Not a HashMap specifically, but there is a way to group them into `Vec<(User, Vec<(Post, Vec<Comment>)>)>`. (Which you can convert into a hashmap pretty easily) http://docs.diesel.rs/diesel/associations/index.html for details.
That's actually what I was looking for. I'm was too out of practice in general with signatures in static languages and unfamiliar with Rust's syntax in specific to figure it out and express what I was looking for correctly. ;)