serde_derive guarantees are not documented

[ZoeyR]

I can't seem to find any documentation on any guarantees that serde_derive may have for the manner in which it serializes structs and enums. I'm mostly curious if the order in which it will call the different serialize_* and deserialize_* methods is guaranteed to be stable or not.

[dtolnay]

Or are they all documented, because there are none. ;)

Derived Serialize impls will call serialize_field according to the order of fields in the struct definition.

[ZoeyR]

If there are actually no guarantees that would be good to know since formats that don't self-describe could potentially break based on serde_derive versions. serde_cbor in packed mode would also be affected since it uses indexes for the fields.

[ZoeyR]

@dtolnay can you please clarify if serde_derive is guaranteed to call the fields in declaration order? If its just the current behavior and not actually part of the stability contract I need to know for documenting my format.

[dtolnay]

Yes, that can be considered stable.