Move classification of newline after multi-line string quote and escaped newline to lexer

This simplifies parsing of string literals while only making the lexer slightly more complex. It also fixes two bugs where we incorrectly identified a trailing `\` as escaped even if it wasn’t.

Author: ahoppen

Sources/SwiftParser/Lexer/Cursor.swift

@@ -259,17 +259,22 @@ extension Lexer {
     /// error occurred
     let error: (kind: LexerError.Kind, position: Lexer.Cursor)?
     let stateTransition: StateTransition?
+    /// If set, overritdes the trailing trivia lexing mode of the current state
+    /// for this lexeme.
+    let trailingTriviaLexingMode: Lexer.Cursor.TriviaLexingMode?
 
     init(
       _ tokenKind: RawTokenKind,
       flags: Lexer.Lexeme.Flags = [],
       error: (kind: LexerError.Kind, position: Cursor)? = nil,
-      stateTransition: StateTransition? = nil
+      stateTransition: StateTransition? = nil,
+      trailingTriviaLexingMode: Lexer.Cursor.TriviaLexingMode? = nil
     ) {
       self.tokenKind = tokenKind
       self.flags = flags
       self.error = error
       self.stateTransition = stateTransition
+      self.trailingTriviaLexingMode = trailingTriviaLexingMode
     }
   }
 }
@@ -317,16 +322,9 @@ extension Lexer.Cursor {
 
     // Trailing trivia.
     let trailingTriviaStart = self
-    let newlineInTrailingTrivia: NewlinePresence
-    if let trailingTriviaMode = currentState.trailingTriviaLexingMode(cursor: self) {
-      newlineInTrailingTrivia = self.lexTrivia(mode: trailingTriviaMode)
-    } else {
-      newlineInTrailingTrivia = .absent
+    if let trailingTriviaMode = result.trailingTriviaLexingMode ?? currentState.trailingTriviaLexingMode(cursor: self) {
+      _ = self.lexTrivia(mode: trailingTriviaMode)
     }
-    assert(
-      newlineInTrailingTrivia == .absent,
-      "trailingTrivia should not have a newline"
-    )
 
     if self.currentState.shouldPopStateWhenReachingNewlineInTrailingTrivia && self.is(at: "\r", "\n") {
       self.stateStack.perform(stateTransition: .pop, stateAllocator: stateAllocator)
@@ -853,7 +851,7 @@ extension Lexer.Cursor {
       UInt8(ascii: "9"):
       return self.lexNumber()
     case UInt8(ascii: #"'"#), UInt8(ascii: #"""#):
-      return self.lexStringQuote(leadingDelimiterLength: 0)
+      return self.lexStringQuote(isOpening: true, leadingDelimiterLength: 0)
 
     case UInt8(ascii: "`"):
       return self.lexEscapedIdentifier()
@@ -878,7 +876,7 @@ extension Lexer.Cursor {
   private mutating func lexAfterRawStringDelimiter(delimiterLength: Int) -> Lexer.Result {
     switch self.peek() {
     case UInt8(ascii: #"'"#), UInt8(ascii: #"""#):
-      return self.lexStringQuote(leadingDelimiterLength: delimiterLength)
+      return self.lexStringQuote(isOpening: true, leadingDelimiterLength: delimiterLength)
     case nil:
       return Lexer.Result(.eof)
     default:
@@ -889,7 +887,7 @@ extension Lexer.Cursor {
   private mutating func lexAfterStringLiteral() -> Lexer.Result {
     switch self.peek() {
     case UInt8(ascii: #"'"#), UInt8(ascii: #"""#):
-      return self.lexStringQuote(leadingDelimiterLength: 0)
+      return self.lexStringQuote(isOpening: false, leadingDelimiterLength: 0)
     case nil:
       return Lexer.Result(.eof)
     default:
@@ -984,9 +982,28 @@ extension Lexer.Cursor {
     case normal
     /// Don't lex newlines (`\r` and `\r`) as trivia
     case noNewlines
+    /// Lex the characters that escape a newline in a multi-line string literal
+    /// as trivia.
+    ///
+    /// Matches the following regex: `\\?#*[ \t]*(\r\n|\r|\n)
+    case escapedNewlineInMultiLineStringLiteral
   }
 
   fileprivate mutating func lexTrivia(mode: TriviaLexingMode) -> NewlinePresence {
+    if mode == .escapedNewlineInMultiLineStringLiteral {
+      _ = self.advance(matching: "\\")
+      self.advance(while: { $0 == "#" })
+      self.advance(while: { $0 == " " || $0 == "\t" })
+      if self.advance(matching: "\r") {
+        _ = self.advance(matching: "\n")
+        return .present
+      } else if self.advance(matching: "\n") {
+        return .present
+      } else {
+        return .absent
+      }
+    }
+
     var hasNewline = false
     while true {
       let start = self
@@ -1662,7 +1679,9 @@ extension Lexer.Cursor {
     }
   }
 
-  mutating func lexStringQuote(leadingDelimiterLength: Int) -> Lexer.Result {
+  /// `isOpening` is `true` if this string quote is the opening quote of a string
+  /// literal and `false` if we are lexing the closing quote of a string literal.
+  mutating func lexStringQuote(isOpening: Bool, leadingDelimiterLength: Int) -> Lexer.Result {
     if self.advance(matching: "'") {
       return Lexer.Result(.singleQuote, stateTransition: stateTransitionAfterLexingStringQuote(kind: .singleQuote))
     }
@@ -1696,7 +1715,20 @@ extension Lexer.Cursor {
       }
 
       self = lookingForMultilineString
-      return Lexer.Result(.multilineStringQuote, stateTransition: stateTransitionAfterLexingStringQuote(kind: .multiLine))
+      let trailingTriviaLexingMode: TriviaLexingMode?
+      if isOpening && self.is(at: "\n", "\r") {
+        // The opening quote of a multi-line string literal must be followed by
+        // a newline that's not part of the represented string.
+        trailingTriviaLexingMode = .escapedNewlineInMultiLineStringLiteral
+      } else {
+        trailingTriviaLexingMode = nil
+      }
+
+      return Lexer.Result(
+        .multilineStringQuote,
+        stateTransition: stateTransitionAfterLexingStringQuote(kind: .multiLine),
+        trailingTriviaLexingMode: trailingTriviaLexingMode
+      )
     } else {
       return Lexer.Result(.stringQuote, stateTransition: stateTransitionAfterLexingStringQuote(kind: .singleLine))
     }
@@ -1714,6 +1746,23 @@ extension Lexer.Cursor {
     return tmp.advanceIfStringDelimiter(delimiterLength: delimiterLength) && tmp.is(at: "(")
   }
 
+  /// Returns `true` if we are positioned at a backslash that escapes the newline
+  /// character in a multi-line string literal.
+  private func isAtEscapedNewline(delimiterLength: Int) -> Bool {
+    guard self.is(at: "\\") else {
+      return false
+    }
+
+    var tmp = self
+    let backslashConsumed = tmp.advance(matching: "\\")  // Skip over the '\' to look for '#' and '('
+    assert(backslashConsumed)
+    guard tmp.advanceIfStringDelimiter(delimiterLength: delimiterLength) else {
+      return false
+    }
+    tmp.advance(while: { $0 == " " || $0 == "\t" })
+    return tmp.is(at: "\r", "\n")
+  }
+
   mutating func lexInStringLiteral(stringLiteralKind: StringLiteralKind, delimiterLength: Int) -> Lexer.Result {
     /*
     if IsMultilineString && *CurPtr != '\n' && *CurPtr != '\r' {
@@ -1729,6 +1778,11 @@ extension Lexer.Cursor {
             .stringSegment,
             stateTransition: .push(newState: .inStringInterpolationStart(stringLiteralKind: stringLiteralKind))
           )
+        } else if self.isAtEscapedNewline(delimiterLength: delimiterLength) {
+          return Lexer.Result(
+            .stringSegment,
+            trailingTriviaLexingMode: .escapedNewlineInMultiLineStringLiteral
+          )
         }
       case UInt8(ascii: "\r"), UInt8(ascii: "\n"):
         if stringLiteralKind == .multiLine {