don't suggest non-deriveable traits for unions#154118

malezjaa · 2026-03-19T22:26:15Z

Before, the compiler suggested adding #[derive(Debug)] (other traits too) for unions,
which is misleading because some traits can't be automatically derived.

Only traits that are still suggested are Copy and Clone.

I noticed the error label changed after removing the suggestion. I hope this isn't a big deal, but let me know if that's an issue.

original example:

union Union {
    member: usize,
}

impl PartialEq<u8> for Union {
    fn eq(&self, rhs: &u8) -> bool {
        unsafe { self.member == (*rhs).into() }
    }
}

fn main() {
    assert_eq!(Union { member: 0 }, 0);
}

before:

error[E0277]: `Union` doesn't implement `Debug`                                                                       
  --> src\main.rs:13:5
   |
13 |     assert_eq!(Union { member: 0 }, 0);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `Union`
   |
   = note: add `#[derive(Debug)]` to `Union` or manually `impl Debug for Union`
   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider annotating `Union` with `#[derive(Debug)]`
   |
 2 + #[derive(Debug)]
 3 | union Union {
   |

after (the message doesn't suggest adding #[derive(Debug)] to unions)

error[E0277]: `Union` doesn't implement `Debug`                                                                       
  --> src\main.rs:13:5
   |
13 |     assert_eq!(Union { member: 0 }, 0);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound
   |
help: the trait `Debug` is not implemented for `Union`
  --> src\main.rs:2:1
   |
 2 | union Union {
   | ^^^^^^^^^^^
   = note: manually `impl Debug for Union`

rustbot · 2026-03-19T22:26:18Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-03-19T22:26:20Z

r? @dingxiangfei2009

rustbot has assigned @dingxiangfei2009.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler, types
compiler, types expanded to 69 candidates
Random selection from 14 candidates

Kivooeo · 2026-03-20T01:03:16Z

Can you please add a test to reflect new behaviour

library/core/src/fmt/mod.rs

compiler/rustc_attr_parsing/src/attributes/diagnostic/mod.rs

compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

compiler/rustc_attr_parsing/src/attributes/diagnostic/mod.rs

rustbot · 2026-03-20T10:06:25Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

JonathanBrouwer · 2026-03-20T15:49:09Z

cc @mejrs
if you feel like it, could you also take a look?

mejrs

The changes to on_unimplemented look good. I have some suggestions.

Please also update this test

rust/tests/ui/diagnostic_namespace/on_unimplemented/do_not_accept_options_of_the_internal_rustc_attribute.rs

Lines 28 to 43 in bfc05d6

    
           #[diagnostic::on_unimplemented( 
        
               message = "{from_desugaring}{direct}{cause}{integral}{integer}", 
        
               //~^WARN there is no parameter `from_desugaring` on trait `Baz` 
        
               //~|WARN there is no parameter `direct` on trait `Baz` 
        
               //~|WARN there is no parameter `cause` on trait `Baz` 
        
               //~|WARN there is no parameter `integral` on trait `Baz` 
        
               //~|WARN there is no parameter `integer` on trait `Baz` 
        
               label = "{float}{_Self}{crate_local}{Trait}{ItemContext}{This}" 
        
               //~^WARN there is no parameter `float` on trait `Baz` 
        
               //~|WARN there is no parameter `_Self` on trait `Baz` 
        
               //~|WARN there is no parameter `crate_local` on trait `Baz` 
        
               //~|WARN there is no parameter `Trait` on trait `Baz` 
        
               //~|WARN there is no parameter `ItemContext` on trait `Baz` 
        
               //~|WARN there is no parameter `This` on trait `Baz` 
        
           )] 
        
           trait Baz {}

View changes since this review

tests/ui/union/union-no-derive-suggestion.rs

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

malezjaa · 2026-03-20T20:44:45Z

I forgot to rebless the test. Will do that in a sec

malezjaa · 2026-03-20T20:57:17Z

I updated this test (do_not_accept_options_of_the_internal_rustc_attribute.rs). The diagnostic uses raw identifier for struct and enum, but displays union normally. Is that expected?

mejrs · 2026-03-20T21:54:45Z

I updated this test (do_not_accept_options_of_the_internal_rustc_attribute.rs). The diagnostic uses raw identifier for struct and enum, but displays union normally. Is that expected?

Interesting. I hadn't realized it does this with keywords. It does that with other keywords too.

#[diagnostic::on_unimplemented(message = "{impl}")]
trait Baz{}

warning: there is no parameter `r#impl` on trait `Baz`

Do note that union is a weak keyword; that's probably why it doesn't do it for union.

It at least is consistent:

fn main(){
    println!("{impl} {union}")
}

error[E0425]: cannot find value `r#impl` in this scope
 --> src/main.rs:2:16
  |
2 |     println!("{impl} {union}")
  |                ^^^^ not found in this scope

error[E0425]: cannot find value `union` in this scope
 --> src/main.rs:2:23
  |
2 |     println!("{impl} {union}")
  |                       ^^^^^ not found in this scope

so I suppose we should leave it this way.

library/core/src/fmt/mod.rs

Don't suggest non-deriveable traits for unions. This also adds enum, struct and union markers to rustc_on_unimplemented

JonathanBrouwer · 2026-03-20T23:24:04Z

It is a bit weird the r# prefix is there, it could be intended but not sure, but regardless out of scope for this PR.

JonathanBrouwer · 2026-03-20T23:29:28Z

@bors r+ rollup

rust-bors · 2026-03-20T23:29:31Z

📌 Commit 27212bb has been approved by JonathanBrouwer

It is now in the queue for this repository.

…for-unions, r=JonathanBrouwer don't suggest non-deriveable traits for unions cc rust-lang#137587 Before, the compiler suggested adding `#[derive(Debug)]` (other traits too) for unions, which is misleading because some traits can't be automatically derived. Only traits that are still suggested are Copy and Clone. I noticed the error label changed after removing the suggestion. I hope this isn't a big deal, but let me know if that's an issue. original example: ```rs union Union { member: usize, } impl PartialEq<u8> for Union { fn eq(&self, rhs: &u8) -> bool { unsafe { self.member == (*rhs).into() } } } fn main() { assert_eq!(Union { member: 0 }, 0); } ``` before: ``` error[E0277]: `Union` doesn't implement `Debug` --> src\main.rs:13:5 | 13 | assert_eq!(Union { member: 0 }, 0); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `Union` | = note: add `#[derive(Debug)]` to `Union` or manually `impl Debug for Union` = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info) help: consider annotating `Union` with `#[derive(Debug)]` | 2 + #[derive(Debug)] 3 | union Union { | ``` after (the message doesn't suggest adding #[derive(Debug)] to unions) ``` error[E0277]: `Union` doesn't implement `Debug` --> src\main.rs:13:5 | 13 | assert_eq!(Union { member: 0 }, 0); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound | help: the trait `Debug` is not implemented for `Union` --> src\main.rs:2:1 | 2 | union Union { | ^^^^^^^^^^^ = note: manually `impl Debug for Union` ```

JonathanBrouwer · 2026-03-20T23:35:20Z

@malezjaa For in the future, if you put "Fixes #..." rather than "CC #..." it will automatically close the issue :)
I fixed it in the PR description for this PR

malezjaa · 2026-03-20T23:43:25Z

Thank you both for the help with this PR :D

…uwer Rollup of 6 pull requests Successful merges: - #154154 (Emit fewer errors for incorrect rtn and `=` -> `:` typos in bindings) - #154155 (tests/ui/async-await/drop-option-future.rs: New regression test) - #146961 (Allow passing `expr` metavariable as `cfg` predicate) - #154118 (don't suggest non-deriveable traits for unions) - #154120 (Start migrating `DecorateDiagCompat::Builtin` items to `DecorateDiagCompat::Dynamic`) - #154156 (Moving issue-52049 to borrowck)

rustbot assigned dingxiangfei2009 Mar 19, 2026

malezjaa changed the title ~~don't suggest non-deriveable for unions~~ don't suggest non-deriveable traits for unions Mar 19, 2026

malezjaa force-pushed the dont-suggest-some-traits-for-unions branch from 8fd2da8 to ccaae21 Compare March 20, 2026 08:24

JonathanBrouwer assigned JonathanBrouwer and unassigned dingxiangfei2009 Mar 20, 2026

JonathanBrouwer requested changes Mar 20, 2026

View reviewed changes

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 20, 2026

malezjaa force-pushed the dont-suggest-some-traits-for-unions branch from ccaae21 to dd844f3 Compare March 20, 2026 20:03

mejrs approved these changes Mar 20, 2026

View reviewed changes

tests/ui/union/union-no-derive-suggestion.rs Show resolved Hide resolved

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs Outdated Show resolved Hide resolved

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs Outdated Show resolved Hide resolved

This comment has been minimized.

Sign in to view

malezjaa force-pushed the dont-suggest-some-traits-for-unions branch from dd844f3 to 927e225 Compare March 20, 2026 21:19

mejrs reviewed Mar 20, 2026

View reviewed changes

library/core/src/fmt/mod.rs Outdated Show resolved Hide resolved

Don't suggest non-deriveable traits for unions.

27212bb

Don't suggest non-deriveable traits for unions. This also adds enum, struct and union markers to rustc_on_unimplemented

malezjaa force-pushed the dont-suggest-some-traits-for-unions branch from 927e225 to 27212bb Compare March 20, 2026 22:39

JonathanBrouwer approved these changes Mar 20, 2026

View reviewed changes

rust-bors bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 20, 2026

JonathanBrouwer mentioned this pull request Mar 20, 2026

Rollup of 5 pull requests #154159

Closed

JonathanBrouwer mentioned this pull request Mar 20, 2026

Rollup of 6 pull requests #154160

Merged

malezjaa mentioned this pull request Mar 21, 2026

new rustc_on_unimplemented match values rust-lang/rustc-dev-guide#2801

Merged

rust-bors bot merged commit 70d1c58 into rust-lang:main Mar 21, 2026
11 checks passed

rustbot added this to the 1.96.0 milestone Mar 21, 2026

JonathanBrouwer mentioned this pull request Mar 21, 2026

drop derive helpers during attribute parsing #153540

Merged

	#[diagnostic::on_unimplemented(
	message = "{from_desugaring}{direct}{cause}{integral}{integer}",
	//~^WARN there is no parameter `from_desugaring` on trait `Baz`
	//~\|WARN there is no parameter `direct` on trait `Baz`
	//~\|WARN there is no parameter `cause` on trait `Baz`
	//~\|WARN there is no parameter `integral` on trait `Baz`
	//~\|WARN there is no parameter `integer` on trait `Baz`
	label = "{float}{_Self}{crate_local}{Trait}{ItemContext}{This}"
	//~^WARN there is no parameter `float` on trait `Baz`
	//~\|WARN there is no parameter `_Self` on trait `Baz`
	//~\|WARN there is no parameter `crate_local` on trait `Baz`
	//~\|WARN there is no parameter `Trait` on trait `Baz`
	//~\|WARN there is no parameter `ItemContext` on trait `Baz`
	//~\|WARN there is no parameter `This` on trait `Baz`
	)]
	trait Baz {}

Uh oh!

Conversation

malezjaa commented Mar 19, 2026 • edited by JonathanBrouwer Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Mar 19, 2026

Uh oh!

rustbot commented Mar 19, 2026

Uh oh!

Kivooeo commented Mar 20, 2026

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

rustbot commented Mar 20, 2026

Uh oh!

JonathanBrouwer commented Mar 20, 2026

Uh oh!

mejrs left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

This comment has been minimized.

malezjaa commented Mar 20, 2026

Uh oh!

malezjaa commented Mar 20, 2026

Uh oh!

mejrs commented Mar 20, 2026

Uh oh!

Uh oh!

JonathanBrouwer commented Mar 20, 2026

Uh oh!

JonathanBrouwer commented Mar 20, 2026

Uh oh!

rust-bors bot commented Mar 20, 2026

Uh oh!

JonathanBrouwer commented Mar 20, 2026

Uh oh!

malezjaa commented Mar 20, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

malezjaa commented Mar 19, 2026 •

edited by JonathanBrouwer

Loading

mejrs left a comment •

edited by rustbot

Loading