Improve compiler errors

[manuels]

If you use traits error messages can become very long and complicated to read.

Consider this error message:

error[E0599]: no method named `into_stream` found for type `futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>>` in the current scope
   --> src/main.rs:220:19
    |
220 |     let foo = foo.into_stream();
    |                   ^^^^^^^^^^^ method not found in `futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>>`
    |
    = note: the method `into_stream` exists but the following trait bounds were not satisfied:
            `&futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::future::future::FutureExt`
            `&futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::stream::try_stream::TryStreamExt`
            `&mut futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::future::future::FutureExt`
            `&mut futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::stream::try_stream::TryStreamExt`
            `futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::future::future::FutureExt`
            `futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> : futures_util::stream::try_stream::TryStreamExt`

Especially people new to rust can be overwhelmed by all the information and a lot of information here is redundant because the type of foo is repeated over and over again. pas_mtts on reddit came up with the idea to replace the actual type of foo (futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>> ) with typeof(foo), so the error message would look like this:

error[E0599]: typeof(foo) = futures_util::stream::stream::forward::Forward<impl futures_core::stream::TryStream, futures_util::sink::map_err::SinkMapErr<futures_util::sink::fanout::Fanout<futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>, futures_channel::mpsc::UnboundedSender<(bytes::bytes::Bytes, std::net::SocketAddr)>>, [closure@src/main.rs:218:47: 218:101]>>

no method named `into_stream` found for type `typeof(foo)` in the current scope
   --> src/main.rs:220:19
    |
220 |     let foo = foo.into_stream();
    |                   ^^^^^^^^^^^ method not found in `typeof(foo)`
    |
    = note: the method `into_stream` exists but the following trait bounds were not satisfied:
            `&typeof(foo) : futures_util::future::future::FutureExt`
            `&typeof(foo) : futures_util::stream::try_stream::TryStreamExt`
            `&mut typeof(foo) : futures_util::future::future::FutureExt`
            `&mut typeof(foo) : futures_util::stream::try_stream::TryStreamExt`
            `typeof(foo) : futures_util::future::future::FutureExt`
            `typeof(foo) : futures_util::stream::try_stream::TryStreamExt`

which is much nicer to read.
It would be great if rust would report errors like this and would make it much easier to read error messages in the console.

Meta

rustc --version --verbose:

rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-unknown-linux-gnu
release: 1.42.0
LLVM version: 9.0

[jonas-schievink]

Duplicate of #50310