`TcpStream::connect` freezes `tokio` for one second

Question

Consider the following snippet:

use std::net::Ipv4Addr;
use std::time::{Duration, Instant};

use tokio::net::{TcpListener, TcpStream};
use tokio::time;

#[tokio::main(flavor = "current_thread")]
async fn main() {
    tokio::spawn(async {
        loop {
            println!("[{:?}] I am alive!", Instant::now());
            tokio::time::sleep(Duration::from_millis(100)).await;
        }
    });

    time::sleep(Duration::from_millis(1000)).await;

    let listener = TcpListener::bind((Ipv4Addr::UNSPECIFIED, 0)).await.unwrap();
    let address = listener.local_addr().unwrap();

    drop(listener);

    time::sleep(Duration::from_millis(1000)).await;

    println!("Connecting");
    let _connection = TcpStream::connect(address).await;
    println!("Done");

    time::sleep(Duration::from_millis(1000)).await;
}

In summary, on a single-threaded run of tokio I do the following:

• I spawn a task to print something every 100 ms, just know that everything keeps running.
• I bind a TcpListener anywhere, get its address.
• I drop the TcpListener.
• I then try to connect to the TcpListener's former address.

Now, here is the output that I get:

[Instant { t: 322033533953690 }] I am alive!
[Instant { t: 322033639639772 }] I am alive!
[Instant { t: 322033740157897 }] I am alive!
[Instant { t: 322033844392865 }] I am alive!
[Instant { t: 322033948457992 }] I am alive!
[Instant { t: 322034051559169 }] I am alive!
[Instant { t: 322034155284597 }] I am alive!
[Instant { t: 322034257879982 }] I am alive!
[Instant { t: 322034361229691 }] I am alive!
[Instant { t: 322034462803458 }] I am alive!
[Instant { t: 322034564346433 }] I am alive!
[Instant { t: 322034668035678 }] I am alive!
[Instant { t: 322034772372462 }] I am alive!
[Instant { t: 322034877014330 }] I am alive!
[Instant { t: 322034978911485 }] I am alive!
[Instant { t: 322035080462270 }] I am alive!
[Instant { t: 322035181321674 }] I am alive!
[Instant { t: 322035282532413 }] I am alive!
[Instant { t: 322035384651311 }] I am alive!
[Instant { t: 322035488278141 }] I am alive!
Connecting
Done
[Instant { t: 322036543294463 }] I am alive!
[Instant { t: 322036644255096 }] I am alive!
[Instant { t: 322036747256653 }] I am alive!
[Instant { t: 322036853860504 }] I am alive!
[Instant { t: 322036955935656 }] I am alive!
[Instant { t: 322037057575980 }] I am alive!
[Instant { t: 322037162679980 }] I am alive!
[Instant { t: 322037264386141 }] I am alive!
[Instant { t: 322037365479256 }] I am alive!
[Instant { t: 322037468083176 }] I am alive!

As expected, the connection does not succeed. Somewhat reasonably, it takes around one second for the connection attempt to return (some timeout might have been put in place, I guess?).

HOWEVER, as you can notice by the lack of I am alive messages between Connecting and Done, the call to TcpStream::connect seems to be blocking...! The task I spawned to print every 100 ms does not execute (and nor does anything else) for the whole time the connection attempt is in progress.

I am baffled: tokio calls are supposed to be asynchronous, and not block the entire event loop.. right? Am I missing something here? Or did I stumble upon some sort of bug?

From my Cargo.lock:

...

[[package]]
name = "tokio"
version = "1.13.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "588b2d10a336da58d877567cd8fb8a14b463e2104910f8132cd054b4b96e29ee"

...

Update: About my system

• Operating System: MacOs Big Sur (11.1)
• Computer: MacBook Pro, Intel 64 bits
• Rust version: 1.56.0

Update: Additional observations

In the above code, changing Ipv4Addr::UNSPECIFIED to Ipv4Addr::LOCALHOST significantly shortens the time to (fail to) establish the connection. It is still unclear to me whether or not it blocks while doing so.