Detect missing `if let` or `let-else` #145582

estebank · 2025-08-18T16:52:42Z

During let binding parse error and encountering a block, detect if there is a likely missing if or else:

error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{`
  --> $DIR/missing-if-let-or-let-else.rs:14:25
   |
LL |     let Some(x) = foo() {
   |                         ^ expected one of `.`, `;`, `?`, `else`, or an operator
   |
help: you might have meant to use `if let`
   |
LL |     if let Some(x) = foo() {
   |     ++
help: alternatively, you might have meant to use `let else`
   |
LL |     let Some(x) = foo() else {
   |                         ++++

Fix #107806.

rustbot · 2025-08-18T16:52:47Z

r? @SparrowLii

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chenyukang · 2025-08-19T04:17:47Z

I'd like to have a review on it
r? @chenyukang

During `let` binding parse error and encountering a block, detect if there is a likely missing `if` or `else`: ``` error: expected one of `.`, `;`, `?`, `else`, or an operator, found `{` --> $DIR/missing-if-let-or-let-else.rs:14:25 | LL | let Some(x) = foo() { | ^ expected one of `.`, `;`, `?`, `else`, or an operator | help: you might have meant to use `if let` | LL | if let Some(x) = foo() { | ++ help: alternatively, you might have meant to use `let else` | LL | let Some(x) = foo() else { | ++++ ```

estebank · 2025-08-22T18:16:10Z

If we wanted to improve the handling of these, then we would have to actually add a new error recovery AST node, with all the information that an if-let would have and perform validation at the resolve or HIR stage. The purely parser approach I think is likely at the right trade-off between complexity and results.

chenyukang · 2025-08-25T06:06:50Z

tests/ui/uninhabited/missing-if-let-or-let-else.stderr

+   |
+help: you might have meant to use `let else`
+   |
+LL |     let Some(x) = foo() else {


if we suggest add else here, a following error missing ; will comes after applying the fix.

so the correct code maybe:

let Some(x) = foo() else { return; }; // need `;` here

i'm not sure whether we should also suggest the ; at the same time, maybe goes too far.

emm, this code is also valid 😀

fn b() { if let Some(x) = foo() { return; }; }

The last example is valid, but unless x is used, then the intent seems muddled. We can of course just suggest if-let in all cases.

Keep in mind that a person is still in the loop. If they didn't mean let-else, then it would give them enough information to realize that the code they wrote wasn't if-let.

yeah, make sense.

compiler/rustc_parse/src/parser/stmt.rs

chenyukang · 2025-08-31T01:49:14Z

@bors r=chenyukang

bors · 2025-08-31T01:49:17Z

📌 Commit 3af81cf has been approved by chenyukang

It is now in the queue for this repository.

bors · 2025-08-31T03:00:58Z

⌛ Testing commit 3af81cf with merge 64a99db...

bors · 2025-08-31T06:08:50Z

☀️ Test successful - checks-actions
Approved by: chenyukang
Pushing 64a99db to master...

github-actions · 2025-08-31T06:12:17Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing cd60c60 (parent) -> 64a99db (this PR)

Test differences

Show 4 test diffs

Stage 1

[ui] tests/ui/uninhabited/missing-if-let-or-let-else.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/uninhabited/missing-if-let-or-let-else.rs: [missing] -> pass (J0)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 64a99db105f45ea3304732ffb51066c3b5193bc7 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

pr-check-1: 1359.6s -> 1704.1s (25.3%)
dist-x86_64-apple: 9040.3s -> 7027.6s (-22.3%)
x86_64-gnu-llvm-19: 2432.2s -> 2817.5s (15.8%)
dist-aarch64-apple: 7104.0s -> 6090.5s (-14.3%)
aarch64-apple: 6902.0s -> 5928.7s (-14.1%)
x86_64-rust-for-linux: 2587.8s -> 2931.8s (13.3%)
x86_64-gnu-llvm-19-1: 3377.1s -> 3727.4s (10.4%)
i686-gnu-1: 7526.2s -> 8304.4s (10.3%)
x86_64-gnu-tools: 3389.8s -> 3725.8s (9.9%)
dist-aarch64-msvc: 5554.4s -> 6082.2s (9.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-08-31T07:50:47Z

Finished benchmarking commit (64a99db): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results (primary 2.0%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 466.008s -> 466.641s (0.14%)
Artifact size: 388.55 MiB -> 388.58 MiB (0.01%)

rustbot assigned SparrowLii Aug 18, 2025

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 Aug 18, 2025

rustbot assigned chenyukang and unassigned SparrowLii Aug 19, 2025

estebank force-pushed the issue-107806 branch from 59a2490 to 1d39706 Compare August 20, 2025 18:17

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estebank force-pushed the issue-107806 branch from 1d39706 to d216ca0 Compare August 20, 2025 18:26

chenyukang reviewed Aug 25, 2025

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3af81cf

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 31, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 31, 2025

bors merged commit 64a99db into rust-lang:master Aug 31, 2025
11 checks passed

rustbot added this to the 1.91.0 milestone Aug 31, 2025

Detect missing if let or let-else #145582

Detect missing if let or let-else #145582

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chenyukang commented Aug 31, 2025

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github-actions bot commented Aug 31, 2025

Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Aug 31, 2025

Overall result: ❌ regressions - no action needed

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Detect missing `if let` or `let-else` #145582

Detect missing `if let` or `let-else` #145582

estebank commented Aug 18, 2025 •

edited by rustbot

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estebank Aug 30, 2025 •

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