Deserialization implementation for Document doesn't handle documents with extjson keys

[PenguinToast]

Describe the bug

See reproduction below:

    let test_doc = doc! {
        "acutalTimestamp": bson::Timestamp {increment: 0, time: 12345},
        "operationTime": {"$timestamp": "7248047827882870681"},
    };
    let mut bson_bytes = Vec::<u8>::new();
    bson::Document::to_writer(&test_doc, &mut bson_bytes)?;
    let deserialized = bson::from_slice::<bson::Document>(&bson_bytes)?;

This errors with:

Error: invalid type: string "7248047827882870681", expected struct TimestampBody

test_doc is a valid BSON document, however the deserializer doesn't make any distinction between a $timestamp from an actual Timestamp vs. just a document with a $timestamp key. This causes issues with custom Deserialize implementations for data structures, where I can't figure out how to differentiate between a real Timestamp and a Document.

BE SPECIFIC:

• What is the expected behavior and what is actually happening?
  • This should not error, but it does.

[abr-egn]

Hi!

You're right, that shouldn't be an error. This is an unfortunate side effect of how our implementation interacts with serde; the information that the value is a subdocument rather than a timestamp is lost in the parsing. I've filed RUST-1693 to track this.

In the meanwhile, you can use the raw bson types to work around this - if you change your final line to

let deserialized = bson::from_slice::<bson::RawDocumentBuf>(&bson_bytes)?.to_document()?;

it works as expected.