Add unnecessary_trailing_comma lint #16530

nyurik · 2026-02-07T08:30:52Z

This matches cargo fmt behavior but for format!(...) -style macro calls.

To make this PR easier to review, it moves all linting of the clippy code to #16533

Suggest removing an unnecessary trailing comma before the closing parenthesis in single-line format-like macro invocations (e.g. println!, format!, write!). The lint currently only runs on format-like macros because it relies on format-argument parsing; arbitrary user macros are not supported to avoid incorrect suggestions.

Lint is in the style group (allow-by-default)
Single-line only: multi-line macro invocations are not linted
Machine-applicable fix: removes the trailing comma

// catches this
debug!("Foo={}", 1,);

// suggests this
debug!("Foo={}", 1);

Closes #13965

changelog: [unnecessary_trailing_comma]: new lint to remove trailing commas in a single-line format macro usage

rustbot · 2026-02-07T08:30:57Z

r? @Jarcho

rustbot has assigned @Jarcho.
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, samueltardieu

github-actions · 2026-02-07T08:47:00Z

Lintcheck changes for d6d1b45

Lint	Added	Removed	Changed
`clippy::unnecessary_trailing_comma`	17	0	0

This comment will be updated if you push new changes

rustbot · 2026-02-07T09:07:09Z

Some changes occurred in clippy_lints/src/doc

cc @notriddle

notriddle · 2026-02-07T17:01:46Z

clippy_lints/src/format_args.rs

+        let sm = self.cx.sess().source_map();
+        let span = self.macro_call.span.source_callsite();
+        if !sm.is_multiline(span)
+            && let span = sm.span_extend_to_prev_char_before(span.shrink_to_hi(), ')', false)


This means println!["hi",], which is allowed, won't lint.

Thx, great catch, totally forgot!!

thx @notriddle , fixed

This still doesn't work right in all cases. This test case gives an incorrect "correction":

println!["Foo(,)", ]; //~ unnecessary_trailing_comma

@notriddle thx for catching all the edge cases! It took me a bit longer than anticipated. Seem to work now with all the new test cases

samueltardieu · 2026-02-08T08:53:08Z

The style category is warn-by-default. There is a risk that this lint is too opinionated for a warn-by-default category. Also, we don't usually lint for formatting as this is handled by rustfmt.

nyurik · 2026-02-08T09:15:33Z

@samueltardieu this lint is essentially the missing piece for a single line macro calls that cargo fmt already does for regular function calls. I don't think anyone would ever want to have println!("...",) -- most of these are simply accidental leftovers for when multiline formats get reformatted as a single line. The only reason cargo fmt cannot do it is that it requires type resolution to ensure correct detection. Note that --fix should work perfectly for this lint. I honestly think this is a "style" as anything could possibly get.

Jarcho · 2026-02-09T19:04:13Z

FCP started: https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/FCP.3A.20.60unnecessary_trailing_comma.60/with/572863765

…g#16533) This noop PR makes the new comma-removing lint PR less noisy, allowing it focus only on the new functionality See rust-lang#16530 changelog: none

…n, r=mati865 style: remove unneeded trailing commas Make format-like macro calls look similar to what `cargo fmt` does automatically - remove trailing commas. When removing a comma, I also inlined some variables for consistency and clarity. I'm working on a [clippy lint](rust-lang/rust-clippy#16530) to make this process automatic.

Suggest removing an unnecessary trailing comma before the closing parenthesis in single-line format-like macro invocations (e.g. println!, format!, write!). The lint currently only runs on format-like macros because it relies on format-argument parsing; arbitrary user macros are not supported to avoid incorrect suggestions. - Lint is in the `style` group (allow-by-default) - Single-line only: multi-line macro invocations are not linted - Machine-applicable fix: removes the trailing comma

rustbot · 2026-02-11T20:44:15Z

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.

Rollup merge of #152372 - nyurik:clippy-rustc_trait_selection, r=mati865 style: remove unneeded trailing commas Make format-like macro calls look similar to what `cargo fmt` does automatically - remove trailing commas. When removing a comma, I also inlined some variables for consistency and clarity. I'm working on a [clippy lint](rust-lang/rust-clippy#16530) to make this process automatic.

rustbot assigned Jarcho Feb 7, 2026

rustbot added needs-fcp S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Feb 7, 2026

This comment has been minimized.

Sign in to view

nyurik force-pushed the unnecessary_trailing_comma branch from 46ad29b to 0f7b216 Compare February 7, 2026 08:36

This comment has been minimized.

Sign in to view

nyurik force-pushed the unnecessary_trailing_comma branch from 0f7b216 to f3f0517 Compare February 7, 2026 08:37

nyurik force-pushed the unnecessary_trailing_comma branch from f3f0517 to f180a19 Compare February 7, 2026 09:07

notriddle reviewed Feb 7, 2026

View reviewed changes

nyurik mentioned this pull request Feb 7, 2026

chore: remove unnecessary trailing commas in format strings #16533

Merged

nyurik force-pushed the unnecessary_trailing_comma branch from f180a19 to c8541cc Compare February 7, 2026 19:05

This comment has been minimized.

Sign in to view

nyurik mentioned this pull request Feb 9, 2026

style: remove unneeded trailing commas rust-lang/rust#152372

Merged

nyurik force-pushed the unnecessary_trailing_comma branch from ccd5535 to 1f35186 Compare February 9, 2026 21:35

This comment has been minimized.

Sign in to view

nyurik added 5 commits February 11, 2026 15:43

work with every type of macro delimeters

1451a50

fix ci issue

7fdf908

fix span computation

0b8e139

address clippy lints

c434930

switch to pedantic

d6d1b45

nyurik force-pushed the unnecessary_trailing_comma branch from 1f35186 to d6d1b45 Compare February 11, 2026 20:44

Add unnecessary_trailing_comma lint #16530

Are you sure you want to change the base?

Add unnecessary_trailing_comma lint #16530

Conversation

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

Uh oh!

Uh oh!

rustbot commented Feb 7, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

github-actions bot commented Feb 7, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Feb 7, 2026

Uh oh!

notriddle Feb 7, 2026

Choose a reason for hiding this comment

Uh oh!

nyurik Feb 7, 2026

Choose a reason for hiding this comment

Uh oh!

nyurik Feb 7, 2026

Choose a reason for hiding this comment

Uh oh!

notriddle Feb 8, 2026

Choose a reason for hiding this comment

Uh oh!

nyurik Feb 8, 2026

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

samueltardieu commented Feb 8, 2026

Uh oh!

nyurik commented Feb 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Jarcho commented Feb 9, 2026

Uh oh!

This comment has been minimized.

rustbot commented Feb 11, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

nyurik commented Feb 7, 2026 •

edited by rustbot

Loading

github-actions bot commented Feb 7, 2026 •

edited

Loading

nyurik commented Feb 8, 2026 •

edited

Loading