I seem to recall an old RFC that would make this code legal:

pub struct NoDefault;

pub struct Mixed {
    a: u32,
    b: NoDefault,
    c: Option<NoDefault>,
}

fn foo() -> Mixed {
    Mixed {
        b: NoDefault,
        ..Default::default()
    }
}

This unfortunately isn't covered by default_field_values because as far as I can tell, that will require an explicit = default() on each field going through #[derive(Default)]. In this case I want to specify values manually for all of the non-default fields, and use Default to fill in the remaining ones, without explicitly marking up the struct*. Does such an RFC or proposal/discussion exist?

* - Say, for example, this struct is a generated builder struct from a macro that can't tell which field is Default or not.

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