Ensure inferred let pattern types are well-formed by s7tya · Pull Request #157013 · rust-lang/rust

s7tya · 2026-05-27T10:10:58Z

This registers a well-formedness obligation for inferred let pattern types when the pattern contains ref bindings.

Previously, let PAT; without an explicit type annotation could infer a non-well-formed pattern input type, such as [str; 2] or (str, str). Some bindings inside the pattern can still have well-formed local types, for example ref x gives x: &str, so checking only the binding locals missed the enclosing array or tuple type. As a result, invalid unsized array/tuple pattern types were accepted and could later lead to layout ICEs.

The new check registers a WF obligation for the inferred declaration type after pattern checking.

rustbot · 2026-05-27T10:11:03Z

r? @mejrs

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mejrs · 2026-05-27T10:17:06Z

r? @lcnr

lcnr · 2026-05-28T08:11:29Z

+                decl.pat.span,
+                ObligationCauseCode::WellFormed(None),
+            );
+        }


is there a reason we need to limit this to pat_has_ref_binding? 🤔 This feels somewhat brittle even if it fixes the existing known issues.

We could do it for all non-trivial pattern. Or personally, if perf doesn't care, just for all locals which don't have an explicitly provided type

View changes since the review

It was only for perf, and I was assuming that there was no way to create invalid types without ref bindings. However, I wasn't sure about patterns coming from unstable features, and I agree with removing the limit. I've removed the limit and applied the change to all patterns.

lcnr · 2026-05-28T08:11:44Z

@@ -0,0 +1,53 @@
+#![allow(unused)]
+


please comment explaining this test and linking to the relevant github issue

View changes since the review

I've added the comment. Please let me know if you need a more detailed description for each test case, and I'd be happy to provide that.

lcnr · 2026-05-28T08:11:59Z

@bors try @rust-timer queue

Ensure inferred let pattern types are well-formed

rust-bors · 2026-05-28T10:22:45Z

☀️ Try build successful (CI)
Build commit: 7f60c49 (7f60c4952189d0f4b52f688acb28979a2c537c2f, parent: 8f02e856be945d5d879b2ff9b7d19dd6bfb1c0e5)

rust-timer · 2026-05-28T11:03:48Z

Finished benchmarking commit (7f60c49): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary -0.6%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	2.8%	[2.0%, 3.6%]	3
Improvements ✅ (primary)	-1.8%	[-2.0%, -1.7%]	2
Improvements ✅ (secondary)	-2.3%	[-3.3%, -1.9%]	4
All ❌✅ (primary)	-0.6%	[-2.0%, 2.0%]	3

Cycles

Results (secondary 18.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	18.2%	[18.2%, 18.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 511.209s -> 509.706s (-0.29%)
Artifact size: 400.71 MiB -> 400.76 MiB (0.01%)

rustbot · 2026-05-28T11:31:31Z

This PR was rebased onto a different main 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.

s7tya · 2026-06-04T05:42:33Z

@lcnr I've updated the PR to check all non-trivial patterns for let stmts without an initializer and explicitly provided type. Could you please re-run the perf and review? (Sorry for the ping; please ignore this if you've already noticed it.)

lcnr · 2026-06-09T13:21:53Z

@bors try @rust-timer queue

Ensure inferred let pattern types are well-formed

rust-bors · 2026-06-09T15:36:52Z

☀️ Try build successful (CI)
Build commit: 6523e71 (6523e71f525206bf155d5424814da93f3e3bb98d, parent: 057ed8ba87418a77d5758f2d5e13f84b89b197e9)

rust-timer · 2026-06-09T16:45:25Z

Finished benchmarking commit (6523e71): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 2.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 517.542s -> 519.208s (0.32%)
Artifact size: 400.80 MiB -> 401.29 MiB (0.12%)

lcnr · 2026-06-10T12:30:08Z

thanks a lot!

@bors r+ rollup

rust-bors · 2026-06-10T12:30:12Z

📌 Commit d780821 has been approved by lcnr

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

Ensure inferred let pattern types are well-formed Fixes rust-lang#150040 This registers a well-formedness obligation for inferred `let` pattern types when the pattern contains `ref` bindings. Previously, `let PAT;` without an explicit type annotation could infer a non-well-formed pattern input type, such as `[str; 2]` or `(str, str)`. Some bindings inside the pattern can still have well-formed local types, for example `ref x` gives `x: &str`, so checking only the binding locals missed the enclosing array or tuple type. As a result, invalid unsized array/tuple pattern types were accepted and could later lead to layout ICEs. The new check registers a WF obligation for the inferred declaration type after pattern checking.

…uwer Rollup of 14 pull requests Successful merges: - #157280 (traits: Allow escaping self types in ExistentialTraitRef::with_self_ty) - #157282 (Fix post-monomorphization error note race in the parallel frontend) - #157352 (Make the retained dep graph deterministic under the parallel frontend) - #157601 (Emit error for unused target expression in glob and list delegations) - #157626 (Autogenerate unstable compiler flag stubs for unstable-book) - #157647 (Start using comptime for reflection intrinsics and their wrapper functions) - #157013 (Ensure inferred let pattern types are well-formed) - #157288 (platform support: add SNaN erratum to MIPS targets) - #157355 (Add `or_try_*` variants for `HashMap` and `BTreeMap` Entry APIs) - #157577 (Fix parser error recovery treating 'dyn' as a strict keyword in Rust …) - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N)) - #157691 (Move symbol hiding code to a new file) - #157700 (Rename `errors.rs` file to `diagnostics.rs` (5/N)) - #157703 (Fix doc link to Instant sub in saturating caveat) Failed merges: - #157699 (Arg splat experiment - hir FnDecl impl)

rustbot assigned mejrs May 27, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 27, 2026

rustbot assigned lcnr and unassigned mejrs May 27, 2026

mejrs added the needs-crater This change needs a crater run to check for possible breakage in the ecosystem. label May 27, 2026

lcnr reviewed May 28, 2026

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