You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: join.rs
    path of file: ./repos/Rocket/core/lib/src/util
    
    the code of the file until where you have to start completion: use std::pin::Pin;
use std::task::{Poll, Context};

use pin_project_lite::pin_project;

use futures::stream::Stream;
use futures::ready;

/// Join two streams, `a` and `b`, into a new `Join` stream that returns items
/// from both streams, biased to `a`, until `a` finishes. The joined stream
/// completes when `a` completes, irrespective of `b`. If `b` stops producing
/// values, then the jo
    
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (left_low, left_high) = self.a.size_hint();
        let (right_low, right_high) = self.b.as_ref()
            .map(|b| b.size_hint())
            .unwrap_or_default();

        let low = left_low.saturating_add(right_low);
        let high = match (left_high, right_high) {
            (Some(h1), Some(h2)) => h1.checked_add(h2),
            _ => None,
        };

        (low, high)
    }