Improve builder string escape insertion

The `StringLiteralExpr.init(… content: …)` initializer was attempting to escape newlines, but it did not insert pound delimiters after the backslashes as Swift requires for raw literals. It also was not escaping control characters, which (except for tab) can only be written as escapes in Swift source code. Modify the initializer’s logic to correct these issues.

Author: beccadax

Sources/SwiftSyntaxBuilder/ConvenienceInitializers.swift

@@ -199,24 +199,39 @@ extension MemberAccessExpr {
 // MARK: - StringLiteralExpr
 
 extension String {
-  /// Replace literal newlines with "\r", "\n".
-  fileprivate func replacingNewlines() -> String {
-    var result = ""
-    var input = self[...]
-    while let firstNewline = input.firstIndex(where: { $0.isNewline }) {
+  /// Replace literal newlines with "\r", "\n", "\u{2028}", and ASCII control characters with "\0", "\u{7}"
+  fileprivate func escapingForStringLiteral(usingDelimiter delimiter: String) -> String {
+    // String literals cannot contain "unprintable" ASCII characters (control
+    // characters, etc.) besides tab. As a matter of style, we also choose to
+    // escape Unicode newlines like "\u{2028}" even though swiftc will allow
+    // them in string literals.
+    func needsEscaping(_ scalar: UnicodeScalar) -> Bool {
+      return (scalar.isASCII && scalar != "\t" && !scalar.isPrintableASCII)
+              || Character(scalar).isNewline
+    }
+
+    // Work at the Unicode scalar level so that "\r\n" isn't combined.
+    var result = String.UnicodeScalarView()
+    var input = self.unicodeScalars[...]
+    while let firstNewline = input.firstIndex(where: needsEscaping(_:)) {
       result += input[..<firstNewline]
-      if input[firstNewline] == "\r" {
-        result += "\\r"
-      } else if input[firstNewline] == "\r\n" {
-        result += "\\r\\n"
-      } else {
-        result += "\\n"
+
+      result += "\\\(delimiter)".unicodeScalars
+      switch input[firstNewline] {
+      case "\r":
+        result += "r".unicodeScalars
+      case "\n":
+        result += "n".unicodeScalars
+      case "\0":
+        result += "0".unicodeScalars
+      case let other:
+        result += "u{\(String(other.value, radix: 16))}".unicodeScalars
       }
       input = input[input.index(after: firstNewline)...]
-      continue
     }
+    result += input
 
-    return result + input
+    return String(result)
   }
 }
 
@@ -269,10 +284,6 @@ extension StringLiteralExpr {
     closeQuote: Token = .stringQuote,
     closeDelimiter: Token? = nil
   ) {
-    let contentToken = Token.stringSegment(content.replacingNewlines())
-    let segment = StringSegment(content: contentToken)
-    let segments = StringLiteralSegments([.stringSegment(segment)])
-
     var openDelimiter = openDelimiter
     var closeDelimiter = closeDelimiter
     if openDelimiter == nil, closeDelimiter == nil {
@@ -285,6 +296,11 @@ extension StringLiteralExpr {
       }
     }
 
+    let escapedContent = content.escapingForStringLiteral(usingDelimiter: closeDelimiter?.text ?? "")
+    let contentToken = Token.stringSegment(escapedContent)
+    let segment = StringSegment(content: contentToken)
+    let segments = StringLiteralSegments([.stringSegment(segment)])
+
     self.init(
       openDelimiter: openDelimiter,
       openQuote: openQuote,

Tests/SwiftSyntaxBuilderTest/StringLiteralTests.swift

@@ -85,8 +85,38 @@ final class StringLiteralTests: XCTestCase {
 
   func testNewlines() {
     AssertBuildResult(
-      StringLiteralExpr(content: "linux\nwindows\r\na"),
-      #""linux\nwindows\r\na""#
+      StringLiteralExpr(content: "linux\nwindows\r\nunicode\u{2028}a"),
+      #""linux\nwindows\r\nunicode\u{2028}a""#
+    )
+
+    AssertBuildResult(
+      StringLiteralExpr(content: "\\linux\nwindows\r\nunicode\u{2028}a"),
+      ##"#"\linux\#nwindows\#r\#nunicode\#u{2028}a"#"##
+    )
+  }
+
+  func testNul() {
+    AssertBuildResult(
+      StringLiteralExpr(content: "before\0after"),
+      #""before\0after""#
+    )
+
+    AssertBuildResult(
+      StringLiteralExpr(content: "\\before\0after"),
+      ##"#"\before\#0after"#"##
+    )
+  }
+
+  func testControlChars() {
+    // Note that tabs do *not* get escaped.
+    AssertBuildResult(
+      StringLiteralExpr(content: "before\u{07}\t\u{7f}after"),
+      #""before\u{7}\#t\u{7f}after""#
+    )
+
+    AssertBuildResult(
+      StringLiteralExpr(content: "\\before\u{07}\t\u{7f}after"),
+      ##"#"\before\#u{7}\##t\#u{7f}after"#"##
     )
   }
 }