Avoid dropping null byte in format string (#4732)

* Avoid dropping null byte in format string

* Add a test for null byte in format string

Author: user202729

include/fmt/base.h

@@ -2690,16 +2690,16 @@ template <typename... T> struct fstring {
     static_assert(count<(is_view<remove_cvref_t<T>>::value &&
                          std::is_reference<T>::value)...>() == 0,
                   "passing views as lvalues is disallowed");
-    if (FMT_USE_CONSTEVAL) parse_format_string<char>(s, checker(s, arg_pack()));
+    if (FMT_USE_CONSTEVAL)
+      parse_format_string<char>(str, checker(str, arg_pack()));
     constexpr bool unused = detail::enforce_compile_checks<sizeof(s) != 0>();
     (void)unused;
   }
   template <typename S,
             FMT_ENABLE_IF(std::is_convertible<const S&, string_view>::value)>
   FMT_CONSTEVAL FMT_ALWAYS_INLINE fstring(const S& s) : str(s) {
-    auto sv = string_view(str);
     if (FMT_USE_CONSTEVAL)
-      detail::parse_format_string<char>(sv, checker(sv, arg_pack()));
+      detail::parse_format_string<char>(str, checker(str, arg_pack()));
     constexpr bool unused = detail::enforce_compile_checks<sizeof(s) != 0>();
     (void)unused;
   }

test/compile-error-test/CMakeLists.txt

@@ -239,6 +239,16 @@ if (CMAKE_CXX_STANDARD GREATER_EQUAL 20)
   #endif
     "
     ERROR)
+  expect_compile(
+    format-string-embedded-nul-error
+    "
+    #if FMT_USE_CONSTEVAL
+      fmt::format(\"a\\0{}\");
+    #else
+      #error
+    #endif
+    "
+    ERROR)
 
   # Compile-time argument name check
   expect_compile(