New lint: `manual_next_multiple_of` by qdot3 · Pull Request #17517 · rust-lang/rust-clippy

qdot3 · 2026-08-07T06:25:39Z

Implementation of #16728 and part of #14543.

This lint suggests (checked_)next_multiple_of instead of manual reimplementation. These two patterns are supported:

let _ = a.div_ceil(b) * b;
let _ = a + (b - a % b) % b;

Note that (a + b - 1) / b * b is for manual_bit_width.

changelog:

changelog: new lint: [`manual_next_multiple_of`]

rustbot · 2026-08-07T06:25:43Z

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. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

@rustbot author: the review is finished, PR author should check the comments and take action accordingly
@rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

CommanderStorm

community review:
I have a few comments on the code, but mostly nits (should not do them this much likely).

There are a few cases where I think there might be a missing comment why not in a few cases (or a false negative), but that should be easy to clear up for you.

Overall, pretty clean and well done.
@rustbot author

View changes since this review

CommanderStorm · 2026-08-09T17:02:44Z

+            _ => return,
+        };
+
+        let mut app = Applicability::MaybeIncorrect;


in what kind of cases might it be incorrect? 🤔

Have you done an lintcheck run?
What are the results/rationale here?

There is no way to detect no-op macro like this:

macro_rules! identity { ( $e:expr ) => { $e }; }

It leads to false positives because no-op macros must be modified in the futuer.

If macros are the only source of fails, can we not lint on these macros then?

False positives are not ideal.

False positives are not ideal.

I agree with you but they seem to be inevitable in this case. Please see #17472 (comment) for reference.

CommanderStorm · 2026-08-09T18:06:58Z

+    let (a, b, x) = if let Some((lhs, rhs)) = unpack_bin_op(lhs1, BinOpKind::Rem) {
+        (rhs1, rhs, lhs)
+    } else
+    // lhs = a
+    // rhs = x % b
+    if let Some((lhs, rhs)) = unpack_bin_op(rhs1, BinOpKind::Rem) {
+        (lhs1, rhs, lhs)
+    } else {
+        return None;
+    };


Is checked_rem not possible here?

There are two patterns worth considering:

if u2 > 0 { u1.div_ceil(u2) * u2 } else { /* do something */ }
The ideal suggestion depends on the else block. IMO, this might be too much.

u1.checked_next_power_of_two(u2).unwrap_or(x) if the else expression has no side effects

u1.checked_next_power_of_two(u2).unwrap_or_else(|| foo()) if the else expression has side effects

normal suggestion if the else block is complex

u1.checked_add((u2 - u1.checked_rem(u2)?).checked_rem(u2)?)
I can implement this, but it will take some time.

Are there any other patterns we should consider?

CommanderStorm · 2026-08-09T18:07:48Z

+    // x = b - a % b
+    if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)
+        && eq_expr_value(cx, expr.span.ctxt(), lhs, b)
+        && let Some((lhs, rhs)) = unpack_bin_op(rhs, BinOpKind::Rem)
+        && eq_expr_value(cx, expr.span.ctxt(), lhs, a)
+        && eq_expr_value(cx, expr.span.ctxt(), rhs, b)
+    {
+        Some((a, b))
+    } else {
+        None
+    }


is checked_sub and checked_rem not possible here?

Since a - b % a can never overflow, checked_sub should ideally be handled by a separate lint such as needless_checked_sub (which does not exist yet). In contrast, checked_rem should be considered.

CommanderStorm · 2026-08-09T18:17:37Z

+    // `v = 2^k - 1`
+    if integer_const(cx, c, expr.span.ctxt()).is_some_and(|v| v & v.wrapping_add(1) == 0) {
+        if eq_expr_value(cx, expr.span.ctxt(), b, c) {
+            Some((a, c))
+        } else if eq_expr_value(cx, expr.span.ctxt(), a, c) {
+            Some((b, c))
+        } else {
+            None
+        }


is this doing what it should? Are you here not testing if v + 1 == 0?

add: test cases

Co-authored-by: Frank Elsinga <frank@elsinga.de>

rustbot · 2026-08-10T12:19:54Z

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

Co-authored-by: Frank Elsinga <frank@elsinga.de>

qdot3 · 2026-08-11T14:14:58Z

@rustbot review

CommanderStorm

community review:
LGTM, lets add the comments from the discussions as code comments, since this might not be clear for future readers.

Also, don't forget to sure to squash your changes into one commit 😉

View changes since this review

CommanderStorm · 2026-08-11T19:32:08Z

+        };
+
+        // x = b - a % b
+        if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)


lets document #17517 (comment)

Suggested change

if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)

// Since a - b % a can never overflow, checked_sub is not hanled and intentionally

if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)

CommanderStorm · 2026-08-11T19:38:14Z

+            Some(Self::DivCeil { a, b })
+        } else
+        // lhs = b
+        // rhs = a.div_ceil(b)


Suggested change

// rhs = a.div_ceil(b)

// rhs = a.div_ceil(b)

// no handling of checked_div_ceil, because does not exist

CommanderStorm · 2026-08-11T19:49:26Z

+            $e
+        };
+    }
+    // This is an inevitable false positive


actually document the reason 😉
Future readers will thank you why this is an "inevitable" false positive

rustbot · 2026-08-11T23:28:34Z

☔ The latest upstream changes (possibly #17538) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added the S-waiting-on-community-reviews Status: This is awaiting for positive reviews from the community before a maintainer is assigned. label Aug 7, 2026

rustbot added needs-fcp PRs that add, remove, or rename lints and need an FCP S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Aug 7, 2026

This comment has been minimized.

Sign in to view

CommanderStorm suggested changes Aug 9, 2026

View reviewed changes

qdot3 marked this pull request as draft August 10, 2026 04:49

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Aug 10, 2026

qdot3 and others added 15 commits August 10, 2026 14:51

add: test cases

909f4e4

add: test cases

e8ebbb3

typo

ffb78ec

add: lint

e91cd70

add: test cases

refactor

19a0646

add lint doc & refactor

cf991d3

support (a + b) & !b where b = 2^k - 1

7be3bbd

support a.div_ceil(b).checked_mul(b)

7e937f0

support checked ops

97292ab

lower applicability due to transparent no-op macro

1cd247e

minor fixes

6fbe31d

fix typo

50f30d6

Co-authored-by: Frank Elsinga <frank@elsinga.de>

add link to issue

24fb618

Co-authored-by: Frank Elsinga <frank@elsinga.de>

change a comment to a doc string

e80800e

Co-authored-by: Frank Elsinga <frank@elsinga.de>

refactor

58e12e2

qdot3 force-pushed the wip_manual_next_multiple_of branch from 6193d70 to 58e12e2 Compare August 10, 2026 05:55

qdot3 marked this pull request as ready for review August 10, 2026 08:42

qdot3 marked this pull request as draft August 11, 2026 03:56

qdot3 and others added 5 commits August 11, 2026 12:56

fix typo

aef194d

Co-authored-by: Frank Elsinga <frank@elsinga.de>

refine tests

f1bcf91

refactor

abcca2b

fix special power_of_two patterns

bf1261f

minor fix

ea58380

qdot3 marked this pull request as ready for review August 11, 2026 11:33

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Aug 11, 2026

qdot3 added 2 commits August 11, 2026 22:38

fix suggestion of power_of_two pattern

62e5027

support more checked_ops

e2c5501

qdot3 marked this pull request as draft August 11, 2026 14:00

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Aug 11, 2026

minor fixes

82e3cdc

qdot3 marked this pull request as ready for review August 11, 2026 14:14

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 Aug 11, 2026

CommanderStorm approved these changes Aug 11, 2026

View reviewed changes

	if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)
	// Since a - b % a can never overflow, checked_sub is not hanled and intentionally
	if let Some((lhs, rhs)) = unpack_bin_op(x, BinOpKind::Sub)

	// rhs = a.div_ceil(b)
	// rhs = a.div_ceil(b)
	// no handling of checked_div_ceil, because does not exist

Uh oh!

Uh oh!

Conversation

qdot3 commented Aug 7, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Aug 7, 2026

Uh oh!

This comment has been minimized.

CommanderStorm left a comment • edited 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!

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!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

qdot3 Aug 11, 2026 • edited 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!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

rustbot commented Aug 10, 2026

Uh oh!

qdot3 commented Aug 11, 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!

rustbot commented Aug 11, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

qdot3 commented Aug 7, 2026 •

edited by rustbot

Loading

CommanderStorm left a comment •

edited

Loading

qdot3 Aug 11, 2026 •

edited

Loading

CommanderStorm left a comment •

edited by rustbot

Loading