Format `match` exprs even when or-patterns have comments by estebank · Pull Request #6893 · rust-lang/rustfmt

estebank · 2026-05-12T18:23:17Z

When encountering comments in between elements of an or-pattern in a match expression, format the matched value, the arm body and the patterns of all other arms. Only skip the pattern with embedded comments.

Fix #4119, fix #6491, cc #6044, cc #6663.

An alternative approach would be to make pre and post comments part of all patterns, which would allow us to keep them around and format or patterns properly, but wanted to have the minimal possible change that could address a common papercut I've noticed in rustc.

When encountering comments in between elements of an or-pattern in a `match` expression, format the matched value, the arm body and the patterns of all other arms. *Only* skip the pattern with embedded comments.

estebank · 2026-05-12T18:24:18Z

+        () => {}
+        () // Comment
+| () => {
+            println!("Foo");


This highlights the outstanding problem with this approach: the pattern is kept with bad formatting, where this should instead have been indented to the proper level.

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ytmimi · 2026-05-12T20:07:21Z

Diff-Check failed ❌. We probably have to gate this change on style_edition=2027.

ytmimi · 2026-05-12T20:08:33Z

@rustbot author

rustbot · 2026-05-12T20:08:38Z

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

estebank · 2026-05-13T01:13:21Z

@rustbot ready

@ytmimi I added (not yet existing) Edition 2027 and gated the new behavior behind it. Is that the usual modus operandi in rustfmt when introducing behavior changes that have big fan-out potential? I think the big diff in CI goes to show just how prevalent a gap this is :)

Using rust-lang/rustfmt#6893, reformat the codebase. The result is that matches that *would have* been formatted under normal circumstances get their expected format. These match expressions were being entirely skipped because they contain or-patterns with comments in between patterns, causing rustfmt to bail out entirely. The or-patterns with comments themselves remain untouched, but now the match arm bodies and other patterns without comments do get formatted under that PR. Because the fix in rustfmt isn't landed yet, I reworked some of the or-patterns with comments so that formatting doesn't regress. Tried doing this only in larger blocks that are more likely to regress in the meantime.

[style] rustfmt `match`es with comments in or-patterns Using rust-lang/rustfmt#6893, I reformatted the whole codebase. The result is that `match`es that *should have* been formatted under normal circumstances but are getting skipped now got their expected format. These match expressions were being entirely skipped because they contain or-patterns with comments in between patterns, causing rustfmt to bail out entirely. The or-patterns with comments themselves remain untouched, but now the match arm bodies and other patterns without comments do get formatted under that PR. Because the fix in rustfmt isn't landed yet, I reworked some of the or-patterns with comments so that formatting doesn't regress. Tried doing this only in larger blocks that are more likely to regress in the meantime. (Introduced and) removed a bunch of stray backticks \` likely left after an editor autoclosed the intended closing \`, resulting in <code>\`name\`\`</code> in comments.

ytmimi · 2026-05-22T02:58:40Z

+    /// rust edition 2027
+    Edition2027,


Wait, you mentioned in the FIXME below that rustc doesn't have an edition 2027 yet. In that case I'd prefer we don't add that here yet. It's fine that rustfmt has it's own style_edition to gate formatting changes outside of editions that are needed for parsing.

Please revert the Edition::Edition2027 changes from this PR. As long as things are gated on StyleEdition::Edition2027 then we should be good to move forward.

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ytmimi · 2026-05-22T03:03:04Z

Is that the usual modus operandi in rustfmt when introducing behavior changes that have big fan-out potential? I think the big diff in CI goes to show just how prevalent a gap this is :)

Yeah, anything that could potentially introduce breaking formatting changes needs to be gated. From what I understand that's how the team has pretty much always operated. Things used to be gated on version=Two, but since the 2024 edition we've gated things on style_edition instead.

ytmimi · 2026-05-22T03:03:17Z

@rustbot author

[style] rustfmt `match`es with comments in or-patterns Using rust-lang/rustfmt#6893, I reformatted the whole codebase. The result is that `match`es that *should have* been formatted under normal circumstances but are getting skipped now got their expected format. These match expressions were being entirely skipped because they contain or-patterns with comments in between patterns, causing rustfmt to bail out entirely. The or-patterns with comments themselves remain untouched, but now the match arm bodies and other patterns without comments do get formatted under that PR. Because the fix in rustfmt isn't landed yet, I reworked some of the or-patterns with comments so that formatting doesn't regress. Tried doing this only in larger blocks that are more likely to regress in the meantime. (Introduced and) removed a bunch of stray backticks \` likely left after an editor autoclosed the intended closing \`, resulting in <code>\`name\`\`</code> in comments.

estebank · 2026-06-19T20:44:21Z

Please verify whether the new code is what you requested.

@rustbot review

ytmimi · 2026-06-23T03:31:06Z

+    #[value = "2027"]
+    #[doc_hint = "2027"]
+    /// Edition 2027.
+    Edition2027,


Please revert this Edition2027 change.

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ytmimi · 2026-06-23T03:31:24Z

+            // FIXME: rustc doesn't have an edition 2027 yet.
+            Edition::Edition2027 => Self::Edition2024,


Please revert this edition 2027 change

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ytmimi · 2026-06-23T03:32:22Z

+    // rustfmt style_edition 2027
+    Edition2027,


Currently, the diff check is meant to test against stable formatting changes. We don't need to test Edition2027 because it's unstable. Please revert all Edition2027 changes made to check_diff

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ytmimi · 2026-06-23T03:33:10Z

        "style-edition",
        "The edition of the Style Guide (unstable).",
-        "[2015|2018|2021|2024]",
+        "[2015|2018|2021|2024|2027]",


Personally, I'd rather we kept these changes focused on match formatting. Please revert this change. Happy to make this change in a different PR.

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ytmimi · 2026-06-23T03:33:35Z

            Edition::Edition2018 => StyleEdition::Edition2018,
            Edition::Edition2021 => StyleEdition::Edition2021,
            Edition::Edition2024 => StyleEdition::Edition2024,
+            Edition::Edition2027 => StyleEdition::Edition2027,


Please revert this edition 2027 change

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ytmimi · 2026-06-23T03:34:31Z

-    // Style Edition 2024
+    // Style Edition 2027


Thank you for catching this typo. As mentioned in earlier comments I'd prefer to keep this PR focused on match formatting. Please revert this change.

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ytmimi · 2026-06-23T03:35:22Z


        fn empty_result(s: &RewriteResult) -> bool {
-            !matches!(*s, Ok(ref s) if !s.is_empty())
+            !matches!(*s, Ok(ref s) | Err(RewriteError::InlineComment(ref s)) if !s.is_empty())


Can you help me better understand this change.

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ytmimi · 2026-06-23T03:37:18Z

+            Err(RewriteError::InlineComment(item)) => {
+                // Keep the original contents and recover, allowing the subsequent items to be
+                // formatted while keeping the original formatting of this item.
+                // This is currently only relevant to or-patterns with comments in between patterns.
+                // We gate this under future edition 2027 because the amount of code that needs to
+                // be reformatted in the wild is quite high.
+                item
+            }


Where is the 2027 gate that you're referring to? Shouldn't this be gated on StyleEdition::Edition2027?

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ytmimi · 2026-06-23T03:39:04Z

+    /// We will keep the original code, report an error, but recover and format code that comes
+    /// later properly.
+    #[error("Formatting was skipped due to a comment that would have been removed.")]
+    InlineComment(String),


Is the String meant to represent the entire rewrite or just the comment? Maybe that should be made clearer.

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ytmimi · 2026-06-23T03:39:47Z

Please don't modify existing tests. Create a new 2027 test case.

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ytmimi · 2026-06-23T03:39:58Z

Please don't modify existing tests. Create a new 2027 test case.

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ytmimi

Please revert all Edition2027 changes that are outside the scope of match arm formatting. Also, please don't modify existing test cases. Create new ones for the 2027 case.

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estebank added 2 commits May 12, 2026 03:16

Add test for comment in pattern causing entire match to not be formatted

fdcf891

Format match exprs even when or-patterns have comments

094b75b

When encountering comments in between elements of an or-pattern in a `match` expression, format the matched value, the arm body and the patterns of all other arms. *Only* skip the pattern with embedded comments.

rustbot added the S-waiting-on-review Status: awaiting review from the assignee but also interested parties. label May 12, 2026

estebank commented May 12, 2026

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ytmimi added the F-impacts-stable-unformatted-code Expected formatting impact: affects stable + unformatted code (caution) label May 12, 2026

rustbot added S-waiting-on-author Status: awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: awaiting review from the assignee but also interested parties. labels May 12, 2026

Gate new match formatting behavior under edition 2027

8888485

rustbot added the A-CI Area: CI label May 13, 2026

rustbot added S-waiting-on-review Status: awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: awaiting some action (such as code changes or more information) from the author. labels May 13, 2026

estebank mentioned this pull request May 13, 2026

[style] rustfmt matches with comments in or-patterns rust-lang/rust#156518

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ytmimi reviewed May 22, 2026

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review comments

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		// FIXME: rustc doesn't have an edition 2027 yet.
		Edition::Edition2027 => Self::Edition2024,

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