registry+https://github.com/rust-lang/crates.io-index#mongodb@3.2.3
registry+https://github.com/rust-lang/crates.io-index#bson@2.14.0

#[derive(Debug, Deserialize, Serialize, Hash, PartialEq, Eq)]
struct NewType(String);
#[derive(Debug, Deserialize, Serialize, PartialEq, Eq)]
struct Foo {
 map: HashMap<NewType, u64>,
}

#[cfg(test)]
mod test {
 use std::collections::HashMap;
 use mongodb::bson::{self, doc};
 use serde::{Deserialize, Serialize};
 async fn connect_to_database() -> mongodb::error::Result<mongodb::Database> {
 let client_options =
 mongodb::options::ClientOptions::parse(std::env::var("MONGODB_URL").unwrap()).await?;
 let default_database = client_options.default_database.as_ref().unwrap().clone();
 let client = mongodb::Client::with_options(client_options)?;
 Ok(client.database(&default_database))
 }
 #[derive(Debug, Deserialize, Serialize, Hash, PartialEq, Eq)]
 struct NewType(String);
 #[derive(Debug, Deserialize, Serialize, PartialEq, Eq)]
 struct Foo {
 map: HashMap<NewType, u64>,
 }
 #[test]
 fn to_from_bson() {
 let foo = Foo {
 map: [(NewType("abc".to_string()), 1)].into(),
 };
 let doc = bson::to_document(&foo).unwrap();
 let foo2: Foo = bson::from_document(doc).unwrap();
 assert_eq!(foo2, foo);
 }
 #[tokio::test]
 async fn to_from_bson_via_db() {
 let foo = Foo {
 map: [(NewType("abc".to_string()), 1)].into(),
 };
 let db = connect_to_database().await.unwrap();
 let coll = db.collection::<Foo>("_mongodb-serialization-test");
 coll.insert_one(&foo).await.unwrap();
 let foo2 = coll.find_one(doc! {}).await.unwrap().unwrap();
 assert_eq!(foo2, foo);
 }
}

running 2 tests
test test::to_from_bson ... ok
test test::to_from_bson_via_db ... FAILED
failures:
---- test::to_from_bson_via_db stdout ----
thread 'test::to_from_bson_via_db' panicked at src/lib.rs:46:49:
called `Result::unwrap()` on an `Err` value: Error { kind: BsonDeserialization(DeserializationError { message: "invalid type: string \"abc\", expected tuple struct NewType" }), labels: {}, wire_version: None, source: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
 test::to_from_bson_via_db
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.73s
error: test failed, to rerun pass `--lib`