The inner forestay, when tensioned, will bend the mast and sag, unless the mast is supported from the opposite side. This is usually achieved with running backstays (runners) - a pair of wires attaching to the mast near the inner (or, on fractional rigs, only) forestay and tensioned alternately, with the windward one taking the load while the leeward one is eased. The running backstays are needed to fly the staysail on the inner stay, just like the main backstay must be tensioned to fly the jib on the main forestay.
Running backstay (right) supporting the mast