Function Lchecklstring 

pub unsafe extern "C" fn Lchecklstring(
    state: lua_State,
    index: i32,
    length: &mut usize,
) -> *const u8

Checks whether the function argument index is a string and returns this string; if length is not NULL fills *length with the string’s length.

This function uses tolstring (lua_tolstring) to get its result, so all conversions and caveats of that function apply here.