Fix multiline string indentation

Author: kimdv

Sources/SwiftBasicFormat/Trivia+Indented.swift

@@ -13,12 +13,12 @@
 import SwiftSyntax
 
 extension Trivia {
-  func indented(indentation: TriviaPiece) -> Trivia {
+    func indented(indentation: TriviaPiece, hasContentNewline: Bool = false) -> Trivia {
     var indentedPieces: [TriviaPiece] = []
     for (index, piece) in self.enumerated() {
       let nextPiece = index < pieces.count - 1 ? pieces[index + 1] : nil
       indentedPieces.append(piece)
-      if piece.isNewline {
+      if piece.isNewline || hasContentNewline && index == 0 {
         switch (nextPiece, indentation) {
         case (.spaces(let nextPieceSpaces)?, .spaces(let indentationSpaces)):
           if nextPieceSpaces < indentationSpaces {

Sources/SwiftBasicFormat/generated/BasicFormat.swift

@@ -44,7 +44,7 @@ open class BasicFormat: SyntaxRewriter {
       indentationLevel -= 1
     }
   }
-  
+
   open override func visit(_ node: TokenSyntax) -> TokenSyntax {
     var leadingTrivia = node.leadingTrivia
     var trailingTrivia = node.trailingTrivia
@@ -57,8 +57,12 @@ open class BasicFormat: SyntaxRewriter {
     if let keyPath = getKeyPath(Syntax(node)), requiresLeadingNewline(keyPath), !(leadingTrivia.first?.isNewline ?? false) {
       leadingTrivia = .newline + leadingTrivia
     }
-    leadingTrivia = leadingTrivia.indented(indentation: indentation)
-    trailingTrivia = trailingTrivia.indented(indentation: indentation)
+
+      leadingTrivia = leadingTrivia.indented(indentation: indentation,
+                                             hasContentNewline: hasTrailingNewlineInContent(lastRewrittenToken)
+      )
+      trailingTrivia = trailingTrivia.indented(indentation: indentation)
+
     let rewritten = TokenSyntax(
         node.tokenKind, 
         leadingTrivia: leadingTrivia, 
@@ -70,6 +74,15 @@ open class BasicFormat: SyntaxRewriter {
     putNextTokenOnNewLine = false
     return rewritten
   }
+
+    open func hasTrailingNewlineInContent(_ node: TokenSyntax?) -> Bool {
+        switch node?.tokenKind {
+        case .stringSegment(let text):
+            return text.hasSuffix("\n")
+        default:
+            return false
+        }
+    }
 
   open func shouldIndent(_ keyPath: AnyKeyPath) -> Bool {
     switch keyPath {
@@ -176,7 +189,7 @@ open class BasicFormat: SyntaxRewriter {
       return false
     }
   }
-  
+
   /// If this returns a value that is not `nil`, it overrides the default
   /// trailing space behavior of a token.
   open func requiresTrailingSpace(_ keyPath: AnyKeyPath) -> Bool? {

Tests/SwiftSyntaxBuilderTest/StringLiteralTests.swift

@@ -249,7 +249,7 @@ final class StringLiteralTests: XCTestCase {
   func testMultiStringLiteralInIfExpr() {
     let buildable = ExprSyntax(
       #"""
-      if nonNilErrors.count == errors.count {
+      if true {
         assertionFailure("""
           Error validating child at index
           Node did not satisfy any node choice requirement.
@@ -262,7 +262,7 @@ final class StringLiteralTests: XCTestCase {
     AssertBuildResult(
       buildable,
       #"""
-      if nonNilErrors.count == errors.count {
+      if true {
           assertionFailure("""
                   Error validating child at index
                   Node did not satisfy any node choice requirement.
@@ -287,7 +287,7 @@ final class StringLiteralTests: XCTestCase {
       }
       """#
     )
-
+      
     AssertBuildResult(
       buildable,
       #"""