Deriving decodable is useless for struct with anonymous fields

[tomaka]

For example:

#[deriving(Decodable)]
struct Duration(int);

The decodable implementation created here tries to decode an object with a field named _field0 and of type int, like { "_field0": 5 }

There two problems with this:

• Most people won't expect this behavior (I would expect this code to decode a single int, not an object)
• The fact that the field name is arbitrary makes this trait impl useless anyway

I think that the deriving implementation of decodable for structs with one anonymous field should be equivalent to decoding the type of the field itself.

Structs with multiple anonymous fields are ambiguous, so I think deriving decodable for them should simply trigger a compilation error.

[huonw]

cc me

[reem]

Probably relevant to the newtype RFC (or vice versa). rust-lang/rfcs#186

[bluss]

Related to #17158 -- there is a separate method for encoding tuple structs, but it is never used by deriving.

[steveklabnik]

Encode was moved out of tree, so if this is still an issue, please pursue it over at https://github.com/rust-lang/rustc-serialize