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Reject union default field values #142570

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@jieyouxu jieyouxu commented Jun 16, 2025 •
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Fixes #142555.

The default_field_values RFC does not specify that default field values may be used on unions, and it's not clear how default field values may be used with unions without an design extension to the RFC. So, for now, reject trying to use default field values with unions.

Review notes

  • The first commit adds the union with default field values test case to tests/ui/structs/default-field-values/failures.rs, where unions 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 unions 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

@jieyouxu
Add union with default field values case test
32c0cb0 
In particular, there should be no additional errors (default field
values for `union` fields are currently erroneously accepted).
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rustbot commented Jun 16, 2025

r? @petrochenkov

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. F-default_field_values `#![feature(default_field_values)]` labels Jun 16, 2025
estebank
estebank reviewed Jun 16, 2025
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r? @estebank

r=me after addressing typo

@rustbot rustbot assigned estebank and unassigned petrochenkov Jun 16, 2025
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jieyouxu commented Jun 16, 2025 •
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Fixed typo.
@bors r=estebank rollup

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bors commented Jun 16, 2025

📌 Commit 2dd9cc1 has been approved by estebank

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 16, 2025
jhpratt added a commit to jhpratt/rust that referenced this pull request Jun 17, 2025
@jhpratt
Rollup merge of rust-lang#142570 - jieyouxu:disunion, r=estebank
78e2974 
Reject union default field values

Fixes rust-lang#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
@jhpratt jhpratt mentioned this pull request Jun 17, 2025
Closed
bors added a commit that referenced this pull request Jun 17, 2025
@bors
Auto merge of #142604 - jhpratt:rollup-77fsgte, r=jhpratt
896d9e4 
Rollup of 11 pull requests

Successful merges:

 - #140809 (Reduce special casing for the panic runtime)
 - #141608 (Add support for repetition to `proc_macro::quote`)
 - #141864 (Handle win32 separator for cygwin paths)
 - #142216 (Miscellaneous RefCell cleanups)
 - #142517 (Windows: Use anonymous pipes in Command)
 - #142570 (Reject union default field values)
 - #142584 (Handle same-crate macro for borrowck semicolon suggestion)
 - #142585 (Update books)
 - #142586 (Fold unnecessary `visit_struct_field_def` in AstValidator)
 - #142595 (Revert overeager warning for misuse of `--print native-static-libs`)
 - #142598 (Set elf e_flags on ppc64 targets according to abi)

r? `@ghost`
`@rustbot` modify labels: rollup
workingjubilee added a commit to workingjubilee/rustc that referenced this pull request Jun 17, 2025
@workingjubilee
Rollup merge of rust-lang#142570 - jieyouxu:disunion, r=estebank
d4f23cd 
Reject union default field values

Fixes rust-lang#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
@workingjubilee workingjubilee mentioned this pull request Jun 17, 2025
Draft
rust-bors bot added a commit that referenced this pull request Jun 17, 2025
@rust-bors
Auto merge of #142613 - workingjubilee:rollup-yuod2hg, r=<try>
69b2373 
Rollup of 13 pull requests

Successful merges:

 - #138538 (Make performance description of String::{insert,insert_str,remove} more precise)
 - #141946 (std: refactor explanation of `NonNull`)
 - #142216 (Miscellaneous RefCell cleanups)
 - #142542 (Manually invalidate caches in SimplifyCfg.)
 - #142563 (Refine run-make test ignores due to unpredictable `i686-pc-windows-gnu` unwind mechanism)
 - #142570 (Reject union default field values)
 - #142584 (Handle same-crate macro for borrowck semicolon suggestion)
 - #142585 (Update books)
 - #142586 (Fold unnecessary `visit_struct_field_def` in AstValidator)
 - #142587 (Make sure to propagate result from `visit_expr_fields`)
 - #142595 (Revert overeager warning for misuse of `--print native-static-libs`)
 - #142598 (Set elf e_flags on ppc64 targets according to abi)
 - #142601 (Add a comment to `FORMAT_VERSION`.)

r? `@ghost`
`@rustbot` modify labels: rollup
<!-- homu-ignore:start -->
[Create a similar rollup](https://bors.rust-lang.org/queue/rust?prs=138538,141946,142216,142542,142563,142570,142584,142585,142586,142587,142595,142598,142601)
<!-- homu-ignore:end -->
try-job: dist-apple-various
workingjubilee added a commit to workingjubilee/rustc that referenced this pull request Jun 17, 2025
@workingjubilee
Rollup merge of rust-lang#142570 - jieyouxu:disunion, r=estebank
0945b2b 
Reject union default field values

Fixes rust-lang#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
@workingjubilee workingjubilee mentioned this pull request Jun 17, 2025
Open
bors added a commit that referenced this pull request Jun 17, 2025
@bors
Auto merge of #142614 - workingjubilee:rollup-g5qf7y3, r=workingjubilee
1b4aa42 
Rollup of 10 pull requests

Successful merges:

 - #138538 (Make performance description of String::{insert,insert_str,remove} more precise)
 - #141946 (std: refactor explanation of `NonNull`)
 - #142216 (Miscellaneous RefCell cleanups)
 - #142371 (avoid `&mut P<T>` in `visit_expr` etc methods)
 - #142377 (Try unremapping compiler sources)
 - #142517 (Windows: Use anonymous pipes in Command)
 - #142542 (Manually invalidate caches in SimplifyCfg.)
 - #142563 (Refine run-make test ignores due to unpredictable `i686-pc-windows-gnu` unwind mechanism)
 - #142570 (Reject union default field values)
 - #142584 (Handle same-crate macro for borrowck semicolon suggestion)

r? `@ghost`
`@rustbot` modify labels: rollup
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Compiler should reject union field definitions having default values
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