All the documentation and examples given describe a nil type can be passed as a parameter however when doing so the output is: 

$ go run main.go 
ID ObjectID("5c454e3cfd1361d7957038bf")
2019/01/21 14:44:44 bigerr: %sdocument is nil
exit status 1

The error comes from a call to collection.Find()

// cant pass nil     
// filter := bson.D{}
cur, err := collection.Find(context.Background(), nil)     
if err != nil {
         log.Fatal("bigerr: %s", err)     
}

collection.Find() passes the filter value downard to transformDocument()

https://github.com/mongodb/mongo-go-driver/blob/5514df920614a2ffba26ec3539d8509e8ad2344e/mongo/mongo.go#L133
 
 

And subsequently it is checked and if the value is nil it is returned. 

 

if val == nil {         return nil, ErrNilDocument     } 

It would make sense to be able to pass a nil to filter to get all the results from a colleciton however passing an empty bson.D{} object works also.