I have reached the part where I have to go through the Rusturf Tunnel and there are boulders that are breakable. The NPC that said he was digging the tunnel by himself said the rocks are too hard and he can't get through and when I walked closer to them, the player mentioned "if only I could smash these rocks".
Where can I acquire Rock Smash?
Best Answer
You are automatically given Rock Smash after defeating Wally in Mauville city, in front of the gym and you will need the Dynamo badge to use it outside of battle.
At the point described in the question, however, you don't have to go through the Rusturf tunnel yet, instead you have to go to Dewford town, south from the Rustboro city (credit goes to @Jerry for pointing this out).