No way for custom formats to initialize default values.

[ConradIrwin]

I am trying to write a serve deserializer for an (in-progress) language that omits default values from struct serialization.

This is not a problem for fields like strings, numbers, booleans, lists and maps, because I can use the well-defined default of 0, false or empty. That said, I found no way to ask serde to default the values for structs/tuples and enums in the presence of missing keys.

The things I tried so far:

• Recursively initializing struct/tuple fields to their primitive types' default values. This works, but does not respect any #[serde(default) or rust-level default values.
• Asking the user to remember to include #[serde(default)] on all fields/structs. This works, but is quite tedious.

My ideal solution would be something like adding an optional visitor.visit_default() to the Visitor protocol, so that I could correctly initialize default values for this file format.