Special case `matches!` macro by ytmimi · Pull Request #5554 · rust-lang/rustfmt

ytmimi · 2022-10-01T17:03:14Z

Closes #4462
Closes #5547
Closes #5709
Closes #5860
Closes #6650
Closes #6714

Related / duplicate issues #4885 #5176

This PR adds support to parse and format the arguments of the matches! macro. Now matches! formatting more closely resembles match expressions. This update introduces breaking formatting changes and is only supported when formatting with style_edition=2027.

r? @calebcartwright

I've done my best to logically group commits and I think this PR would be best reviewed 1 commit at a time.

cynecx · 2022-10-02T01:25:21Z

+    matches!(
+        something,
+        Some(_)
+            if method(


The indentation here looks weird in my opinion. The indentation makes sense in match context because a match might contain multiple arms but the matches! macro does not.

I appreciate the feedback. My goal with this PR was to make matches! formatting consistent with match expression formatting, and this particular formatting is a result of reusing rewrite_guard to not have to reimplement guard formatting.

@calebcartwright maybe t-style would want to chime in on the guard formatting here

Has this been discussed somewhere or yet. If so, what was the conclusion?

calebcartwright · 2023-06-19T15:37:12Z

I feel like we discussed this a tad offline some time back, but probably worth posting here too that I think a big consideration around this is whether it needs to be version gated.

A question i've had teed up to the style team for a while is if/how macro calls should have style guide prescriptions or whether it's going to be something completely deferred to rustfmt.

In either case, I suspect it's highly likely this will need to be gated, and could perhaps become the new default as part of the 2024 style edition

ytmimi · 2023-06-20T02:13:33Z

+    } else if macro_name == "matches!" && context.config.version() == Version::Two {
+        if let success @ Some(..) = format_matches(context, shape, &macro_name, mac) {
+            return success;
+        }


@calebcartwright per your comment #5554 (comment), I just wanted to let you know that the changes are version gated.

I don't think there's a rush to get this in, and we can definitely hold off until style_edition lands to gate this around the 2024 style edition.

ytmimi · 2023-07-31T16:47:16Z

note to self, add a test case for #5860

cynecx · 2024-06-12T15:42:38Z

Sorry for the bump, but there hasn't been any public news on this for almost a year now. Have there been any discussions or anything about whether to include this in the 2024 edition or what the next steps are here?

ytmimi · 2024-06-12T16:07:59Z

Sorry for the bump, but there hasn't been any public news on this for almost a year now. Have there been any discussions or anything about whether to include this in the 2024 edition or what the next steps are here?

This isn't something that the team has discussed in quite some time so I don't have any updated to share from rustfmt. To the best of my knowledge the style-team has had some discussions on the 2024 edition, and these are the tracking issues for rustfmt.

freerig · 2025-10-25T15:31:39Z

Hi, I also found this bug, which is produced when two ranges are in a matches!, separated by |. I don't think it's a duplicate because it's specific to ranges (when you remove a range from the code, it works).

The first step in being able to format the `matches!` macro is to parse it into ast nodes that we know how to rewrite.

The guard rewrite rules vary based on how the pattern was rewritten. That logic was previously written in `rewrite_match_arm`, but now it's fully encapsulates in `rewrite_guard`.

These traits will make it easier to implement rewriting matches! in terms of `overflow::rewrite_with_*` methods.

Now that we're able to parse the TokenStream of `matches!` calls into a `Matches` struct and then convert that `Matches` struct into an iterable which implments `IntoOverflowableItem` we're able to implement `matches!` rewriting in terms of `overflow::rewrite_with_*` calls.

The unit tests cover version One and Two formatting

Since rustfmt uses version two formatting the self tests were failing due to the new special case handling for `matches!`. various `matches!` calls in the codebase were updated to address this issue.

bjorn3 · 2026-02-23T15:23:06Z

Why does there need to be a special case for matches!? I don't think rustfmt should ever edit the actual tokens inside an unknown macro? It is fine to heuristically assume that the content is an expression with respect to whitespace insertion/deletion, at worst that would just result in an ugly output. Changing the actual tokens on the other hand risks changing the runtime behavior of the code or creating a compilation error if rustfmt guessed wrong.

rustbot · 2026-03-13T15:32:33Z

☔ The latest upstream changes (possibly #6823) made this pull request unmergeable. Please resolve the merge conflicts.

anatawa12 · 2026-05-08T10:11:21Z

                _ => return Err(err),
            },
        }
+    } else if macro_name == "matches!"


I feel it might be better to support (debug_)assert_matches will be stabilized in 1.96 macros in similar bases.

View changes since the review

ytmimi added the pr-not-reviewed label Oct 1, 2022

ytmimi force-pushed the special_case_matches_macro branch 2 times, most recently from 5e5c766 to e834841 Compare October 1, 2022 17:14

cynecx reviewed Oct 2, 2022

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ytmimi force-pushed the special_case_matches_macro branch from e834841 to 0c5552d Compare March 9, 2023 14:04

ytmimi mentioned this pull request Mar 9, 2023

rustfmt thinks that a pattern matching is a closure #5709

Open

Alexendoo mentioned this pull request Apr 26, 2023

Warn on leading pipes in patterns rust-lang/rust-clippy#10717

Closed

ytmimi commented Jun 20, 2023

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ytmimi mentioned this pull request Jul 31, 2023

Invalid code when formatting matches!() expressions with leading pipe also for the first alternative #5860

Open

ytmimi mentioned this pull request Sep 22, 2024

rustfmt leaves behind a too-long line inside a closure #6344

Open

ytmimi force-pushed the special_case_matches_macro branch from 0c5552d to 92afb9d Compare August 31, 2025 03:15

ytmimi added the A-2027-style-edition Area: style edition 2027 label Aug 31, 2025

ytmimi added 11 commits November 20, 2025 13:51

Add helper function to determine if a TokenTree is a comma

46f3b2a

Parse matches! macro

f2d6f85

The first step in being able to format the `matches!` macro is to parse it into ast nodes that we know how to rewrite.

Increase visibility of expr::choose_separator_tactic

d32e72b

Better encapsulate guard rewrite logic in relation to pattern rewirte

5973f0f

The guard rewrite rules vary based on how the pattern was rewritten. That logic was previously written in `rewrite_match_arm`, but now it's fully encapsulates in `rewrite_guard`.

Add MatchesMacroItem and imp Rewrite, Spanned, and IntoOverflowableItem

34a6d17

These traits will make it easier to implement rewriting matches! in terms of `overflow::rewrite_with_*` methods.

Add unit tests for matches! special case formatting

1ccdbc2

The unit tests cover version One and Two formatting

Add matches! test case for issue 5709

e7553df

Apply updated matches! formatting to rustfmt

26e198f

Since rustfmt uses version two formatting the self tests were failing due to the new special case handling for `matches!`. various `matches!` calls in the codebase were updated to address this issue.

Add test case for issue 6650

1a502d0

Add test case for issue 6714

7b12588

ytmimi force-pushed the special_case_matches_macro branch from 92afb9d to 7b12588 Compare November 20, 2025 19:05

ytmimi mentioned this pull request Nov 20, 2025

matches! macro with ranges prevent formating inside other macro #6714

Open

jieyouxu added S-waiting-on-review Status: awaiting review from the assignee but also interested parties. and removed pr-not-reviewed labels Feb 23, 2026

anatawa12 reviewed May 8, 2026

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ytmimi commented Oct 1, 2022 •

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