feature request: Default value for fields && Ignore extra bytes

[kdy1]

Note: This is similar to #29

Hi. I'm the author of the SWC project. We currently use rkyv for Wasm plugins.
We pass serialized bytes to the Wasm plugin and deserialize them from the Wasm plugins, but it's not backward-compatible.

Most of the time, we need to add fields to existing types. I don't want it to be a breaking change.
Is it possible with bitcode?

I think what we need here is ignoring extra data (from a new version of host) and using a default value if when not existing

[finnbear]

Hello!

we need to add fields to existing types. I don't want it to be a breaking change.
Is it possible with bitcode?

No, that will never be possible with bitcode::{Encode, Decode}. bitcode is not self-describing, at all. You would need some abstraction like encoding a HashMap<String, Field> or Vec<Field>. As an exception, you can add variants to the end of an enum if you use the serde version of bitcode.

I think what we need here is ignoring extra data (from a new version of host) and using a default value if when not existing

When the total encoded length doesn't match decoding expectations, that is a very good indication that the data is corrupted. It is not enough information to reconstruct which part of the data-structure is missing or extra. Your suggestion to use defaults would only work for the outer-most struct, which we don't have much motivation to optimize for at the expense of much more complexity.

Note: This is similar to #29

#29 is different because it deals with unconditionally ignoring fields when encoding/decoding.

[kdy1]

Thank you!