Make interpolated string with no formatting a syntax error

Ast/include/Luau/Lexer.h

@@ -86,6 +86,7 @@ struct Lexeme
         BrokenUnicode,
 
         BrokenInterpDoubleBrace,
+        BrokenInterpNoFormat,
 
         Error,
 

Ast/src/Lexer.cpp

@@ -187,6 +187,9 @@ std::string Lexeme::toString() const
     case BrokenInterpDoubleBrace:
         return "'{{', which is invalid (did you mean '\\{'?)";
 
+    case BrokenInterpNoFormat:
+        return "interpolated string with no formatting";
+
     case BrokenUnicode:
         if (codepoint)
         {
@@ -631,7 +634,7 @@ Lexeme Lexer::readInterpolatedStringBegin()
 
     if (!readSectionOpt)
     {
-        LUAU_ASSERT(!"INTERP TODO: Error if there was no interpolated expression");
+        return Lexeme(Location(start, position()), Lexeme::BrokenInterpNoFormat);
     }
 
     return *readSectionOpt;

Ast/src/Parser.cpp

@@ -2683,6 +2683,8 @@ AstExpr* Parser::parseInterpString()
             return reportExprError(location, {}, "Malformed interpolated string");
         case Lexeme::BrokenInterpDoubleBrace:
             return reportExprError(location, {}, "Double braces are not permitted within interpolated strings. Did you mean '\\{'?");
+        default:
+            break;
         }
     } while (true);
 }

tests/Parser.test.cpp

@@ -1065,6 +1065,21 @@ TEST_CASE_FIXTURE(Fixture, "parse_interpolated_string_double_brace_mid")
     }
 }
 
+TEST_CASE_FIXTURE(Fixture, "parse_interpolated_string_without_format")
+{
+    try
+    {
+        parse(R"(
+            _ = `doge`
+        )");
+        FAIL("Expected ParseErrors to be thrown");
+    }
+    catch (const ParseErrors& e)
+    {
+        CHECK_EQ("Expected identifier when parsing expression, got interpolated string with no formatting", e.getErrors().front().getMessage());
+    }
+}
+
 TEST_CASE_FIXTURE(Fixture, "parse_nesting_based_end_detection")
 {
     try