Allow empty lines without indenation in multi-line string literals

Author: ahoppen

Sources/SwiftParser/StringLiterals.swift

@@ -210,6 +210,8 @@ extension Parser {
               segment.unexpectedAfterContent,
               arena: self.arena
             )
+          } else if segment.content.tokenText == "" || segment.content.tokenText.triviaPieceIfNewline != nil {
+            // Empty lines don't need to be indented and there's no indentation we need to strip away.
           } else {
             let actualIndentation = segment.content.tokenText.prefix(while: { $0 == UInt8(ascii: " ") || $0 == UInt8(ascii: "\t") })
             let actualIndentationTrivia = TriviaParser.parseTrivia(SyntaxText(rebasing: actualIndentation), position: .leading)

Tests/SwiftParserTest/ExpressionTests.swift

@@ -1025,6 +1025,76 @@ final class ExpressionTests: XCTestCase {
       """#
     )
   }
+
+  func testEmptyLineInMultilineStringLiteral() {
+    AssertParse(
+      #"""
+        """
+        line 1
+
+        line 2
+        """
+      """#,
+      substructure: Syntax(
+        StringLiteralExprSyntax(
+          openDelimiter: nil,
+          openQuote: .multilineStringQuoteToken(leadingTrivia: [.spaces(2)], trailingTrivia: .newline),
+          segments: StringLiteralSegmentsSyntax([
+            .stringSegment(StringSegmentSyntax(content: .stringSegment("line 1\n", leadingTrivia: [.spaces(2)]))),
+            .stringSegment(StringSegmentSyntax(content: .stringSegment("\n"))),
+            .stringSegment(StringSegmentSyntax(content: .stringSegment("line 2", leadingTrivia: [.spaces(2)], trailingTrivia: .newline))),
+          ]),
+          closeQuote: .multilineStringQuoteToken(leadingTrivia: [.spaces(2)]),
+          closeDelimiter: nil
+        )
+      ),
+      options: [.substructureCheckTrivia]
+    )
+
+    AssertParse(
+      #"""
+        """
+        line 1
+
+        """
+      """#,
+      substructure: Syntax(
+        StringLiteralExprSyntax(
+          openDelimiter: nil,
+          openQuote: .multilineStringQuoteToken(leadingTrivia: [.spaces(2)], trailingTrivia: .newline),
+          segments: StringLiteralSegmentsSyntax([
+            .stringSegment(StringSegmentSyntax(content: .stringSegment("line 1\n", leadingTrivia: [.spaces(2)]))),
+            .stringSegment(StringSegmentSyntax(content: .stringSegment("", trailingTrivia: .newline))),
+          ]),
+          closeQuote: .multilineStringQuoteToken(leadingTrivia: [.spaces(2)]),
+          closeDelimiter: nil
+        )
+      ),
+      options: [.substructureCheckTrivia]
+    )
+  }
+
+  func testUnderIndentedWhitespaceonlyLineInMultilineStringLiteral() {
+    AssertParse(
+      #"""
+        """
+        line 1
+       1️⃣
+        line 2
+        """
+      """#,
+      diagnostics: [
+        DiagnosticSpec(message: "insufficient indentation of line in multi-line string literal")
+      ],
+      fixedSource: #"""
+          """
+          line 1
+        \#("  ")
+          line 2
+          """
+        """#
+    )
+  }
 }
 
 final class MemberExprTests: XCTestCase {