Better diagnostic for expected escape

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/Diagnostics.swift

@@ -31,6 +31,7 @@ enum ParseError: Error, Hashable {
   case expectedASCII(Character)
 
   case expectedNonEmptyContents
+  case expectedEscape
 
   case unknownGroupKind(String)
 
@@ -70,6 +71,8 @@ extension ParseError: CustomStringConvertible {
       return s
     case .expectedNonEmptyContents:
       return "expected non-empty contents"
+    case .expectedEscape:
+      return "expected escape sequence"
     case let .unknownGroupKind(str):
       return "unknown group kind '(\(str)'"
     case let .invalidMatchingOption(c):

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -105,18 +105,18 @@ extension Source {
   /// Throws an unexpected end of input error if not matched
   ///
   /// Note: much of the time, but not always, we can vend a more specific error.
-  mutating func expectNonEmpty() throws {
+  mutating func expectNonEmpty(
+    _ error: ParseError = .unexpectedEndOfInput
+  ) throws {
     _ = try recordLoc { src in
-      if src.isEmpty { throw ParseError.unexpectedEndOfInput }
+      if src.isEmpty { throw error }
     }
   }
 
   mutating func tryEatNonEmpty<C: Collection>(sequence c: C) throws -> Bool
     where C.Element == Char
   {
-    _ = try recordLoc { src in
-      guard !src.isEmpty else { throw ParseError.expected(String(c)) }
-    }
+    try expectNonEmpty(.expected(String(c)))
     return tryEat(sequence: c)
   }
 
@@ -430,7 +430,7 @@ extension Source {
         // Ignore escapes if we're allowed to. lexUntil will consume the next
         // character.
         if ignoreEscaped, src.tryEat("\\") {
-          try src.expectNonEmpty()
+          try src.expectNonEmpty(.expectedEscape)
         }
         return false
       }.value

Tests/RegexTests/ParseTests.swift

@@ -1093,6 +1093,7 @@ extension RegexTests {
     diagnosticTest("\\Qab\\", .expected("\\E"))
     diagnosticTest(#""ab"#, .expected("\""), syntax: .experimental)
     diagnosticTest(#""ab\""#, .expected("\""), syntax: .experimental)
+    diagnosticTest("\"ab\\", .expectedEscape, syntax: .experimental)
 
     // MARK: Text Segment options