Pre-RFC: User-provided default field values

[scottmcm]

cc @ekuber , since I'd still like to see exactly what was worked out in their previous thread

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…