`std::net::TcpStream`? Interestingly, it doesn't need to be mutable if you know the trick.

* It implements `Write` for `TcpStream` which is likely what you were using. And `Write` requires `&mut`, probably because the same trait is used for writing to application buffers (e.g. through `BufWriter` or just to a `Vec`). So this doesn't compile: [2] I think the error message is pretty good; maybe it's improved since you tried. It doesn't though suggest the trick below.

* It also implements `Write` for `&TcpStream`. So if you use the awkward phrasing `(&stream).write_all(b"asdf")`, you don't need mutable access. [3] This allows you to use it with anything that takes the trait, without requiring mutability. You might need this if say you're reading from the socket from one thread while simultaneously writing to it from another thread.

It's a vaguely similar situation with the most common async equivalent, `tokio::net::TcpStream`. The most straightforward way to write to it is with a trait that needs mutable access, but it is possible to write to a shared reference by not using a trait. They also have these `split` and `into_split` methods to get mutable access to something that implements the read traits and something that implements the write traits.

[1] https://doc.rust-lang.org/std/net/struct.TcpStream.html

[2] https://play.rust-lang.org/?version=stable&mode=debug&editio...

[2] https://play.rust-lang.org/?version=stable&mode=debug&editio...