Allow interpolated strings without expressions, add TODO test for failing interp strings as types

Ast/src/Lexer.cpp

@@ -601,7 +601,7 @@ Lexeme Lexer::readInterpolatedStringBegin()
     Position start = position();
     consume();
 
-    return readInterpolatedStringSection(start, Lexeme::InterpStringBegin, Lexeme::QuotedString);
+    return readInterpolatedStringSection(start, Lexeme::InterpStringBegin, Lexeme::InterpStringEnd);
 }
 
 Lexeme Lexer::readInterpolatedStringSection(Position start, Lexeme::Type formatType, Lexeme::Type endType)

Ast/src/Parser.cpp

@@ -2203,7 +2203,7 @@ AstExpr* Parser::parseSimpleExpr()
             }
         }
     }
-    else if (lexer.current().type == Lexeme::RawString || lexer.current().type == Lexeme::QuotedString)
+    else if (lexer.current().type == Lexeme::RawString || lexer.current().type == Lexeme::QuotedString || (FFlag::LuauInterpolatedStringBaseSupport && lexer.current().type == Lexeme::InterpStringEnd))
     {
         return parseString();
     }
@@ -2603,11 +2603,11 @@ AstArray<AstTypeOrPack> Parser::parseTypeParams()
 
 std::optional<AstArray<char>> Parser::parseCharArray()
 {
-    LUAU_ASSERT(lexer.current().type == Lexeme::QuotedString || lexer.current().type == Lexeme::RawString);
+    LUAU_ASSERT(lexer.current().type == Lexeme::QuotedString || lexer.current().type == Lexeme::RawString || lexer.current().type == Lexeme::InterpStringEnd);
 
     scratchData.assign(lexer.current().data, lexer.current().length);
 
-    if (lexer.current().type == Lexeme::QuotedString)
+    if (lexer.current().type == Lexeme::QuotedString || lexer.current().type == Lexeme::InterpStringEnd)
     {
         if (!Lexer::fixupQuotedString(scratchData))
         {

tests/Parser.test.cpp

@@ -1110,6 +1110,24 @@ TEST_CASE_FIXTURE(Fixture, "parse_interpolated_string_after_prefixexp")
     }
 }
 
+TEST_CASE_FIXTURE(Fixture, "parse_interpolated_string_as_type_fail")
+{
+    ScopedFastFlag sff{"LuauInterpolatedStringBaseSupport", true};
+
+    try
+    {
+        parse(R"(
+            local a: `what` = `what`
+        )");
+        FAIL("Expected ParseErrors to be thrown");
+    }
+    catch (const ParseErrors& e)
+    {
+        // CHECK_EQ("Interpolated strings cannot be used alone to call a function. Wrap this in parentheses.", e.getErrors().front().getMessage());
+        CHECK_EQ("TODO", e.getErrors().front().getMessage());
+    }
+}
+
 TEST_CASE_FIXTURE(Fixture, "parse_nesting_based_end_detection")
 {
     try

tests/conformance/stringinterp.lua

@@ -31,7 +31,10 @@ assertEq(`This {localName} does not exist`, "This nil does not exist")
 assertEq(`Welcome to \
 {name}!`, "Welcome to \nLuau!")
 
+assertEq(`empty`, "empty")
+
 assertEq(`Escaped brace: \{}`, "Escaped brace: {}")
+assertEq(`Escaped brace \{} with {"expression"}`, "Escaped brace {} with expression")
 assertEq(`Backslash \ that escapes the space is not a part of the string...`, "Backslash  that escapes the space is not a part of the string...")
 assertEq(`Escaped backslash \\`, "Escaped backslash \\")
 assertEq(`Escaped backtick: \``, "Escaped backtick: `")