Default for a subset of fields

[scottmcm]

Obligatory link to what I'm still hoping we do:

Pre-pre-RFC: syntactic sugar for `Default::default()`

Thanks @spacekookie! I know is a long thread so it is a good time to summarize what the RFC I'll be writing will propose, particularly because it is removed from what I originally wrote: Language support for default values on structs through const expressions: struct Car { wheel_count: usize = 4; } Language support for partially defaulted structs: struct Person { name: String, age: i128 = 0; } Teach #[derive(Default)] to use the default values #[derive(Default)] struct P…