Add %* format specifier

VM/src/lstrlib.cpp

@@ -8,6 +8,8 @@
 #include <string.h>
 #include <stdio.h>
 
+LUAU_FASTFLAGVARIABLE(LuauTostringFormatSpecifier, false);
+
 /* macro to `unsign' a character */
 #define uchar(c) ((unsigned char)(c))
 
@@ -1032,6 +1034,26 @@ static int str_format(lua_State* L)
                     break;
                 }
             }
+            case '*':
+            {
+                if (!FFlag::LuauTostringFormatSpecifier)
+                {
+                    luaL_error(L, "invalid option '%%*' to 'format'");
+                    break;
+                }
+
+                if (formatItemSize != 1)
+                {
+                    luaL_error(L, "'%%*' does not take a form");
+                }
+
+                size_t length;
+                const char* string = luaL_tolstring(L, arg, &length);
+
+                luaL_addlstring(&b, string, length);
+
+                continue; /* skip the `addsize' at the end */
+            }
             default:
             { /* also treat cases `pnLlh' */
                 luaL_error(L, "invalid option '%%%c' to 'format'", *(strfrmt - 1));

tests/Conformance.test.cpp

@@ -291,6 +291,8 @@ TEST_CASE("Clear")
 
 TEST_CASE("Strings")
 {
+    ScopedFastFlag sff{"LuauTostringFormatSpecifier", true};
+
     runConformance("strings.lua");
 }
 

tests/conformance/strings.lua

@@ -130,6 +130,26 @@ assert(string.format('"-%20s.20s"', string.rep("%", 2000)) ==
 -- longest number that can be formated
 assert(string.len(string.format('%99.99f', -1e308)) >= 100)
 
+local function return_one_thing()
+	return "hi"
+end
+local function return_two_nils()
+	return nil, nil
+end
+
+assert(string.format("%*", return_one_thing()) == "hi")
+assert(string.format("%* %*", return_two_nils()) == "nil nil")
+assert(pcall(function()
+	string.format("%* %* %*", return_two_nils())
+end) == false)
+
+assert(string.format("%*", "a\0b\0c") == "a\0b\0c")
+assert(string.format("%*", string.rep("doge", 3000)) == string.rep("doge", 3000))
+
+assert(pcall(function()
+	string.format("%#*", "bad form")
+end) == false)
+
 assert(loadstring("return 1\n--comentário sem EOL no final")() == 1)