d4f23cdc91
Reject union default field values Fixes rust-lang/rust#142555. The [`default_field_values` RFC][rfc] does not specify that default field values may be used on `union`s, and it's not clear how default field values may be used with `union`s without an design extension to the RFC. So, for now, reject trying to use default field values with `union`s. ### Review notes - The first commit adds the `union` with default field values test case to `tests/ui/structs/default-field-values/failures.rs`, where `union`s with default field values are currently accepted. - The second commit rejects trying to supply default field values to `union` definitions. - When `default_field_values` feature gate is disabled, we show the feature gate error when the user tries to write `union`s with default field values. When the feature gate is enabled, we reject this usage with > unions cannot have default field values ``@rustbot`` label: +F-default_field_values [rfc]: https://rust-lang.github.io/rfcs/3681-default-field-values.html
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