Function is_hole 

pub fn is_hole(
    world: &Arc<World>,
    top_pos: BlockPos,
    top_state: BlockStateId,
    bottom_pos: BlockPos,
    bottom_state: BlockStateId,
    fluid_id: FluidRef,
) -> bool

Returns true if the position below is a hole (fluid can flow downward).

Vanilla equivalent: FlowingFluid.isWaterHole(). Checks wall passability, then either same-fluid presence or canHoldFluid.