fix const_item_mutation lint to use needs_drop instead of has_dtor by Rachit2323 · Pull Request #160097 · rust-lang/rust

Rachit2323 · 2026-07-28T17:33:38Z

Description:
The const_item_mutation lint was only checking if the outer type has a direct Drop impl (has_dtor). But if a field inside the type has a Drop impl, that drop logic can also observe the mutation — so the warning should be suppressed in that case too.

This fixes a false positive where the lint would warn on code like:

O.inner.val = 42;

even when Inner has a Drop impl that prints the value — meaning the mutation IS observable and the warning is wrong.

Fix: replace has_dtor check with needs_drop, which checks the whole type including all fields inside it.

rustbot · 2026-07-28T17:33:41Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-07-28T17:33:44Z

r? @jackh726

rustbot has assigned @jackh726.
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
compiler expanded to 74 candidates
Random selection from 17 candidates

rabindra789 · 2026-08-07T13:45:14Z

Nice fix! While testing this branch, I noticed one behaviour change. on the base commit this lint warns for traits associated consts whose type mentions Self, but on this branch it doesn't. Is that expected, or should this case still lint?

Rachit2323 · 2026-08-10T13:13:43Z

Thanks for catching this! It was unintended. Fixed it — for types containing type parameters like Self, needs_drop is too conservative so I now skip the suppression and let the lint fire as before.

rabindra789

Thanks for checking and fixing it! Glad we caught that regression.

View changes since this review

rustbot · 2026-08-22T15:33:31Z

This PR was rebased onto a different main 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.

jackh726 · 2026-08-22T20:07:57Z

If you can squash, r=me.

jackh726 · 2026-09-19T19:06:39Z

@bors r+

rust-bors · 2026-09-19T19:06:41Z

📌 Commit 940498d has been approved by jackh726

It is now in the queue for this repository.

…op, r=jackh726 fix const_item_mutation lint to use needs_drop instead of has_dtor Description: The const_item_mutation lint was only checking if the outer type has a direct Drop impl (has_dtor). But if a field inside the type has a Drop impl, that drop logic can also observe the mutation — so the warning should be suppressed in that case too. This fixes a false positive where the lint would warn on code like: ```O.inner.val = 42;``` even when Inner has a Drop impl that prints the value — meaning the mutation IS observable and the warning is wrong. Fix: replace has_dtor check with needs_drop, which checks the whole type including all fields inside it.

Rollup of 13 pull requests Successful merges: - #158515 (Make let-else respect macro_rules expr metavariable grouping) - #160097 (fix const_item_mutation lint to use needs_drop instead of has_dtor) - #161435 (Provide a `supertrait_def_ids()` function in rustc_type_ir's interner) - #161894 (Do not suppress the fn item uniqueness note for late bound lifetimes) - #162990 (post GH comment on types nominations) - #154665 (add safety section for mem::zeroed) - #162700 ( Remove incorrect parse error recovery code that mistakes `as` casts for the long removed type ascription) - #162705 (Trigger "C array" parse error recovery in far fewer cases) - #162875 (Add .seek_read_exact(), .seek_write_all() to std::os::windows::fs::FileExt) - #162988 (recover `true` and `false` in type position as `bool`) - #162995 (Constify `impl FromStr for NonZero<T>`) - #163006 (Use `end_point` for trailing brace in `let...else` diagnostics) - #163007 (add Dir::try_clone)

…op, r=jackh726 fix const_item_mutation lint to use needs_drop instead of has_dtor Description: The const_item_mutation lint was only checking if the outer type has a direct Drop impl (has_dtor). But if a field inside the type has a Drop impl, that drop logic can also observe the mutation — so the warning should be suppressed in that case too. This fixes a false positive where the lint would warn on code like: ```O.inner.val = 42;``` even when Inner has a Drop impl that prints the value — meaning the mutation IS observable and the warning is wrong. Fix: replace has_dtor check with needs_drop, which checks the whole type including all fields inside it.

…uwer Rollup of 15 pull requests Successful merges: - #158515 (Make let-else respect macro_rules expr metavariable grouping) - #162726 (std: fix unix socket address panic on a full sun_path) - #160028 (Better account for `Self` that might be a typo of `self`) - #160097 (fix const_item_mutation lint to use needs_drop instead of has_dtor) - #161435 (Provide a `supertrait_def_ids()` function in rustc_type_ir's interner) - #161894 (Do not suppress the fn item uniqueness note for late bound lifetimes) - #162990 (post GH comment on types nominations) - #154665 (add safety section for mem::zeroed) - #162700 ( Remove incorrect parse error recovery code that mistakes `as` casts for the long removed type ascription) - #162705 (Trigger "C array" parse error recovery in far fewer cases) - #162875 (Add .seek_read_exact(), .seek_write_all() to std::os::windows::fs::FileExt) - #162988 (recover `true` and `false` in type position as `bool`) - #162995 (Constify `impl FromStr for NonZero<T>`) - #163006 (Use `end_point` for trailing brace in `let...else` diagnostics) - #163007 (add Dir::try_clone)

…op, r=jackh726 fix const_item_mutation lint to use needs_drop instead of has_dtor Description: The const_item_mutation lint was only checking if the outer type has a direct Drop impl (has_dtor). But if a field inside the type has a Drop impl, that drop logic can also observe the mutation — so the warning should be suppressed in that case too. This fixes a false positive where the lint would warn on code like: ```O.inner.val = 42;``` even when Inner has a Drop impl that prints the value — meaning the mutation IS observable and the warning is wrong. Fix: replace has_dtor check with needs_drop, which checks the whole type including all fields inside it.

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. labels Jul 28, 2026

rustbot assigned jackh726 Jul 28, 2026

rabindra789 approved these changes Aug 10, 2026

View reviewed changes

jackh726 reviewed Aug 19, 2026

View reviewed changes

Comment thread compiler/rustc_mir_transform/src/check_const_item_mutation.rs Outdated

This comment has been minimized.

Sign in to view

Rachit2323 force-pushed the const-mutation-needs-drop branch from b47f970 to 4326ae2 Compare August 22, 2026 15:33

fix const_item_mutation lint to use needs_drop instead of has_dtor

940498d

Rachit2323 force-pushed the const-mutation-needs-drop branch from 4326ae2 to 940498d Compare August 24, 2026 06:55

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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 19, 2026

jhpratt mentioned this pull request Sep 19, 2026

Rollup of 13 pull requests #163035

Closed

JonathanBrouwer mentioned this pull request Sep 19, 2026

Rollup of 15 pull requests #163038

Closed

mu001999 mentioned this pull request Sep 20, 2026

Rollup of 17 pull requests #163043

Open

Navigation Menu

Uh oh!

fix const_item_mutation lint to use needs_drop instead of has_dtor - #160097

fix const_item_mutation lint to use needs_drop instead of has_dtor#160097
Rachit2323 wants to merge 1 commit into
rust-lang:mainfrom
Rachit2323:const-mutation-needs-drop

Rachit2323 commented Jul 28, 2026

Uh oh!

rustbot commented Jul 28, 2026

Uh oh!

rustbot commented Jul 28, 2026

Uh oh!

rabindra789 commented Aug 7, 2026

Uh oh!

Rachit2323 commented Aug 10, 2026

Uh oh!

rabindra789 left a comment •

edited by rustbot

Loading

Uh oh!

Uh oh!

This comment has been minimized.

rustbot commented Aug 22, 2026

Uh oh!

jackh726 commented Aug 22, 2026

Uh oh!

jackh726 commented Sep 19, 2026

Uh oh!

rust-bors Bot commented Sep 19, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Uh oh!

Conversation

Rachit2323 commented Jul 28, 2026

Uh oh!

rustbot commented Jul 28, 2026

Uh oh!

rustbot commented Jul 28, 2026

Uh oh!

rabindra789 commented Aug 7, 2026

Uh oh!

Rachit2323 commented Aug 10, 2026

Uh oh!

rabindra789 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!

This comment has been minimized.

rustbot commented Aug 22, 2026

Uh oh!

jackh726 commented Aug 22, 2026

Uh oh!

jackh726 commented Sep 19, 2026

Uh oh!

rust-bors Bot commented Sep 19, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

rabindra789 left a comment •

edited by rustbot

Loading