After obtaining HM06 Rock Smash from Wattson, head west from Mauville City to Verdanturf Town. In the northern part of Verdanturf, you'll find the entrance to Rusturf Tunnel. Inside, you'll encounter Wally's uncle, who is trying to clear a path but is blocked by rocks. Use Rock Smash on the rocks to clear the path, connecting Verdanturf Town to Rustboro City. As a reward, Wally's uncle will give you HM04 Strength. This is an optional but recommended detour, as Strength is a useful HM for later in the game. You can also catch Whismur in Rusturf Tunnel, a Normal-type Pokémon with good Special Attack.
