Fix handling of raw multiline string newline-suppressing continuations.

Sources/SwiftFormat/Core/Trivia+Convenience.swift

@@ -80,13 +80,19 @@ extension Trivia {
       })
   }
 
-  /// Returns `true` if this trivia contains any backslashes (used for multiline string newline
-  /// suppression).
-  var containsBackslashes: Bool {
-    return contains(
-      where: {
-        if case .backslashes = $0 { return true }
-        return false
-      })
+  /// Returns the prefix of this trivia that corresponds to the backslash and pound signs used to
+  /// represent a non-line-break continuation of a multiline string, or nil if the trivia does not
+  /// represent such a continuation.
+  var multilineStringContinuation: String? {
+    var result = ""
+    for piece in pieces {
+      switch piece {
+      case .backslashes, .pounds:
+        piece.write(to: &result)
+      default:
+        break
+      }
+    }
+    return result.isEmpty ? nil : result
   }
 }

Sources/SwiftFormat/PrettyPrint/TokenStreamCreator.swift

@@ -2637,11 +2637,13 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       emitSegmentTextTokens(segmentText[...])
     }
 
-    if node.trailingTrivia.containsBackslashes && !config.reflowMultilineStringLiterals.isAlways {
-      // Segments with trailing backslashes won't end with a literal newline; the backslash is
-      // considered trivia. To preserve the original text and wrapping, we need to manually render
-      // the backslash and a break into the token stream.
-      appendToken(.syntax("\\"))
+    if !config.reflowMultilineStringLiterals.isAlways,
+      let continuation = node.trailingTrivia.multilineStringContinuation
+    {
+      // Segments with trailing backslashes won't end with a literal newline; the backslash and any
+      // `#` delimiters for raw strings are considered trivia. To preserve the original text and
+      // wrapping, we need to manually render them break into the token stream.
+      appendToken(.syntax(continuation))
       appendToken(.break(breakKind, newlines: .hard(count: 1)))
     }
     return .skipChildren

Tests/SwiftFormatTests/PrettyPrint/StringTests.swift

@@ -739,4 +739,41 @@ final class StringTests: PrettyPrintTestCase {
 
     assertPrettyPrintEqual(input: input, expected: input + "\n", linelength: 20)
   }
+
+  func testMultilineStringWithContinuations() {
+    let input =
+      ##"""
+      let someString =
+        """
+        lines \
+        \nare \
+        short.
+        """
+      let someString =
+        #"""
+        lines \#
+        \#nare \#
+        short.
+        """#
+      """##
+
+    let expected =
+      ##"""
+      let someString =
+        """
+        lines \
+        \nare \
+        short.
+        """
+      let someString =
+        #"""
+        lines \#
+        \#nare \#
+        short.
+        """#
+
+      """##
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 30)
+  }
 }