Fixes a case where `collapsible_match` suggested a transformation that changes runtime behavior. by Souradip121 · Pull Request #16878 · rust-lang/rust-clippy

Souradip121 · 2026-04-17T21:30:36Z

What was wrong

The lint suggested collapsing pattern => { if test { body } } into pattern if test => { body }. These are not equivalent. In the original, once pattern matches, the match exits regardless of test. In the suggestion, a failing guard allows fall-through to subsequent arms, which can match values that previously never reached them.

// Before (original)
match a {
    Some(_) => {
        if b == 0 { res = 1 }
    }
    _ if b == 1 => res = 2,  // unreachable when a is Some(_)
    _ => {}
}

// After (suggestion — WRONG, changes behavior)
match a {
    Some(_) if b == 0 => res = 1,
    _ if b == 1 => res = 2,  // now reachable when a is Some(_) and b != 0
    _ => {}
}

Fix

The if-guard collapsing is only safe when there are no non-wildcard arms between the current arm and the wildcard arm. The fix adds a check that all arms after the current one are "wild-like" (a bare _, a binding, or None, all without guards) before suggesting the transformation.

As a side effect, three #[expect(clippy::collapsible_match)] suppressions in clippy_lints/src/methods/mod.rs are removed. They had been added to suppress this exact false positive and are no longer needed.

Still lints correctly

When only wildcard arms follow, the transformation is safe and the lint still fires:

// This still lints — only `_ => {}` follows, fall-through is harmless
match a {
    Some(_) => {
        if b == 0 { res = 1 }  // triggers collapsible_match
    }
    _ => {}
}

changelog: [collapsible_match]: no longer suggests collapsing pattern => { if test { body } } into a match guard when non-wildcard arms follow, as this changes the semantics of the match.

Summary Notes

Beta-nomination by samueltardieu

Managed by @rustbot—see help for details

…rd arm present

rustbot · 2026-04-17T21:30:42Z

r? @dswij

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github-actions · 2026-04-17T21:39:22Z

Lintcheck changes for c7cadb6

Lint	Added	Removed	Changed
`clippy::collapsible_match`	0	7	2

This comment will be updated if you push new changes

rustbot · 2026-04-18T07:47:39Z

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

Co-authored-by: Samuel Tardieu <sam@rfc1149.net>

Souradip121 · 2026-04-18T08:16:55Z

@rustbot ready

samueltardieu

@rustbot label +beta-nominated
@rustbot note Beta-nomination

This fixes a I-suggestion-causes-bug regression in 1.95. This could even be backported in a point release for 1.95.1.

Cc @flip1995

View changes since this review

profetia · 2026-04-20T03:21:07Z

Can you check if this also fixes #16716 and #16717? If not, can you borrow from #16749 to fix them? I'm thinking to put everything on this PR since it is beta nominated.

Souradip121 · 2026-04-20T10:41:54Z

Alright @profetia checking right now.

samueltardieu · 2026-04-20T11:14:55Z

Please keep beta-nominated PR at minimal size, so that they are easy to review. Those PRs are merged very late into the beta cycle, almost right before the stable release, so they must be very very readable.

This one fixes a I-suggestion-causes-bug kind of problem which is more critical than I-suggestion-causes-error.

samueltardieu · 2026-04-20T11:16:01Z

If not, can you borrow from #16749 to fix them? I'm thinking to put everything on this PR since it is beta nominated.

This is not a good idea.

Souradip121 · 2026-04-20T11:28:40Z

@samueltardieu @profetia should I push the new commits or not :)

samueltardieu · 2026-04-20T11:43:21Z

@samueltardieu @profetia should I push the new commits or not :)

Not in this PR.

Souradip121 · 2026-04-20T11:46:28Z

@samueltardieu Alright, not doing it, kindly merge my PR then.

samueltardieu · 2026-04-20T12:33:19Z

Given that's it's beta-nominated and possibly considered for stable backporting, I want to let a second maintainer a chance to have a look to it first.

Ping @rust-lang/clippy

profetia · 2026-04-20T16:58:36Z

This PR now already fixes #16717. I will reopen #16749 for #16716 solely, after it is merged.

The `clippy::collapsible_match` lint [1] can make code harder to read in certain cases [2], e.g. CLIPPY P rust/libmacros.so - due to command line change warning: this `if` can be collapsed into the outer `match` --> rust/pin-init/internal/src/helpers.rs:91:17 | 91 | / if nesting == 1 { 92 | | impl_generics.push(tt.clone()); 93 | | impl_generics.push(tt); 94 | | skip_until_comma = false; 95 | | } | |_________________^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match = note: `-W clippy::collapsible-match` implied by `-W clippy::all` = help: to override `-W clippy::all` add `#[allow(clippy::collapsible_match)]` help: collapse nested if block | 90 ~ TokenTree::Punct(p) if skip_until_comma && p.as_char() == ',' 91 ~ && nesting == 1 => { 92 | impl_generics.push(tt.clone()); 93 | impl_generics.push(tt); 94 | skip_until_comma = false; 95 ~ } | The lint does not have much upside -- when the suggestion may be a good one, it would still read fine when nested anyway. And it is the kind of lint that may easily bias people to just apply the suggestion instead of allowing it. [ In addition, as Gary points out [3], the suggestion is also wrong [4] and in the process of being fixed [5], possibly for Rust 1.97.0: Link: https://lore.kernel.org/rust-for-linux/DI3YV94TH9I3.1SOHW51552497@garyguo.net/ [3] Link: rust-lang/rust-clippy#16875 [4] Link: rust-lang/rust-clippy#16878 [5] - Miguel ] Thus just let developers decide on their own. Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs). Link: https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match [1] Link: https://lore.kernel.org/rust-for-linux/CANiq72nWYJna_hdFxjQCQZK6yJBrr1Mb86iKavivV0U0BgufeA@mail.gmail.com/ [2] Reviewed-by: Gary Guo <gary@garyguo.net> Link: https://patch.msgid.link/20260426144201.227108-1-ojeda@kernel.org Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Souradip121 added 2 commits April 18, 2026 02:47

fix collapsible_match: skip if-guard lint when non-wildcard arms follow

0c14721

add tests for collapsible_match: no lint when intermediate non-wildca…

2e7b2ba

…rd arm present

rustbot assigned dswij Apr 17, 2026

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Apr 17, 2026

samueltardieu requested changes Apr 18, 2026

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Comment thread clippy_lints/src/matches/collapsible_match.rs Outdated

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 Apr 18, 2026

Update clippy_lints/src/matches/collapsible_match.rs

c7cadb6

Co-authored-by: Samuel Tardieu <sam@rfc1149.net>

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 Apr 18, 2026

samueltardieu approved these changes Apr 18, 2026

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rustbot added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Apr 18, 2026

samueltardieu added the I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way label Apr 18, 2026

profetia mentioned this pull request Apr 19, 2026

Fix collapsible_match suggests wrongly when match body has no braces #16749

Open

Manishearth approved these changes Apr 20, 2026

View reviewed changes

samueltardieu added this pull request to the merge queue Apr 20, 2026

Merged via the queue into rust-lang:master with commit 87e4c91 Apr 20, 2026
13 checks passed

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Apr 20, 2026

This was referenced Apr 20, 2026

Handle different root contexts in SpanlessEq #15793

Merged

Fix collapsible_match false negative on if let chains (#16570) #16624

Open

Fix collapsible_match suggestion causing non-exhaustive pattern error #16865

Open

profetia mentioned this pull request Apr 20, 2026

collapsible_match FP when scrutinee could match following arm #16717

Open

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What was wrong

Fix

Still lints correctly

Summary Notes

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