Fix indentation of nested multi-line string literals

Author: ahoppen

Sources/SwiftBasicFormat/BasicFormat.swift

@@ -498,10 +498,18 @@ open class BasicFormat: SyntaxRewriter {
       }
     }
 
-    let isEmptyLine = token.leadingTrivia.isEmpty && leadingTriviaIsFollowedByNewline
-    if leadingTrivia.indentation(isOnNewline: isInitialToken || previousTokenWillEndWithNewline) == [] && !isEmptyLine {
+    let isStringSegment: Bool
+    if case .stringSegment = token.tokenKind {
+      isStringSegment = true
+    } else {
+      isStringSegment = false
+    }
+    if leadingTrivia.indentation(isOnNewline: isInitialToken || previousTokenWillEndWithNewline) == [] && !isStringSegment {
       // 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.
+      // But never consider string segments as anchor points since you can’t
+      // indent individual lines of a multi-line string literals without breaking
+      // their integrity.
       anchorPoints[token] = currentIndentationLevel
     }
 
@@ -529,14 +537,17 @@ open class BasicFormat: SyntaxRewriter {
     var leadingTriviaIndentation = self.currentIndentationLevel
     var trailingTriviaIndentation = self.currentIndentationLevel
 
-    // If the trivia contain user-defined indentation, find their anchor point
+    // If the trivia contains user-defined indentation, find their anchor point
     // and indent the token relative to that anchor point.
-    if leadingTrivia.containsIndentation(isOnNewline: previousTokenWillEndWithNewline),
+    // Always indent string segments relative to their anchor point because
+    // their indentation has structural meaning and we just want to maintain
+    // what the user wrote.
+    if leadingTrivia.containsIndentation(isOnNewline: previousTokenWillEndWithNewline) || isStringSegment,
       let anchorPointIndentation = self.anchorPointIndentation(for: token)
     {
       leadingTriviaIndentation = anchorPointIndentation
     }
-    if combinedTrailingTrivia.containsIndentation(isOnNewline: previousTokenWillEndWithNewline),
+    if combinedTrailingTrivia.containsIndentation(isOnNewline: previousTokenWillEndWithNewline) || isStringSegment,
       let anchorPointIndentation = self.anchorPointIndentation(for: token)
     {
       trailingTriviaIndentation = anchorPointIndentation

Tests/SwiftBasicFormatTest/BasicFormatTests.swift

@@ -285,6 +285,68 @@ final class BasicFormatTest: XCTestCase {
     assertFormatted(source: source, expected: source)
   }
 
+  func testIndentNestedMultilineStringLiterals() throws {
+    let stringLiteral: ExprSyntax = #"""
+      """
+
+        \("""
+          First Line
+        """)
+      """
+      """#
+
+    assertFormatted(tree: stringLiteral, expected: stringLiteral.description)
+
+    let tree = try FunctionDeclSyntax("func test()") {
+      stringLiteral
+    }
+
+    assertFormatted(
+      tree: tree,
+      expected: #"""
+        func test() {
+            """
+
+              \("""
+                First Line
+              """)
+            """
+        }
+        """#
+    )
+  }
+
+  func testFooIndentNestedMultilineStringLiterals() throws {
+    let stringLiteral: ExprSyntax = #"""
+      _ = """
+
+            \("""
+              First Line
+            """)
+          """
+      """#
+
+    assertFormatted(tree: stringLiteral, expected: stringLiteral.description)
+
+    let tree = try FunctionDeclSyntax("func test()") {
+      stringLiteral
+    }
+
+    assertFormatted(
+      tree: tree,
+      expected: #"""
+        func test() {
+            _ = """
+
+                  \("""
+                    First Line
+                  """)
+                """
+        }
+        """#
+    )
+  }
+
   func testNestedUserDefinedIndentation() {
     assertFormatted(
       source: """