tokio: tokio_stream::StreamMap allows duplicate keys

Version List the versions of all tokio crates you are using. This is version independent, but here you go:

$ cargo tree | grep tokio
tokio_stream--StreamMap v0.1.0 (/home/user/git/tmp/tokio_stream--StreamMap)
└── tokio-stream v0.1.9
    └── tokio v1.19.2

Platform The output of uname -a (UNIX), or version and 32 or 64-bit (Windows) This is platform independent, but here you go:

$ uname -a
Linux archlinux 5.18.3-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 09 Jun 2022 16:14:10 +0000 x86_64 GNU/Linux

Description [short summary of the bug] tokio_stream::StreamMap::insert followed by tokio_stream::StreamMap::extend allows for duplicate keys.

[code sample that causes the bug]

use tokio_stream;

fn main() {
    let mut streams = tokio_stream::StreamMap::new();
    streams.insert("key", tokio_stream::pending::<u8>());
    streams.extend(vec![
        ("key", tokio_stream::pending::<u8>())
    ]);

    for (key, value) in streams.iter() {
        println!("{}: {:?}", key, value);
    }
}

I expected that the duplicate keys would overwrite existing ones like they are with from_iter (which calls insert, which calls remove). However, I think it’d be best if both methods raised errors (see #4775) or at least returned a list of the replaced keys to match insert’s functionality. This way, the calling code would at least know what keys were replaced.

Instead, you can see that the StreamMap now contains duplicate keys:

key: Pending(PhantomData)
key: Pending(PhantomData)