How do you scrape and process data from sites with complex navigation using Rust?

Scraping and processing data from sites with complex navigation using Rust involves several steps: sending HTTP requests, parsing HTML, and handling navigation logic. Since complex navigation often requires interaction with JavaScript, handling cookies, or maintaining session states, you might also need to consider using headless browsers or browser automation tools.

Here's a general outline of the steps you'd take:

1. Sending HTTP Requests: Use an HTTP client library to send requests to the website.
2. Parsing HTML: Utilize an HTML parsing library to extract data.
3. Handling Navigation: Implement logic to deal with pagination, form submissions, or any AJAX-based content loading.
4. Persisting State: Maintain cookies or session information if required.
5. Data Extraction: Define selectors to extract the relevant pieces of data.

For Rust, some popular libraries for these tasks are:

• reqwest: An HTTP client for making requests.
• scraper: A library for parsing HTML, built on top of html5ever.
• select: Another library for parsing HTML which provides jQuery-like selectors.

In the following example, we will scrape data from a hypothetical website with complex navigation using Rust. This example will focus on sending requests and parsing HTML. For more complex navigation involving JavaScript, you might need a browser automation tool like fantoccini which is a Rust wrapper around WebDriver.

First, add the necessary dependencies to your Cargo.toml:

[dependencies]
reqwest = { version = "0.11", features = ["json", "cookies"] }
scraper = "0.12"
tokio = { version = "1", features = ["full"] }

Now, let's write a Rust program to perform the web scraping:

use reqwest::Client;
use scraper::{Html, Selector};
use std::error::Error;

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    let client = Client::builder()
        .cookie_store(true)
        .build()?;

    // Navigate to the initial page.
    let initial_url = "https://example.com";
    let res = client.get(initial_url).send().await?;
    let body = res.text().await?;

    // Parse the initial page.
    let document = Html::parse_document(&body);
    let some_selector = Selector::parse(".some-class").unwrap();

    for element in document.select(&some_selector) {
        let data = element.text().collect::<Vec<_>>();
        println!("{:?}", data);
    }

    // Here you would have logic to find the next page or handle other navigation:
    // let next_page = document.select(Selector::parse(".next-page").unwrap());

    // Continue scraping other pages or sections as required.

    Ok(())
}

In this example, we:

• Create an async main function using Tokio as the runtime, which is required by reqwest for asynchronous operations.
• Build a Client with cookie support enabled for maintaining session state.
• Send a GET request to the initial URL and await the response.
• Parse the response body as HTML.
• Create a Selector to target elements with a specific class.
• Iterate over all elements matching the selector and collect their text content.
• Print the extracted data.

For complex navigation, you may need to:

• Handle pagination by looping through page numbers and modifying the URL or request parameters accordingly.
• Interact with forms by sending POST requests with appropriate form data.
• Deal with JavaScript by using a real browser or a headless browser such as headless Chrome with fantoccini.

Remember that web scraping should be performed responsibly and in compliance with the terms of service or robots.txt of the target website. Always check the website's policy on automated access before scraping.