`S { foo: val, ..Default::default() }` gives an error if the other fields aren't public

[Boscop]

S { foo: val, ..Default::default() } gives an error if the other fields aren't public, but it should work (given S: Default).

[petrochenkov]

This is unsound because Default::default() is not consumed with current language rules.
This can work if Default::default() is consumed, but that would be a breaking change unless the syntax is changed to something else (e.g. S { ..Default::default(), foo: val }).

[zackmdavis]

A more complete code sample:

mod my_module {
    #[derive(Default)]
    pub struct MyStruct {
        pub public_field: usize,
        private_field: bool,
        another_private_field: bool,
    }
}
 
use my_module::MyStruct;
 
fn main() {
    let s = MyStruct { public_field: 2, ..Default::default() };
}

Arguably not a bug insofar as we regard ..Default::default() as mere syntactic sugar for private_field: Default::default().private_field?

[Boscop]

But why can't

let s = MyStruct { public_field: 2, ..Default::default() };

be syntactic sugar for

let mut s: S = Default::default();
s.public_field = 2;
let s = s;

Then this would also work:

let s: MyStruct = returning_mystruct();
let s = MyStruct { public_field: 2, ..s };

when only public_field is pub.
IMO this should work..

It would at least be expected behavior for most people, and didn't we say that in 2018 we should make Rust more user-friendly / newbie-friendly? :)

[petrochenkov]

@Boscop
Because it's a real breaking change, I made an experiment (see the link in #47472 (comment)).

[sfackler]

Reversing rust-lang/rfcs#736 requires an RFC.