Default for a subset of fields

[idanarye]

I think the compiler should check, when doing an impl PartialUnpack, that every field in FooParams has a field in Foo with the same name and the same type, and that that field is accessible from the module that does the impl.

[scottmcm]

@cklein

Let's say I have a struct with 20 fields, all 19 but 1 fields implemented Default, thus I can't derive Default for my struct. Then When I instantiate my struct, I can't do the following: let x = MyStruct { mandatoryfields: 1, ... Default::default() }

Obligatory reference to this proposal that 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…

which would allow

let x = MyStruct { mandatory_field: 1, .. };