Rust: Functional record update: private fields should not throw errors if not explicitly used in literals

[RReverser]

I tried this code:

mod module {
    #[derive(Default)]
    pub struct S {
        pub public_field: u32,
        _private_field: u32,
    }
}

use module::S;

pub fn f() -> S {
    S {
        public_field: 42,
        ..S::default()
    }
}

I expected to see this happen: code compiles successfully.

Instead, this happened: rustc throws an error

error[E0451]: field `_private_field` of struct `module::S` is private

  --> <source>:14:11

   |

14 |         ..S::default()

   |           ^^^^^^^^^^^^ field `_private_field` is private

As a user, I field this confusing, because I'm not trying to access or set the private field explicitly anywhere in the literal.

Meta

Applies to all known rustc versions.

I understand why this happens in principle, if the literal is desugared literally (no pun intended) to:

let s_rest = S::default();
let s = S {
  public_field: 42,
  _private_field: s_rest.private_field,
};

However, it seems that this code could be equally expanded to:

let mut s = S::default();
s.public_field = 42;
let s = s;

This way an immutable literal could be used with many more kinds of structures without users manually resorting to the temporarily-mutable variable as in the 2nd example.

Do I miss some other difference between the two that would break some existing usecases if the implementation is changed?

[petrochenkov]

Do I miss some other difference between the two that would break some existing usecases if the implementation is changed?

The desugaring change would break existing use cases, yes.
See https://internals.rust-lang.org/t/proto-rfc-expanded-functional-record-update/3077/2.

[RReverser]

@petrochenkov Interesting. It's hard to understand from the branch code, but is desugaring in your experiment same as I have shown above? I don't really understand why Drop would be an issue here, since I'm not trying to move any fields out.

[RReverser]

I don't really understand why Drop would be an issue here, since I'm not trying to move any fields out.

Tried to add Drop to my S and the desugaring still works as expected. Could you please provide an example code that would break?

[RalfJung]

This discussion also recently came up on IRLO again: https://internals.rust-lang.org/t/pre-rfc-relaxed-non-exhaustive-structs/11977

Tried to add Drop to my S and the desugaring still works as expected. Could you please provide an example code that would break?

As the linked comment says: "FRU from borrowed contexts, you can’t move the whole value, but can copy individual Copy fields"
So, try a situation where the thing after the .. is borrowed, not owned. This commit has some examples.

[RReverser]

@RalfJung Ah, I see, haven't realised it's possible. It could be interesting to enable this at least for owned case, but I suppose that's what makes feature request more nuanced (which explains previous forum discussions).

[jonhoo]

I'll just add to this that if we could use the alternative desugaring, this would also make functional record update syntax work with #[non_exhaustive] types as well, which would be _really_ nice.

[jplatte]

Would it be feasible to make this work without fundamentally changing the desugaring? I.e. somehow "ignore" the non-exhaustiveness of the struct (and privacy of implicitly-copied / -moved fields) when it is used with FRU?

[jplatte]

Also either this or #63538 should probably be closed as a duplicate.