deserialize_in_place equivalent for bincode 2.0

[Jordan Rose]

Switching from Serde's deserialize to deserialize_in_place saves a significant amount of code size for us, due to Result<Self, Error> requiring a memcpy on success if it's part of a larger structure. It would stink to lose that option if/when we update to bincode 2.0.

---

Note that #425 claims that

Internally there is no actual deserialize_in_place. Serde's implementation is:

#[doc(hidden)]
fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
where
    D: Deserializer<'de>,
{
    // Default implementation just delegates to `deserialize` impl.
    *place = Deserialize::deserialize(deserializer)?;
    Ok(())
}

so for bincode 2 we're considering not supporting this (or implementing it properly)

However, that's merely the default implementation, and serde_derive has a feature to generate proper deserialize_in_place implementations.

[Trangar]

If someone wants to write a PR with this, we can merge that. I've never worked with deserialize_in_place. It would be nice to add a test with serde_derive doing the proper implementation too.