Error in returning from a function due to a borrowed value.

gistfile1.txt

error[E0515]: cannot return value referencing local variable `stmt_blocks`
   --> zcash_client_sqlite/src/chain.rs:200:9
    |
194 |           let iter = stmt_blocks.query_map(&[from_height.0], |row| {
    |                      ----------- `stmt_blocks` is borrowed here
...
200 | /         Ok(Box::new({
201 | |             iter.map(|cbr_result| {
202 | |                 let cbr = cbr_result.map_err(Error::from)?;
203 | |                 let block: CompactBlock = parse_from_bytes(&cbr.data).map_err(Error::from)?;
...   |
211 | |             })
212 | |         }))
    | |___________^ returns a value referencing data owned by the current function

return_borrow_failure.rs

pub struct CacheConnection(Connection);

impl CacheOps for CacheConnection {
    type Error = Error;

    fn fetch_compact_blocks<'a>(&'a self, from_height: &BlockHeight) -> 
        Result<Box<dyn Iterator<Item = Result<CompactBlock, Self::Error>> + 'a>, Self::Error> {
        let mut stmt_blocks = self.0.prepare("SELECT height, data FROM compactblocks WHERE height > ? ORDER BY height DESC")
                                    .map_err(Error::from)?;

        let iter = stmt_blocks.query_map(&[from_height.0], |row| {
            let height = row.get(0)?;
            let data = row.get::<_, Vec<_>>(1)?;
            Ok(CompactBlockRow { height, data })
        }).map_err(Error::from)?;

        Ok(Box::new({
            iter.map(|cbr_result| {
                let cbr = cbr_result.map_err(Error::from)?;
                let block: CompactBlock = parse_from_bytes(&cbr.data).map_err(Error::from)?;
                let row_height = BlockHeight(cbr.height.into());
                
                if block.height() == row_height {
                    Ok(block)
                } else {
                    Err(ChainInvalid::block_height_mismatch(row_height, block.height()))
                }
            })
        }))
    }
}