[SwiftParser] Diagnose missing 'in' after closure signature

Tighten closure lookahead to accept signatures without 'in' only when a top-level '->' is present. Add a dedicated ClosureMissingInTests suite for missing-'in' and resync scenarios, and update Recovery test 141 to expect the additional diagnostic and adjusted fixed source.

Author: kwikysw

Sources/SwiftParser/Expressions.swift

@@ -2567,6 +2567,7 @@ extension Parser.Lookahead {
   mutating func canParseClosureSignature() -> Bool {
     // Consume attributes.
     var lookahead = self.lookahead()
+    var sawTopLevelArrowInLookahead = false
     var attributesProgress = LoopProgressCondition()
     while lookahead.consume(if: .atSign) != nil, lookahead.hasProgressed(&attributesProgress) {
       guard lookahead.at(.identifier) else {
@@ -2630,15 +2631,21 @@ extension Parser.Lookahead {
         return false
       }
 
+      sawTopLevelArrowInLookahead = true
+
       lookahead.consumeEffectsSpecifiers()
     }
 
     // Parse the 'in' at the end.
-    guard lookahead.at(.keyword(.in)) else {
-      return false
+    if lookahead.at(.keyword(.in)) {
+      // Okay, we have a closure signature.
+      return true
     }
-    // Okay, we have a closure signature.
-    return true
+
+    // Even if 'in' is missing, the presence of a top-level '->' makes this look like a
+    // closure signature. There's no other valid syntax that could legally
+    // contain '->' at this position.
+    return sawTopLevelArrowInLookahead
   }
 }
 

Tests/SwiftParserTest/ClosureMissingInTests.swift

@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@_spi(ExperimentalLanguageFeatures) @_spi(RawSyntax) import SwiftParser
+@_spi(ExperimentalLanguageFeatures) @_spi(RawSyntax) import SwiftSyntax
+import XCTest
+
+final class ClosureMissingInTests: ParserTestCase {
+
+  func testMissingInAfterSignature() {
+    assertParse(
+      """
+      _ = { (x: Int) -> Int 1️⃣0 }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected 'in' in closure signature",
+          fixIts: ["insert 'in'"]
+        )
+      ],
+      fixedSource: """
+        _ = { (x: Int) -> Int in 0 }
+        """
+    )
+  }
+
+  func testArrayLiteralNotMisparsedAsSignature() {
+    assertParse(
+      """
+      _ = { [x, y] }
+      """
+    )
+  }
+
+  func testAsyncIsNotASignatureGate() {
+    assertParse(
+      """
+      _ = { async }
+      """
+    )
+  }
+
+  func testShorthandParamsWithReturnType() {
+    assertParse(
+      """
+      _ = { x, _ -> Int 1️⃣x }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected 'in' in closure signature",
+          fixIts: ["insert 'in'"]
+        )
+      ],
+      fixedSource: """
+        _ = { x, _ -> Int in x }
+        """
+    )
+  }
+
+  func testResyncTokensBeforeIn() {
+    assertParse(
+      """
+      _ = { () -> Int
+        1️⃣0
+        in
+        1
+      }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "unexpected code '0' in closure signature"
+        )
+      ]
+    )
+  }
+
+  func testMissingInInFunctionArgument() {
+    assertParse(
+      """
+      test(make: { () -> [Int] 1️⃣
+        return [3]
+      }, consume: { _ in
+        print("Test")
+      })
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected 'in' in closure signature",
+          fixIts: ["insert 'in'"]
+        )
+      ],
+      fixedSource:
+        """
+        test(make: { () -> [Int] in
+          return [3]
+        }, consume: { _ in
+          print("Test")
+        })
+        """
+    )
+  }
+}

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -2445,6 +2445,11 @@ final class RecoveryTests: ParserTestCase {
           locationMarker: "3️⃣",
           message: "unexpected code '[' in function"
         ),
+        DiagnosticSpec(
+          locationMarker: "4️⃣",
+          message: "expected 'in' in closure signature",
+          fixIts: ["insert 'in'"]
+        ),
         DiagnosticSpec(
           locationMarker: "4️⃣",
           message: "unexpected code ') -> Int {}' in closure"
@@ -2453,7 +2458,7 @@ final class RecoveryTests: ParserTestCase {
       fixedSource: """
         #if true
         struct Foo19605164 {
-        func a(s: S) [{{g) -> Int {}
+        func a(s: S) [{{g in) -> Int {}
         }}}
         #endif
         """