Decoding composite data items without Default

[chrysn]

With the currently available documentation it is unclear how a composite type (say, point = [x: int, y: int] / struct Point(x, y) would be decoded. The closest trait to implement is CBORDecodable, but that requires a &mut Point as input, which needs to contain a valid representation before parsing. This may work well for complex types where all components are optional (so that before decoding a default empty instance is created), but needs workarounds like allowing some actually-uninitialized state, and extra safeguards to ensure that that doesn't go wrong.

My impression of CBORDecodable is that it conflates two concerns:

• It ensures that the struct doesn't get passed around, but is pre-allocated in its eventual position. (I don't know how well the Rust compiler handles return placement in general, but it seems to do well in simple cases.)

• It simplifies handling of types with many default properties.

I could come up with various suggestions, but think that best progress would be made here if the rationale behind the current API (and possibly examples) were documented. How should a type like the above Point be used with CBORDecodable?