Function num_to_string 

pub fn num_to_string(
    state: &mut LuaState,
    val: &LuaValue,
) -> Result<GcRef<LuaString>, LuaError>

Converts a numeric LuaValue to an interned LuaString, returning a GcRef<LuaString> handle. Callers are responsible for updating the LuaValue (or stack slot) with LuaValue::Str(s).

in place; in Rust we return the string because holding &mut LuaValue across a state.intern_str call would borrow state twice.