Don't use luaL_typename to get a static type name in Luau.

In Luau this function returns heap-allocated string rather than static string,
so accessing this value when Lua state is destroyed is UB.
Fixes #674

Author: khvzak

src/conversion.rs

@@ -1211,7 +1211,8 @@ impl<L: FromLua, R: FromLua> FromLua for Either<L, R> {
             Err(_) => match R::from_stack(idx, lua).map(Either::Right) {
                 Ok(r) => Ok(r),
                 Err(_) => {
-                    let value_type_name = CStr::from_ptr(ffi::luaL_typename(lua.state(), idx));
+                    let value_type_name =
+                        CStr::from_ptr(ffi::lua_typename(lua.state(), ffi::lua_type(lua.state(), idx)));
                     Err(Error::FromLuaConversionError {
                         from: value_type_name.to_str().unwrap(),
                         to: Self::type_name(),

src/state/raw.rs

@@ -1219,7 +1219,10 @@ impl RawLua {
             Ok(type_id) => Ok(type_id),
             Err(Error::UserDataTypeMismatch) if ffi::lua_type(state, idx) != ffi::LUA_TUSERDATA => {
                 // Report `FromLuaConversionError` instead
-                let idx_type_name = CStr::from_ptr(ffi::luaL_typename(state, idx));
+                // In Luau `luaL_typename` return heap-allocated string that is valid only for
+                // the `state` lifetime.
+                // `lua_typename` is used instead to get a truly static string.
+                let idx_type_name = CStr::from_ptr(ffi::lua_typename(state, ffi::lua_type(state, idx)));
                 let idx_type_name = idx_type_name.to_str().unwrap();
                 let message = format!("expected userdata of type '{}'", short_type_name::<T>());
                 Err(Error::from_lua_conversion(idx_type_name, "userdata", message))