Fix indentation of multiline strings when part of a larger expression.

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -3298,7 +3298,8 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
   }
 
   /// Walks the expression and returns the leftmost multiline string literal (which might be the
-  /// expression itself) if the leftmost child is a multiline string literal.
+  /// expression itself) if the leftmost child is a multiline string literal or if it is a unary
+  /// operation applied to a multiline string literal.
   ///
   /// - Parameter expr: The expression whose leftmost multiline string literal should be returned.
   /// - Returns: The leftmost multiline string literal, or nil if the leftmost subexpression was
@@ -3310,8 +3311,28 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       return stringLiteralExpr
     case .infixOperatorExpr(let infixOperatorExpr):
       return leftmostMultilineStringLiteral(of: infixOperatorExpr.leftOperand)
+    case .asExpr(let asExpr):
+      return leftmostMultilineStringLiteral(of: asExpr.expression)
+    case .isExpr(let isExpr):
+      return leftmostMultilineStringLiteral(of: isExpr.expression)
+    case .forcedValueExpr(let forcedValueExpr):
+      return leftmostMultilineStringLiteral(of: forcedValueExpr.expression)
+    case .optionalChainingExpr(let optionalChainingExpr):
+      return leftmostMultilineStringLiteral(of: optionalChainingExpr.expression)
+    case .postfixUnaryExpr(let postfixUnaryExpr):
+      return leftmostMultilineStringLiteral(of: postfixUnaryExpr.expression)
+    case .prefixOperatorExpr(let prefixOperatorExpr):
+      return leftmostMultilineStringLiteral(of: prefixOperatorExpr.postfixExpression)
     case .ternaryExpr(let ternaryExpr):
       return leftmostMultilineStringLiteral(of: ternaryExpr.conditionExpression)
+    case .functionCallExpr(let functionCallExpr):
+      return leftmostMultilineStringLiteral(of: functionCallExpr.calledExpression)
+    case .subscriptExpr(let subscriptExpr):
+      return leftmostMultilineStringLiteral(of: subscriptExpr.calledExpression)
+    case .memberAccessExpr(let memberAccessExpr):
+      return memberAccessExpr.base.flatMap { leftmostMultilineStringLiteral(of: $0) }
+    case .postfixIfConfigExpr(let postfixIfConfigExpr):
+      return postfixIfConfigExpr.base.flatMap { leftmostMultilineStringLiteral(of: $0) }
     default:
       return nil
     }

Tests/SwiftFormatPrettyPrintTests/StringTests.swift

@@ -327,4 +327,72 @@ final class StringTests: PrettyPrintTestCase {
 
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 20)
   }
+
+  func testLeadingMultilineStringsInOtherExpressions() {
+    // The stacked indentation behavior needs to drill down into different node types to find the
+    // leftmost multiline string literal. This makes sure that we cover various cases.
+    let input =
+      #"""
+      let bytes = """
+        {
+          "key": "value"
+        }
+        """.utf8.count
+      let json = """
+        {
+          "key": "value"
+        }
+        """.data(using: .utf8)
+      let slice = """
+        {
+          "key": "value"
+        }
+        """[...]
+      let forceUnwrap = """
+        {
+          "key": "value"
+        }
+        """!
+      let optionalChaining = """
+        {
+          "key": "value"
+        }
+        """?
+      let postfix = """
+        {
+          "key": "value"
+        }
+        """^*^
+      let prefix = +"""
+        {
+          "key": "value"
+        }
+        """
+      let postfixIf = """
+        {
+          "key": "value"
+        }
+        """
+        #if FLAG
+          .someMethod
+        #endif
+
+      // Like the infix operator cases, cast operations force the string's open quotes to wrap.
+      // This could be considered consistent if you look at it through the right lens. Let's make
+      // sure to test it so that we can see if the behavior ever changes accidentally.
+      let cast =
+        """
+        {
+          "key": "value"
+        }
+        """ as NSString
+      let typecheck =
+        """
+        {
+          "key": "value"
+        }
+        """ is NSString
+      """#
+    assertPrettyPrintEqual(input: input, expected: input + "\n", linelength: 100)
+  }
 }