts)
	if err != nil {
		panic(err)
	}

	collection := client.Database("test").Collection("test")

	var filter bson.D
	cursor, err := collection.Find(nil, filter)
	if err != nil {
		panic(err)
	}
	_ = cursor
}