Fix invalid rewrite in UseExplicitNilCheckInConditions by skipping when explicit optional type annotation is present

Author: TTOzzi

Documentation/RuleDocumentation.md

@@ -490,6 +490,10 @@ than binding and immediately discarding the value.
 For example, `if let _ = someValue { ... }` is forbidden. Use `if someValue != nil { ... }`
 instead.
 
+Note: If the conditional binding carries an explicit type annotation (e.g. `if let _: S? = expr`
+or `if let _: S = expr`), we skip the transformation. Such annotations can be necessary to
+drive generic type inference when a function mentions a type only in its return position.
+
 Lint: `let _ = expr` inside a condition list will yield a lint error.
 
 Format: `let _ = expr` inside a condition list will be replaced by `expr != nil`.

Sources/SwiftFormat/Rules/UseExplicitNilCheckInConditions.swift

@@ -19,6 +19,10 @@ import SwiftSyntaxBuilder
 /// For example, `if let _ = someValue { ... }` is forbidden. Use `if someValue != nil { ... }`
 /// instead.
 ///
+/// Note: If the conditional binding carries an explicit type annotation (e.g. `if let _: S? = expr`
+/// or `if let _: S = expr`), we skip the transformation. Such annotations can be necessary to
+/// drive generic type inference when a function mentions a type only in its return position.
+///
 /// Lint: `let _ = expr` inside a condition list will yield a lint error.
 ///
 /// Format: `let _ = expr` inside a condition list will be replaced by `expr != nil`.
@@ -29,7 +33,8 @@ public final class UseExplicitNilCheckInConditions: SyntaxFormatRule {
     case .optionalBinding(let optionalBindingCondition):
       guard
         let initializerClause = optionalBindingCondition.initializer,
-        isDiscardedAssignmentPattern(optionalBindingCondition.pattern)
+        isDiscardedAssignmentPattern(optionalBindingCondition.pattern),
+        optionalBindingCondition.typeAnnotation == nil
       else {
         return node
       }

Tests/SwiftFormatTests/Rules/UseExplicitNilCheckInConditionsTests.swift

@@ -160,4 +160,18 @@ final class UseExplicitNilCheckInConditionsTests: LintOrFormatRuleTestCase {
       ]
     )
   }
+
+  func testTypeAnnotations() {
+    assertFormatting(
+      UseExplicitNilCheckInConditions.self,
+      input: """
+        if let _: F = foo() {}
+        if let _: S? = foo() {}
+        """,
+      expected: """
+        if let _: F = foo() {}
+        if let _: S? = foo() {}
+        """
+    )
+  }
 }