Add unnecessary_trailing_comma lint by nyurik · Pull Request #16530 · rust-lang/rust-clippy

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.

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The reviewer was selected based on:

Owners of files modified in this PR: 7 candidates
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Random selection from Jarcho, dswij, llogiq, samueltardieu

github-actions · 2026-02-07T08:47:00Z

Lintcheck changes for 87d5992

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.

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.

Jarcho · 2026-02-13T18:54:26Z

clippy_lints/src/format_args.rs

+    #[allow(clippy::result_large_err, reason = "due to span_to_source()")]
+    fn check_trailing_comma(&self) {
+        let sm = self.cx.sess().source_map();
+        let span = self.macro_call.span.source_callsite();


Why are you calling source_callsite here? macro_call.span is already the call site of the macro.

Jarcho · 2026-02-13T19:04:04Z

clippy_lints/src/format_args.rs

+        if !sm.is_multiline(span)
+            && let Ok(removal_span) = sm.span_to_source(span, |s, start, end| {
+                // This fn returns a span between the last non-whitespace character
+                // and the closing parenthesis, but only if it contains a ',' char.
+                // Iterates in reverse, checking last char is a closing parenthesis,
+                // then looking for a comma before it, ignoring whitespace.
+                if let Some(text) = s.get(start..end)
+                    && let mut chars = text.char_indices().rev()
+                    && let Some((last_char_index, ')' | ']' | '}')) = chars.next()
+                {
+                    let mut has_comma = false;
+                    for (index, c) in chars {
+                        if c == ',' {
+                            has_comma = true;
+                        } else if c.is_whitespace() {
+                            // keep iterating
+                        } else if has_comma {
+                            return Ok(span
+                                .with_lo(span.lo() + BytePos::from_usize(index + c.len_utf8()))
+                                .with_hi(span.lo() + BytePos::from_usize(last_char_index)));
+                        } else {
+                            break;
+                        }
+                    }
+                }
+                Err(SpanSnippetError::IllFormedSpan(span))
+            })


This whole thing could be:

let span_data = span.data(); if let Some(src) = span_data.get_source_text(cx) && let Some(src) = src.strip_suffix([')', ']', '}']) && let Some(src) = src.trim_end_matches(|c| c.is_whitespace() && c != '\n').strip_suffix(',') { let removal_sp = SpanData { lo: span_data.lo + src.len(), hi: span_data.hi - 1, ..span_data }.span(); }

This will also remove the comma on weird formatting like:

format!("{}", arg,);

thank you!!! I spent over an hour trying to figure out how to optimize it, and didn't notice that strip_suffix supports a pattern!

Jarcho · 2026-02-13T19:38:25Z

clippy_lints/src/format_args.rs

+                self.cx,
+                UNNECESSARY_TRAILING_COMMA,
+                removal_span,
+                format!("unnecessary trailing comma in `{name}!` macro"),


Calling out the macro name doesn't seem like adds anything useful to the message.

Jarcho · 2026-02-13T19:40:04Z

clippy_lints/src/format_args.rs

+    /// single-line macro invocations.
+    ///
+    /// ### Why is this bad?
+    /// The trailing comma is redundant and removing it keeps the code cleaner.


I wouldn't call this cleaner, but more consistent with regular function calls.

rustbot · 2026-02-13T19:40:51Z

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

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 · 2026-02-13T22:31:29Z

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.

nyurik

thx, fixed

@rustbot ready.

View changes since this review

nyurik · 2026-02-13T22:31:34Z

clippy_lints/src/format_args.rs

+    #[allow(clippy::result_large_err, reason = "due to span_to_source()")]
+    fn check_trailing_comma(&self) {
+        let sm = self.cx.sess().source_map();
+        let span = self.macro_call.span.source_callsite();


nyurik · 2026-02-13T22:32:12Z

clippy_lints/src/format_args.rs

+        if !sm.is_multiline(span)
+            && let Ok(removal_span) = sm.span_to_source(span, |s, start, end| {
+                // This fn returns a span between the last non-whitespace character
+                // and the closing parenthesis, but only if it contains a ',' char.
+                // Iterates in reverse, checking last char is a closing parenthesis,
+                // then looking for a comma before it, ignoring whitespace.
+                if let Some(text) = s.get(start..end)
+                    && let mut chars = text.char_indices().rev()
+                    && let Some((last_char_index, ')' | ']' | '}')) = chars.next()
+                {
+                    let mut has_comma = false;
+                    for (index, c) in chars {
+                        if c == ',' {
+                            has_comma = true;
+                        } else if c.is_whitespace() {
+                            // keep iterating
+                        } else if has_comma {
+                            return Ok(span
+                                .with_lo(span.lo() + BytePos::from_usize(index + c.len_utf8()))
+                                .with_hi(span.lo() + BytePos::from_usize(last_char_index)));
+                        } else {
+                            break;
+                        }
+                    }
+                }
+                Err(SpanSnippetError::IllFormedSpan(span))
+            })


thank you!!! I spent over an hour trying to figure out how to optimize it, and didn't notice that strip_suffix supports a pattern!

nyurik · 2026-02-13T22:32:26Z

clippy_lints/src/format_args.rs

+                self.cx,
+                UNNECESSARY_TRAILING_COMMA,
+                removal_span,
+                format!("unnecessary trailing comma in `{name}!` macro"),


Jarcho · 2026-02-15T07:20:35Z

clippy_lints/src/format_args.rs

+            && let src = src.trim_end_matches(|c: char| c.is_whitespace() && c != '\n')
+            && let Some(src) = src.strip_suffix(',')
+        {
+            let src = src.trim_end_matches(|c: char| c.is_whitespace() && c != '\n');


This should either take all whitespace or not lint if this hits the start of the line.

print!("{}" , arg ,);

Either the parenthesis should be placed right after arg, or the lint shouldn't trigger. I'm fine either way, but not linting is probably better.

@Jarcho thx, I think it is better to only handle single-line cases.

@rustbot ready

All this needed was

&& let src = src.trim_end_matches(|c: char| c.is_whitespace() && c != '\n') && !src.end_with('\n')

This also avoids scanning the whole macro text for line breaks, which will be faster.

rustbot assigned Jarcho Feb 7, 2026

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

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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

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chore: remove unnecessary trailing commas in format strings #16533

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nyurik mentioned this pull request Feb 9, 2026

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

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Jarcho requested changes Feb 13, 2026

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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 Feb 13, 2026

nyurik added 7 commits February 13, 2026 15:30

work with every type of macro delimeters

a2cf91c

fix ci issue

e1af451

fix span computation

7d2ffae

address clippy lints

64c53f3

switch to pedantic

9f84abc

per feedback

ddde8fe

nyurik force-pushed the unnecessary_trailing_comma branch from d6d1b45 to ddde8fe Compare February 13, 2026 22:31

nyurik commented Feb 13, 2026

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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 Feb 13, 2026

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ignore multiline cases

87d5992

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 Feb 15, 2026

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