Fix borrowed_box suggestion for trait objects to mention .as_ref() at call sites by AbhilashG12 · Pull Request #17794 · rust-lang/rust-clippy

AbhilashG12 · 2026-09-26T16:23:28Z

The lint suggested changing &Box to &dyn Trait, but
this breaks compilation at call sites that pass &boxed_value because
the coercion &Box -> &dyn Trait does not happen implicitly.
Extend the message for trait objects to tell users to call .as_ref()
at call sites.

changelog: [borrowed_box]: suggest calling .as_ref() at the call site when the boxed type is a trait object

Fixes #11940

rustbot · 2026-09-26T16:23:32Z

Thanks for the pull request, and welcome!

You should hear from one of our reviewers after this PR gets at least 2 reviews from the community.

Please see the contribution instructions for more information.

rustbot · 2026-09-26T17:05:50Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

CommanderStorm

Community review: there are a few things I found, none that are hard to rectify though, mostly minor stuff

View changes since this review

CommanderStorm · 2026-09-27T06:35:25Z

 LL | pub fn test14(_display: &Box<dyn Display>) {}
   |                         ^^^^^^^^^^^^^^^^^ help: consider using just `&T`: `&dyn Display`
+   |
+   = help: call `.as_ref()` at the call site if it's a `&Box<dyn Trait>`


🤔
why limit this help to traits?
I think if we write the diagnostic like this, it should be generally applicable, or?
I may be missing something xD

Suggested change

= help: call `.as_ref()` at the call site if it's a `&Box<dyn Trait>`

= help: call `.as_ref()` at the call site to convert a `&Box<dyn Display>` to a `&dyn Display`

Good point on the wording. Kept it trait-object-only because sized types like &Box already deref-coerce to &bool automatically, so the note would be misleading there. But you were right that &Box was vague — I've now interpolated the actual type so it reads e.g. call .as_ref()at the call site to convert a&Boxto a&dyn Display``.

CommanderStorm · 2026-09-27T06:35:50Z

                diag.span_suggestion(
                    hir_ty.span,
                    "consider using just `&T`",
                    suggestion,


lets keep the reaon for non-applicability please 😉
Otherwise it is kind of hard on the next person why this is unspecified..

Restored the original comment. Thanks for catching that.

CommanderStorm · 2026-09-27T06:39:20Z

+
+// Test for issue #11940: borrowed_box on boxed trait object should suggest .as_ref()
+pub trait DayTrait {
+    fn get_display(&self) -> String;
+}
+
+pub fn test_borrowed_box_trait_object(day: &Box<dyn DayTrait>) {
+    //~^ borrowed_box
+    let _ = day.get_display();
+}


I think we are already covering the diagnostic in above dyn Display, or what would you like to test here?

Suggested change

// Test for issue #11940: borrowed_box on boxed trait object should suggest .as_ref()

pub trait DayTrait {

fn get_display(&self) -> String;

}

pub fn test_borrowed_box_trait_object(day: &Box<dyn DayTrait>) {

//~^ borrowed_box

let _ = day.get_display();

}

You're right, test14 already covers this path. Removed the extra test.

CommanderStorm · 2026-09-27T06:41:05Z

please remove the changes to CHANGELOG.md, as we do this via the changelog: in the PR description and then centrally once per release ^^

Reverted. Good to know it's handled centrally from the PR description.

CommanderStorm

Community review: LGTM, please make sure to squash your changes

View changes since this review

CommanderStorm

^- was meant as an +1

View changes since this review

… call sites The lint suggested changing \&Box<dyn Trait>\ to \&dyn Trait\, but this breaks compilation at call sites that pass \&boxed_value\ because the coercion from \&Box<dyn Trait>\ to \&dyn Trait\ does not happen implicitly. Extend the help for trait objects to tell users to call \.as_ref()\ at call sites. Signed-off-by: AbhilashG12 <abhilashggg15@gmail.com>

AbhilashG12 · 2026-09-28T05:09:43Z

Squashed into one commit.. Thanks for the review.

AbhilashG12

Squashed

View changes since this review

rustbot added S-waiting-on-community-reviews Status: This is awaiting for positive reviews from the community before a maintainer is assigned. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Sep 26, 2026

This comment has been minimized.

Sign in to view

AbhilashG12 force-pushed the fix-borrowed-box-trait-object-suggestion branch from 799a550 to ac1ee8a Compare September 26, 2026 17:05

CommanderStorm suggested changes Sep 27, 2026

View reviewed changes

CommanderStorm reviewed Sep 27, 2026

View reviewed changes

CommanderStorm approved these changes Sep 27, 2026

View reviewed changes

AbhilashG12 force-pushed the fix-borrowed-box-trait-object-suggestion branch from c2a8424 to 58ab68c Compare September 28, 2026 04:54

This comment has been minimized.

Sign in to view

AbhilashG12 force-pushed the fix-borrowed-box-trait-object-suggestion branch from 58ab68c to 90a441e Compare September 28, 2026 05:00

AbhilashG12 commented Sep 30, 2026

View reviewed changes

	= help: call `.as_ref()` at the call site if it's a `&Box<dyn Trait>`
	= help: call `.as_ref()` at the call site to convert a `&Box<dyn Display>` to a `&dyn Display`

Uh oh!

Conversation

AbhilashG12 commented Sep 26, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Sep 26, 2026

Uh oh!

This comment has been minimized.

rustbot commented Sep 26, 2026

Uh oh!

CommanderStorm 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!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

CommanderStorm 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!

CommanderStorm 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!

This comment has been minimized.

AbhilashG12 commented Sep 28, 2026

Uh oh!

AbhilashG12 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!

This branch has not been deployed

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

AbhilashG12 commented Sep 26, 2026 •

edited by rustbot

Loading

CommanderStorm left a comment •

edited by rustbot

Loading

CommanderStorm left a comment •

edited by rustbot

Loading

CommanderStorm left a comment •

edited by rustbot

Loading

AbhilashG12 left a comment •

edited by rustbot

Loading