Function import 

pub fn import<S, Downloader, Era, PF, B, BB, BH>(
    downloader: Downloader,
    provider_factory: &PF,
    hash_collector: &mut Collector<BlockHash, BlockNumber>,
    to_block: Option<BlockNumber>,
    store_receipts: bool,
    is_receipt_verifiable: &dyn Fn(BlockNumber) -> bool,
) -> Result<BlockNumber>
where
    S: EraBlockReader<BH, BB, <<<PF as DatabaseProviderFactory>::ProviderRW as NodePrimitivesProvider>::Primitives as NodePrimitives>::Receipt>,
    B: Block<Header = BH, Body = BB>,
    BH: FullBlockHeader + Value,
    BB: FullBlockBody<Transaction = <<<PF as DatabaseProviderFactory>::ProviderRW as NodePrimitivesProvider>::Primitives as NodePrimitives>::SignedTx, OmmerHeader = BH>,
    Downloader: Stream<Item = Result<Era>> + Send + 'static + Unpin,
    Era: EraMeta + Send + 'static,
    PF: DatabaseProviderFactory<ProviderRW: BlockWriter<Block = B> + DBProvider + BlockBodyIndicesProvider + BlockHashReader + StaticFileProviderFactory<Primitives: NodePrimitives<Block = B, BlockHeader = BH, BlockBody = BB>> + StageCheckpointReader + StageCheckpointWriter + StorageSettingsCache> + StaticFileProviderFactory<Primitives = <<PF as DatabaseProviderFactory>::ProviderRW as NodePrimitivesProvider>::Primitives>,
    <<<PF as DatabaseProviderFactory>::ProviderRW as NodePrimitivesProvider>::Primitives as NodePrimitives>::Receipt: Compact + Receipt,

Imports blocks from downloader, decoding each file with the EraBlockReader S.

When to_block is set, the import stops after reaching that block height; otherwise it continues until the source has no more files.

store_receipts backfills the Receipts segment from its tip up to the Execution checkpoint, which it must reach exactly: receipts above the checkpoint are pruned on the next node start, and receipts below it leave the node unable to start. Only a missing tail is detected, never a gap inside the existing segment.

is_receipt_verifiable decides which blocks have their receipts checked against the header, as pre-Byzantium receipts can’t be recomputed here.

Returns current block height.