Escape U+2028/U+2029 in regex source generator XML doc comments

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -43,7 +43,8 @@ private static string EscapeXmlComment(string text)
                         case '>': sb.Append(">"); break;
 
                         // Propagate all other valid XML characters as-is. Control chars are considered invalid.
-                        case (>= 0x20 and <= 0x7F) or (>= 0xA0 and <= 0xD7FF) or (>= 0xE000 and <= 0xFFFD): sb.Append(c); break;
+                        // U+2028 and U+2029 are valid XML but are C# line terminators, so they'd break /// comments.
+                        case (>= 0x20 and <= 0x7F) or (>= 0xA0 and <= 0xD7FF and not 0x2028 and not 0x2029) or (>= 0xE000 and <= 0xFFFD): sb.Append(c); break;
 
                         // Use Unicode escape sequences for everything else.
                         default: sb.Append($"\\u{(int)c:X4}"); break;

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs

@@ -46,6 +46,12 @@ public static IEnumerable<object[]> Match_MemberData()
             // Testing control character escapes???: "2", "(\u0032)"
             yield return ("(\u0034)", "4", RegexOptions.None, 0, 1, true, "4");
 
+            // Patterns with literal U+2028, U+2029, and U+FFFE to validate source generator XML doc comment escaping
+            yield return ("ab\u2028", "ab\u2028", RegexOptions.None, 0, 3, true, "ab\u2028");
+            yield return ("ab\u2029", "ab\u2029", RegexOptions.None, 0, 3, true, "ab\u2029");
+            yield return ("ab\uFFFE", "ab\uFFFE", RegexOptions.None, 0, 3, true, "ab\uFFFE");
+            yield return ("[\u2028\u2029\uFFFE]", "x\u2029y", RegexOptions.None, 0, 3, true, "\u2029");
+
             // Using long loop prefix
             yield return (@"a{10}", new string('a', 10), RegexOptions.None, 0, 10, true, new string('a', 10));
             yield return (@"a{100}", new string('a', 100), RegexOptions.None, 0, 100, true, new string('a', 100));