New lint: `missing_must_use` by Kokoro2336 · Pull Request #16919 · rust-lang/rust-clippy

Kokoro2336 · 2026-04-27T08:48:54Z

changelog: New lint: missing_must_use

checklist:

Followed lint naming conventions
Added passing UI tests (including committed .stderr file)
cargo test passes locally
Executed cargo dev update_lints
Added lint documentation
Run cargo dev fmt

github-actions · 2026-04-27T10:43:55Z

Lintcheck changes for c8f3e8f

Lint	Added	Removed	Changed
`clippy::missing_must_use`	4412	0	0

This comment will be updated if you push new changes

rustbot · 2026-04-27T11:59:38Z

r? @llogiq

rustbot has assigned @llogiq.
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: 7 candidates
7 candidates expanded to 7 candidates
Random selection from Jarcho, dswij, llogiq

Kokoro2336 · 2026-06-03T04:30:42Z

@LebedevRI I've added support for Trait/Impl/Fn, with testcases supplied.

rustbot · 2026-06-03T06:41:08Z

Some changes occurred in clippy_lints/src/doc

cc @notriddle

Kokoro2336 · 2026-06-03T06:44:29Z

Oh no, it seems that there're some problems in rebasing. I'll fix it later.

…bute case (rust-lang#17144) changelog: [`double_must_use`]: make the lint machine-applicable in single-attribute case After rust-lang#16919, not having this fix-it will be unfortunate, since that lint will cause many new instances of this diag. Refs rust-lang#16856

LebedevRI · 2026-06-03T20:04:16Z

The PR description should also Close #16859,
at first glance i hadn't realized this wasn't just handling types, but functions too. Nice!

Looking at clippy_lints/src/functions/must_use.rs,
i notice that there are some ignored cases:

proc macros
#[no_mangle] functions

I would suggest having tests for those cases.

I would also suggest adding tests with macros (especially external ones),
there may be unhandled situations there.

rustbot · 2026-06-28T11:05:17Z

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.

Kokoro2336 · 2026-06-28T11:28:24Z

@LebedevRI I've fixed the rebasing

llogiq

This is a good start, but I think the lint is way too trigger happy. I left some suggestions to reduce the churn.

View changes since this review

rustbot · 2026-06-29T06:41:02Z

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

refactor: renaming. chore: cargo dev fmt chore: cargo dev update_lints feat: add help. fix: ignore external macro. test: add test. doc: update documentation. fix: lint message format. enhance: add suggestion. fix: lint Impl/Trait/Fn.

Kokoro2336 · 2026-06-29T14:54:49Z

@llogiq Done @rustbot ready

LebedevRI · 2026-06-29T15:18:00Z

+        // Check if the item is public. If it's not public, we don't need to check it for `#[must_use]`.
+        if !cx.tcx.visibility(item.owner_id).is_public() {
+            return;
+        }
+
+        match item.kind {
+            ItemKind::Struct(_, _, data) if !data.fields().is_empty() => check_item(cx, item.hir_id(), item.span),
+            ItemKind::Enum(_, _, def) if !def.variants.is_empty() => check_item(cx, item.hir_id(), item.span),
+            ItemKind::Union(..) => check_item(cx, item.hir_id(), item.span),


FWIW, this doesn't solve #16858.
I've very explicitly noted that it should diagnose everything.
It's fine if such a behaviour can be toggled off (on?) via config, but it should be possible.

View changes since the review

Just so I understand your use case better, why do you want the lint to mark every type?

Thank you for taking a look!

I do think this check is an improvement even if it only diagnoses non-empty public types,
however, i really do feel like such dichotomy should be configurable,
since then it obviously serves more use-cases that way.

We can of course argue about the defaults,
i'd think it should be more noisy by default, since it's easier to opt-out of something
that you know is there than opt-in into something you don't nessesairly know about.
And it's a restrict check anyways.

Likewise, i've intentionally filed #16858 and #16859 as a separate issues,
so i don't nessesairly think that this check should deal with the fns,
only type/trait definitions.

But to answer the question, mainly, i'm simply of an opinion that Rust got this default wrong,
must-use-ness should have been the other way around.
How often does one have fns that return something that is always discarded at all callsites?
I think that is not a very common pattern...

While I personally like must-use (which is why I wrote the must_use_candidate lint), I disagree about your assertion that the default is wrong. I believe that defaulting to must-use would have led to the false sense of security with regards to the running of drop impls.

Given that those annotations carry a cost, even if it is just one more thing to ignore while reading the code, we should aim to make the most of them. Therefore my aim was to restrict the lint to cases that matter. That is why I asked for those changes. We can always make the lint more configurable, but the default should carefully balance costs and benefits.

I don't disagree.

Given that those annotations carry a cost, even if it is just one more thing to ignore while reading the code, we should aim to make the most of them. Therefore my aim was to restrict the lint to cases that matter. That is why I asked for those changes. We can always make the lint more configurable, but the default should carefully balance costs and benefits.

Note: in previous comment, by opt-out of, i was of course talking about the config option. Of course, silencing each particular lint site would be a problematic solution, given that they'd all have the same root-cause (=empty type, etc)

Just please don't make the current behaviour (no diag) non-configurable.

I would say, discovering that half of a lint is off by default and is hidden
behing a config option is a bit hard to discover, though.
But i don't suppose there's much appetite for issuing those edge-cases
under their own lint names either?
(public-sized; public-empty, non-public-sized, non-public-empty)

rustbot · 2026-07-06T07:11:31Z

☔ The latest upstream changes (possibly #17329) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added the needs-fcp PRs that add, remove, or rename lints and need an FCP label Apr 27, 2026

Kokoro2336 changed the title ~~[type_must_use]: new lint~~ [missing_must_use]: new lint Apr 27, 2026

Kokoro2336 changed the title ~~[missing_must_use]: new lint~~ [missing_must_use]: new restriction-type lint Apr 27, 2026

Kokoro2336 changed the title ~~[missing_must_use]: new restriction-type lint~~ New lint: missing_must_use Apr 27, 2026

Kokoro2336 marked this pull request as ready for review April 27, 2026 11:59

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Apr 27, 2026

rustbot assigned llogiq Apr 27, 2026

This comment has been minimized.

Sign in to view

LebedevRI reviewed Jun 2, 2026

View reviewed changes

Comment thread clippy_lints/src/missing_must_use.rs Outdated

LebedevRI mentioned this pull request Jun 3, 2026

[double_must_use]: make the lint machine-applicable in single-attribute case #17144

Merged

Kokoro2336 force-pushed the feat/type-must-use branch from c8e2a9e to 79cdff5 Compare June 3, 2026 04:29

This comment has been minimized.

Sign in to view

rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jun 3, 2026

Kokoro2336 force-pushed the feat/type-must-use branch from 045bdb1 to 91fc83e Compare June 3, 2026 06:41

This comment has been minimized.

Sign in to view

rustbot removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) has-merge-commits PR has merge commits, merge with caution. labels Jun 3, 2026

This comment has been minimized.

Sign in to view

Kokoro2336 force-pushed the feat/type-must-use branch from 91fc83e to 4b239de Compare June 28, 2026 11:05

Kokoro2336 force-pushed the feat/type-must-use branch from 4b239de to 933f21d Compare June 28, 2026 11:24

llogiq requested changes Jun 29, 2026

View reviewed changes

Comment thread clippy_lints/src/missing_must_use.rs Outdated

Comment thread clippy_lints/src/missing_must_use.rs Outdated

Comment thread tests/ui/missing_must_use.rs

Comment thread tests/ui/missing_must_use.rs

Comment thread tests/ui/missing_must_use.rs

rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jun 29, 2026

Kokoro2336 force-pushed the feat/type-must-use branch 2 times, most recently from f231267 to e14357c Compare June 29, 2026 14:42

feat: add TypeMustUse lint.

c8f3e8f

refactor: renaming. chore: cargo dev fmt chore: cargo dev update_lints feat: add help. fix: ignore external macro. test: add test. doc: update documentation. fix: lint message format. enhance: add suggestion. fix: lint Impl/Trait/Fn.

Kokoro2336 force-pushed the feat/type-must-use branch from e14357c to c8f3e8f Compare June 29, 2026 14:49

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties and removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jun 29, 2026

Kokoro2336 requested a review from llogiq June 29, 2026 14:59

LebedevRI reviewed Jun 29, 2026

View reviewed changes

Uh oh!

Uh oh!

Conversation

Kokoro2336 commented Apr 27, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actions Bot commented Apr 27, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Apr 27, 2026

Uh oh!

This comment has been minimized.

Uh oh!

This comment has been minimized.

Kokoro2336 commented Jun 3, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment has been minimized.

This comment has been minimized.

rustbot commented Jun 3, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

Kokoro2336 commented Jun 3, 2026

Uh oh!

LebedevRI commented Jun 3, 2026

Uh oh!

rustbot commented Jun 28, 2026

Uh oh!

Kokoro2336 commented Jun 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

llogiq 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!

Uh oh!

Uh oh!

rustbot commented Jun 29, 2026

Uh oh!

Kokoro2336 commented Jun 29, 2026

Uh oh!

LebedevRI Jun 29, 2026 • 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!

llogiq Jul 1, 2026

Choose a reason for hiding this comment

Uh oh!

LebedevRI Jul 1, 2026

Choose a reason for hiding this comment

Uh oh!

llogiq Jul 2, 2026

Choose a reason for hiding this comment

Uh oh!

LebedevRI Jul 2, 2026

Choose a reason for hiding this comment

Uh oh!

rustbot commented Jul 6, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Kokoro2336 commented Apr 27, 2026 •

edited by rustbot

Loading

github-actions Bot commented Apr 27, 2026 •

edited

Loading

Kokoro2336 commented Jun 3, 2026 •

edited

Loading

Kokoro2336 commented Jun 28, 2026 •

edited

Loading

llogiq left a comment •

edited by rustbot

Loading

LebedevRI Jun 29, 2026 •

edited by rustbot

Loading