Fix rust 2024 warning when using nserde(skip) field annotations

[Adjective-Object]

At present, the #nserde(skip) attribute on a struct with a derive(DeRon) annotation would cause build warnings. You can see this by running cargo test in the project.

rning: this function depends on never type fallback being `()`
   --> tests/ron.rs:138:14
    |
138 |     #[derive(DeRon, SerRon, PartialEq, Debug)]
    |              ^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
note: in edition 2024, the requirement `!: DeRon` will fail

From looking at the expanded macros, I think this is because in the generated code, type inference relies on the assignment of the local value to its corresponding struct field to know what the value of the type binding is. Because skip() skips this and replaces the value with Default:;default(), the resulting option type is initialized to the "lowest" possible type, Option<!>.

I think that makes the DeSer::de_ron() call to initialize the field values fall back to the DeSer impl for (), which triggers the compiler warning. Not 100% sure about that but either way, adding type annotations makes the warning go away!

[knickish]

Thanks for making this. Seems like it's passing all tests, so am planning to merge this after I have a chance to review it a bit more thoroughly. Did you happen to check if the same thing happens with DeJson?

[Adjective-Object]

I didn't see it happen. It looks like it's because the expanded skip macro for json ends up omitting the field entirely from deserialization when #[nserde(skip]) is set.

I believe the difference between json and ron here is because there's no whole_field equivalent for ron, so the parser has to parse skipped fields in order advance the parser state.

[knickish]

Thanks for doing this!