Understanding what new code does without reading it

// Taken from the basic_example.rs
use tantivy::collector::TopDocs;
use tantivy::query::QueryParser;
use tantivy::schema::*;
use tantivy::{doc, Index, ReloadPolicy};
use tempfile::TempDir;

fn main() -> tantivy::Result<()> {
    let index_path = TempDir::new()?;

    let mut schema_builder = Schema::builder();

    schema_builder.add_text_field("title", TEXT | STORED);

    schema_builder.add_text_field("body", TEXT);

    let schema = schema_builder.build();

    let index = Index::create_in_dir(&index_path, schema.clone())?;

    let mut index_writer = index.writer(50_000_000)?;

    let title = schema.get_field("title").unwrap();
    let body = schema.get_field("body").unwrap();

    let mut old_man_doc = Document::default();
    old_man_doc.add_text(title, "The Old Man and the Sea");
    old_man_doc.add_text(
        body,
        "He was an old man who fished alone in a skiff in the Gulf Stream and \
         he had gone eighty-four days now without taking a fish.",
    );

    index_writer.add_document(old_man_doc);

    index_writer.add_document(doc!(
    title => "Of Mice and Men",
    body => "A few miles south of Soledad, the Salinas River drops in close to the hillside \
            bank and runs deep and green. The water is warm too, for it has slipped twinkling \
            over the yellow sands in the sunlight before reaching the narrow pool. On one \
            side of the river the golden foothill slopes curve up to the strong and rocky \
            Gabilan Mountains, but on the valley side the water is lined with trees—willows \
            fresh and green with every spring, carrying in their lower leaf junctures the \
            debris of the winter’s flooding; and sycamores with mottled, white, recumbent \
            limbs and branches that arch over the pool"
    ));

    index_writer.add_document(doc!(
    title => "Of Mice and Men",
    body => "A few miles south of Soledad, the Salinas River drops in close to the hillside \
            bank and runs deep and green. The water is warm too, for it has slipped twinkling \
            over the yellow sands in the sunlight before reaching the narrow pool. On one \
            side of the river the golden foothill slopes curve up to the strong and rocky \
            Gabilan Mountains, but on the valley side the water is lined with trees—willows \
            fresh and green with every spring, carrying in their lower leaf junctures the \
            debris of the winter’s flooding; and sycamores with mottled, white, recumbent \
            limbs and branches that arch over the pool"
    ));

    index_writer.add_document(doc!(
    title => "Frankenstein",
    title => "The Modern Prometheus",
    body => "You will rejoice to hear that no disaster has accompanied the commencement of an \
             enterprise which you have regarded with such evil forebodings.  I arrived here \
             yesterday, and my first task is to assure my dear sister of my welfare and \
             increasing confidence in the success of my undertaking."
    ));

    index_writer.commit()?;

    let reader = index
        .reader_builder()
        .reload_policy(ReloadPolicy::OnCommit)
        .try_into()?;

    let searcher = reader.searcher();

    // We care about the functions in this block             ------------------|
    let query_parser = QueryParser::for_index(&index, vec![title, body]);  //  |
                                                                           //  |
    let query = query_parser.parse_query("sea whale")?;                    //  |
                                                                           //  |
    let top_docs = searcher.search(&query, &TopDocs::with_limit(10))?;     //  |
    //                                                      -------------------|

    for (_score, doc_address) in top_docs {
        let retrieved_doc = searcher.doc(doc_address)?;
        println!("{}", schema.to_json(&retrieved_doc));
    }

    Ok(())
}