Do not format blank lines in multiline string literals

Sources/SwiftBasicFormat/BasicFormat.swift

@@ -433,6 +433,19 @@ open class BasicFormat: SyntaxRewriter {
       return trailingTrivia + Trivia(pieces: nextTokenLeadingWhitespace)
     }()
 
+    /// Whether the leading trivia of the token is folloed by a newline.
+    let leadingTriviaIsFollowedByNewline: Bool = {
+      if token.text.isEmpty && token.trailingTrivia.startsWithNewline {
+        return true
+      } else if token.text.first?.isNewline ?? false {
+        return true
+      } else if token.text.isEmpty && token.trailingTrivia.isEmpty && nextTokenWillStartWithNewline {
+        return true
+      } else {
+        return false
+      }
+    }()
+
     var leadingTrivia = token.leadingTrivia
     var trailingTrivia = token.trailingTrivia
 
@@ -455,9 +468,11 @@ open class BasicFormat: SyntaxRewriter {
       }
     }
 
-    if leadingTrivia.indentation(isOnNewline: isInitialToken || previousTokenWillEndWithNewline) == [] {
+    let isEmptyLine = token.leadingTrivia.isEmpty && leadingTriviaIsFollowedByNewline
+    if leadingTrivia.indentation(isOnNewline: isInitialToken || previousTokenWillEndWithNewline) == [] && !isEmptyLine {
       // If the token starts on a new line and does not have indentation, this
-      // is the last non-indented token. Store its indentation level
+      // is the last non-indented token. Store its indentation level.
+      // Do not store the indentation level if the line is empty.
       anchorPoints[token] = currentIndentationLevel
     }
 
@@ -501,7 +516,7 @@ open class BasicFormat: SyntaxRewriter {
     leadingTrivia = leadingTrivia.indented(indentation: leadingTriviaIndentation, isOnNewline: previousTokenIsStringLiteralEndingInNewline)
     trailingTrivia = trailingTrivia.indented(indentation: trailingTriviaIndentation, isOnNewline: false)
 
-    leadingTrivia = leadingTrivia.trimmingTrailingWhitespaceBeforeNewline(isBeforeNewline: false)
+    leadingTrivia = leadingTrivia.trimmingTrailingWhitespaceBeforeNewline(isBeforeNewline: leadingTriviaIsFollowedByNewline)
     trailingTrivia = trailingTrivia.trimmingTrailingWhitespaceBeforeNewline(isBeforeNewline: nextTokenWillStartWithNewline)
 
     if leadingTrivia == token.leadingTrivia && trailingTrivia == token.trailingTrivia {

Tests/SwiftBasicFormatTest/BasicFormatTests.swift

@@ -250,6 +250,27 @@ final class BasicFormatTest: XCTestCase {
     )
   }
 
+  func testMultilineStringLiteralWithBlackLines() {
+    assertFormatted(
+      source: #"""
+        assertionFailure("""
+          First line
+
+          Second line
+
+          """)
+        """#,
+      expected: #"""
+        assertionFailure("""
+          First line
+
+          Second line
+
+          """)
+        """#
+    )
+  }
+
   func testNestedUserDefinedIndentation() {
     assertFormatted(
       source: """