Confusing error message related to Sized when actually trait import needed

[alexcrichton]

This gist contains all the code, the relevant file being this one.

When compiling, I get:

$ rustup run nightly cargo run
   Compiling wut v0.1.0 (file:///home/alex/code/wut)
error[E0277]: the trait bound `futures::stream::Stream<Item=(futures_mio::TcpStream, std::net::SocketAddr), Error=std::io::Error>: std::marker::Sized` is not satisfied
  --> src/main.rs:24:32
   |
24 |         let welcomes = clients.and_then(|(socket, _peer_addr)| {
   |                                ^^^^^^^^
   |
   = note: `futures::stream::Stream<Item=(futures_mio::TcpStream, std::net::SocketAddr), Error=std::io::Error>` does not have a constant size known at compile-time

error: no method named `fold` found for type `futures::stream::AndThen<futures::stream::Stream<Item=(futures_mio::TcpStream, std::net::SocketAddr), Error=std::io::Error>, [closure@src/main.rs:24:41: 26:10], futures_io::WriteAll<futures_mio::TcpStream, &[u8; 29]>>` in the current scope
  --> src/main.rs:27:18
   |
27 |         welcomes.fold((), |(), (socket, _welcome)| {
   |                  ^^^^
   |
   = note: the method `fold` exists but the following trait bounds were not satisfied: `futures::stream::AndThen<futures::stream::Stream<Item=(futures_mio::TcpStream, std::net::SocketAddr), Error=std::io::Error>, [closure@src/main.rs:24:41: 26:10], futures_io::WriteAll<futures_mio::TcpStream, &[u8; 29]>> : std::iter::Iterator`

error: aborting due to 2 previous errors

error: Could not compile `wut`.

To learn more, run the command again with --verbose.

The real error here is that I forgot to import the Stream trait, but instead I'm seeing a few points which confused me:

• Talking about Sized is mega-confusing as it's not really related.
• Printing the full type here ends up being somewhat harmful
• At the end it's talking about Iterator, but that's unrelated in this case

cc @jonathandturner
cc @nikomatsakis

[Mark-Simulacrum]

Closing in favor of #35976 duplicate issue.