feat: new lint `non_binding_let_else` by uhiovk · Pull Request #17136 · rust-lang/rust-clippy

uhiovk · 2026-06-01T21:06:59Z

changelog: [non_binding_let_else]: add new lint

rustbot · 2026-06-01T21:07:05Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

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Owners of files modified in this PR: 8 candidates
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Random selection from Jarcho, dswij, llogiq, samueltardieu

uhiovk · 2026-06-01T21:47:36Z

Sorry, I messed up with #16932. I'm not very familiar with Git and GitHub operations, hope this isn't causing any trouble.

github-actions · 2026-06-01T21:56:23Z

Lintcheck changes for 12571df

Lint	Added	Removed	Changed
`clippy::non_binding_let_else`	1	0	0

This comment will be updated if you push new changes

samueltardieu

You should look at this discussion for another lint converting between matches! and equality (or assignment in your case).

View changes since this review

samueltardieu · 2026-06-02T12:59:00Z

+fn can_compare(cx: &LateContext<'_>, pat: &Pat<'_>) -> bool {
+    let is_expr_like = pat.walk_short(|p| {
+        matches!(
+            p.kind,
+            PatKind::Expr(..)
+                | PatKind::Ref(..)
+                | PatKind::Slice(..)
+                | PatKind::Tuple(..)
+                | PatKind::Struct(..)
+                | PatKind::TupleStruct(..)
+        )
+    });
+
+    let ty = cx.typeck_results().pat_ty(pat);
+    let type_impl_partialeq = cx
+        .tcx
+        .lang_items()
+        .eq_trait()
+        .is_some_and(|id| implements_trait(cx, ty, id, &[ty.into()]));
+
+    is_expr_like && type_impl_partialeq
+}


You should check that PartialEq is, recursively, automatically derived. Otherwise nothing guarantees that it will give the same result as a structural match.

See [https://github.com//pull/17018#issuecomment-4467692343](this discussion) in another PR thread.

I'm not sure how to implement this effeciently as you pointed in that PR. For now, I just lint with MaybeIncorrect for all cases where PartialEq is implemented.

samueltardieu · 2026-06-02T13:00:03Z

+            "let pattern doesn't have bindings",
+            help,
+            sugg,
+            Applicability::MachineApplicable,


You can't use MachineApplicable with snippet(), as it may have failed in retrieving the correct source.

samueltardieu · 2026-06-02T13:03:26Z


        let _ = CELL;
        let _ = &CELL; //~ borrow_interior_mutable_const
+        #[allow(clippy::non_binding_let_else)]


Please use #[expect] if you know that the lint will trigger. Ditto in other instances.

samueltardieu · 2026-06-02T13:04:08Z

+    /// if opt != Some(42) { unreachable!() }
+    /// if !matches!(result, Ok(_)) { panic!() }
+    /// ```
+    #[clippy::version = "1.97.0"]


That would be 1.98.0 or later.

samueltardieu · 2026-06-02T13:04:29Z

+    /// if !matches!(result, Ok(_)) { panic!() }
+    /// ```
+    #[clippy::version = "1.97.0"]
+    pub NON_BINDING_LET_ELSE,


I'm not sure this is sufficiently frowned upon to be in style. Maybe restriction.

I think pedantic is fine. This pattern is quite unidiomatic, or at least confusing, so warning about it should not be a kind of restriction.

I think pedantic is fine. This pattern is quite unidiomatic, or at least confusing, so warning about it should not be a kind of restriction.

The proposed solution (matches(funcall(), Ok(_))) is not idiomatic either, that would be funcall().is_ok(). We'll see what others will see about the category when we enter the FCP.

rustbot · 2026-06-02T13:05:14Z

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

samueltardieu · 2026-06-02T13:13:11Z

Also, you should include tests where the let else, or part of it, comes from declarative or procedural macros.

samueltardieu

I'm not sure how to implement this effeciently as you pointed in that PR. For now, I just lint with MaybeIncorrect for all cases where PartialEq is implemented.

This is not enough: when applied, the fix would compile, but it might produce different results. That would be a I-suggestion-causes-bug kind of bug, which is very severe.

Moreover, the implementation of this check could be shared with the other PR in clippy_utils (Cc @GuillaumeGomez).

r? samueltardieu

View changes since this review

rustbot · 2026-06-11T10:06:32Z

☔ The latest upstream changes (possibly f1c1d90) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Jun 1, 2026

rustbot assigned Jarcho Jun 1, 2026

uhiovk force-pushed the non-binding-let-else branch from 126e538 to 3e10f37 Compare June 1, 2026 21:12

rustbot added the needs-fcp PRs that add, remove, or rename lints and need an FCP label Jun 1, 2026

uhiovk force-pushed the non-binding-let-else branch 2 times, most recently from 09839f8 to cfc42c2 Compare June 1, 2026 21:47

uhiovk force-pushed the non-binding-let-else branch from 91bffa7 to e5318ed Compare June 1, 2026 22:44

samueltardieu requested changes Jun 2, 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 Jun 2, 2026

uhiovk added 2 commits June 5, 2026 07:20

feat: new lint non_binding_let_else

907043e

fix: fix dogfood errors from non_binding_let_else

12571df

uhiovk force-pushed the non-binding-let-else branch from e5318ed to 12571df Compare June 4, 2026 23:27

uhiovk requested a review from samueltardieu June 4, 2026 23:52

samueltardieu requested changes Jun 5, 2026

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rustbot assigned samueltardieu and unassigned Jarcho Jun 5, 2026

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